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understanding the nature of the ultimate power source in ultraluminous infrared galaxies ( ulirgs ) has been a primary driver for numerous scientific investigations ever since the discovery in the 1980 s that ulirgs represent the most luminous class of galaxies in the local universe . with total power outputs comparable to those of quasars , characterizing the source of power is of fundamental importance in understanding the composition and evolution of normal galaxies . ulirgs are found to share the common trait that virtually all have recently or are currently experiencing major encounters with other galaxies @xcite . encounters like these may play a significant role in the process of building normal galaxies . with the discovery of faint sources at sub - millimeter wavelengths having characteristic spectral energy distributions like that typically found in ulirgs ( e.g. , arp  220 ) and redshifts @xmath7 , the importance of understanding these systems and their role in the evolution of normal galaxies has been heightened . the present survey was designed to look for near - infrared spectral signatures of obscured active galactic nuclei ( agn ) in ulirgs , either by the presence of velocity - broadened pa@xmath2 emission or by the appearance of the high excitation [ coronal line . a summary of the conclusions of this survey , along with a median ulirg near - infrared spectrum , are presented in @xcite . only two of the 33 sample galaxies show clear indications for agn activity , with the rest appearing to be dominated by star formation processes . either the agn phenomenon is relatively rare among ulirgs , or the extinction at 2 @xmath1 m is still too high to permit a proper assessment . mid infrared studies ( e.g. , * ? ? ? * ; * ? ? ? * ) favor the former conclusion , though it is found in this study that the extinction at 2 @xmath1 m may often be greater than four magnitudes , resulting in over one magnitude of extinction at mid - infrared wavelengths . this paper presents the individual nuclear spectra for the sample galaxies , for both the primary and secondary nuclei where possible . two - dimensional spectra are also presented to give a qualitative view of the line emission distributions typically found in ulirgs . some discussion of extinction measures , molecular hydrogen excitation mechanisms , and a summary of typical line ratios found in ulirgs is included , as is a discussion of the velocity differences found among double nucleus systems . the sample of 33 ulirgs in this survey originates from the 2jy sample of _ iras _ ( infrared astronomical satellite ) galaxies defined by @xcite . the galaxies in the present sample are chosen to meet the following criteria :    1 .   60 @xmath1 m flux density , @xmath8(60 @xmath1 m ) @xmath9 1.94 jy 2 . moderate or high quality iras flux measurements at 60 and 100 @xmath1 m ( as defined in the * ? ? ? * ) 3 .   @xmath10(60 @xmath1m)@xmath11(12 @xmath1m)@xmath12(25 @xmath1 m ) , where @xmath8(@xmath13 ) is the _ iras _ flux density 4 . far - infrared luminosity @xmath14 5 .   absolute galactic latitude , @xmath15 6 . declination , @xmath16 7 . redshift , @xmath17    the first six conditions are those used to define the northern 2 jy ulirg sample in @xcite , such that the current sample is a subset of this work . the seventh condition places both the pa@xmath2 and br@xmath3 lines in the @xmath18 band atmospheric window . the far - infrared luminosity is computed from the iras flux densities by the following conversion:@xmath19 l_{\odot } , \ ] ] @xcite where @xmath20 represents the luminosity distance to the source in mpc .    due to the redshift constraint , the current sample approximately represents a complete volume - limited sample , because even the galaxies with the minimum 60 @xmath1 m flux density are luminous enough to meet the sample criteria at all allowed redshifts . at the outer redshift limit of the survey , corresponding to a luminosity distance of 455 mpc using @xmath21 km  s@xmath6  mpc@xmath6 and @xmath22 ( as is assumed throughout ) , the minimum 60 @xmath1 m flux density of 1.94 jy by itself accounts for @xmath23 of the total far - infrared luminosity . to make up the remaining @xmath24 necessary for sample inclusion , the 100 @xmath1 m flux density needs only be greater than 1.25 jy , implying a maximum @xmath8(60 @xmath1m)@xmath25(100 @xmath1 m ) ratio of 1.55 . ulirgs very rarely have 60 @xmath1 m to 100 @xmath1 m flux density ratios this high , such that the volume limit imposed by the upper redshift cutoff is more stringent than the flux density cutoff , resulting in a sample that is mostly volume - limited rather than flux - limited . taking the 64 ulirgs defined in the northern 2 jy sample of @xcite , one finds an average @xmath8(60 @xmath1m)@xmath25(100 @xmath1 m ) ratio of @xmath26 , with a median value of 0.86 and a maximum of 1.64 ( in iras  08572@xmath273915 ) , which is the only ratio exceeding the maximum of 1.55 compatible with the volume - limited case . the ulirg from the northern 2 jy sample with the second highest ratio is iras  05246@xmath270103 , with @xmath8(60 @xmath1m)@xmath25(100 @xmath1m)@xmath28 . both of these galaxies are contained in the present sample . the redshift limit excludes many of the highest luminosity ulirgs , as these are most likely found at larger distances . for instance , mrk  1014a quasar and a ulirg  would be a member of the sample if not for the upper redshift limit .    of the 35 ulirgs in the redshift survey of @xcite satisfying the above conditions , two are not included here for the following reasons . the spectra of iras  21396@xmath273623 show this galaxy to be at a redshift of @xmath29 , rather than the previously reported @xmath30 . iras  19297@xmath310406 was free of any discernible lines , which may be the result of improper pointing , producing the spectrum of a nearby star in the very crowded field . therefore the entire sample contains 33 galaxies , and is very nearly complete in the volume - limited sense . a list of the sample galaxies appears in table  [ tab : sample ] . lccc    iras 00262@xmath324251 & 29205 & 12.08 & s + iras 01521@xmath325224 & 23959 & 11.95 & 5.5 + iras 04232@xmath321436 & 23640 & 11.99 & s + iras 05246@xmath320103 & 29105 & 12.05 & 6.0 + iras 08030@xmath325243 & 24946 & 11.97 & s + iras 08311@xmath332459 & 30167 & 12.40 & s + iras 08344@xmath325105 & 28999 & 11.94 & s + iras 08572@xmath323915 & 17491 & 12.08 & 5.6 + iras 09061@xmath331248 & 22014 & 11.97 & 5.6 + iras 09111@xmath331007 & 16247 & 11.98 & ( 37 ) + iras 09583@xmath324714 & 25748 & 11.98 & 25.5 + iras 10035@xmath324852 & 19449 & 11.93 & 9.8 + iras 10190@xmath321322 & 22898 & 11.98 & 4.0 + iras 10494@xmath324424 & 27671 & 12.15 & s + iras 11095@xmath330238 & 31936 & 12.20 & s + iras 12112@xmath320305 & 22009 & 12.27 & 2.9 + iras 14348@xmath331447 & 24798 & 12.28 & 3.3 + iras 14352@xmath331954 & 26938 & 11.95 & s + iras 14394@xmath325332 & 31401 & 12.04 & ( 28 ) + iras 15245@xmath321019 & 22629 & 11.96 & 2.6 + iras 15250@xmath323609 & 16515 & 11.99 & s + iras 15462@xmath330450 & 30060 & 12.16 & s + iras 16487@xmath325447 & 31110 & 12.12 & 3.1 + iras 17028@xmath325817 & 31805 & 12.11 & 13.0 + iras 18470@xmath323233 & 23520 & 12.02 & ( 7.0 ) + iras 19458@xmath320944 & 29983 & 12.36 & s + iras 20046@xmath330623 & 25286 & 12.02 & s + iras 20087@xmath330308 & 31613 & 12.39 & s + iras 20414@xmath331651 & 26061 & 12.19 & s + iras 21504@xmath330628 & 23343 & 11.94 & s + iras 22491@xmath331808 & 23264 & 12.12 & 1.6 + iras 23327@xmath322913 & 32225 & 12.03 & 12.5 + iras 23365@xmath323604 & 19305 & 12.13 & s + observations were conducted using the palomar longslit infrared spectrograph @xcite , operating on the 200-inch hale telescope and employing a 256@xmath34256 hgcdte ( nicmos  3 ) array . observations were made over a time period beginning in 1995 july , and ending in 1997 december . each object was observed in three grating settings , with each spanning @xmath35 @xmath1 m at a scale of @xmath36 @xmath1 m  pixel@xmath6 . typical slit widths of @xmath373.75 pixels , or 0625 , resulted in spectra with resolutions of @xmath38 , corresponding to @xmath37300 km  s@xmath6 . the grating settings were chosen to cover the pa@xmath2 line at 1.8751 @xmath1 m , the suite of br@xmath39 , h@xmath40  10 s(3 ) , and [ lines centered at 1.954 @xmath1 m , and the h@xmath40  10 s(1 ) and br@xmath3 lines centered at 2.14 @xmath1 m . the slit was generally rotated such that spectra of both primary and secondary nuclei were obtained simultaneously when possible , or otherwise to coincide with the major axis of the galaxy , where evident . observational details are provided for each source in table  [ tab : obs ] . individual exposure times were 300 s , with total integration times generally around 1800 s. in all cases , the galaxy was dithered between two positions on the slit , generally @xmath41 apart , with a smaller scale dither pattern employed about these points to eliminate the effects of static bad pixels . in this way , the sky integrations were obtained simultaneously , with the effective sky position alternating by @xmath42 to either side . wavelength calibration was provided either by the atmospheric oh airglow spectrum , or by arc lamp spectra for the h@xmath40  10 s(1)@xmath27br@xmath3 grating settings , where the oh lines become unavailable . wavelengths reported here are in air units , with the oh wavelength data coming from @xcite . atmospheric transmission variations as a function of wavelength were compensated via observations of the nearly featureless spectra of g dwarf stars at similar airmass , and similar telescope pointing , when possible . the g star observations were performed either immediately prior to or following each galaxy observation . stellar types and airmasses of the calibrators are listed for each object in table  [ tab : obs ] . the light from the calibrator star was made to uniformly fill the slit aperture by chopping the telescope secondary mirror in a triangle - wave pattern . as such , the atmospheric calibrators perform the dual function of operating as the flat field for the spectral observations . details about the treatment of the spectral calibration appear below . the calibration star observation for iras  12112@xmath270305 was compromised beyond repair , so the spectra for this galaxy were divided by an atmospheric template . each spectrum is reduced pair - wise in the following manner . pairs containing spectra at the two opposite slit positions are subtracted , with cosmic rays and static bad pixels interpolated . the data are divided by the g star spectrum and multiplied by a planck function , described in more detail below . a wavelength reference image is produced using either a median combination of the data frames to produce an oh airglow spectrum , or by using a combination of noble gas arc lamp spectra . the wavelength calibration image is spatially and spectrally rectified onto a rectilinear grid using predetermined maps of the distortion in each of these directions . a cubic function of fixed cubic coefficient and zero quadratic coefficient ( determined empirically to fit the distortion well ) is fit to the wavelength reference , providing a mapping from pixel coordinates to linear wavelength coordinates . this mapping is combined with the predetermined distortion maps , and then applied in a single interpolation to the data frames . once rectified , the pairs are combined into a single spectrum , using gaussian centroids for the spatial registration , or known pixel offsets for galaxies having diffuse or faint continua . this process removes any residual sky subtraction signal , as one effectively subtracts the simultaneous sky at the alternate slit position in the process . the noise at each pixel in the two - dimensional spectrum is computed along with the spectral data by quadrature combination of read noise and poisson shot noise based on signal level for each pixel . pixel replacement , atmospheric calibrator division , and interpolation in the data frames are also properly treated in the noise frames throughout the process . the same extractions applied to the spectral data are performed on the noise image , producing a one - dimensional noise array along with the extracted spectrum . the seeing at the time of observation was assessed by taking short guided exposures typically 10 s  of nearby field stars through the wide - open slit of the spectrograph , with the spectrograph operating in imaging mode . in this way , the point spread function ( psf ) of the telescope - plus - atmosphere during the spectral observations could be estimated . these psf exposures were typically taken 34 times during an observation , usually at the beginnings and ends of the 3040 minute intervals spent at each grating setting . the full - width at half - maximum ( fwhm ) along the slit direction was measured for each psf image , and the nuclear extractions of the spectra were based on this measurement . in each case , the psf fwhm was rounded up to an integral number of pixels , and extractions matching these integer widths were performed . the centers of these apertures were not constrained , however , such that fractional pixels were summed in the production of the nuclear spectra . the widths of the extractions , in arcseconds , are listed in table  [ tab : obs ] . lrcccccc iras 00262@xmath324251 & pa@xmath43 : & 3 jan 1996 & 1800 & @xmath44 & g1.5v & 1.02/1.05 & 1.5 +  & [ si vi ] : & & 1800 & & & 1.11/1.04 & 1.7 +  & h@xmath45br@xmath46 : & & 1800 & & & 1.05/1.05 & 1.5 + iras 01521@xmath325224 & pa@xmath43 : & 2 jan 1996 & 1800 & @xmath47 & g5vb & 1.06/1.09 & 2.0 +  & [ si vi ] : & & 1800 & & & 1.06/1.08 & 1.8 +  & h@xmath45br@xmath46 : & & 1800 & & & 1.07/1.08 & 1.7 + iras 04232@xmath321436 & pa@xmath43 : & 26 nov 1996 & 1800 & @xmath48 & g0v & 1.07/1.04 & 0.7 +  & [ si vi ] : & & 1800 & & & 1.07/1.06 & 0.8 +  & h@xmath45br@xmath46 : & & 1800 & & & 1.06/1.06 & 0.7 + iras 05246@xmath320103 & pa@xmath43 : & 5 dec 1996 & 1200 & @xmath49 & g1v & 1.19/1.18 & 0.8 +  & [ si vi ] : & & 1800 & & & 1.21/1.21 & 0.8 +  & h@xmath45br@xmath46 : & & 1800 & & & 1.18/1.17 & 0.7 + iras 08030@xmath325243 & pa@xmath43 : & 31 dec 1995 & 1800 & @xmath50 & g5v & 1.07/1.07 & 1.5 +  & [ si vi ] : & & 1800 & & & 1.07/1.06 & 1.3 +  & h@xmath45br@xmath46 : & & 1800 & & & 1.06/1.07 & 1.3 + iras 08311@xmath332459 & pa@xmath43 : & 8 apr 1996 & 1800 & @xmath44 & g3v & 1.91/1.99 & 1.7 +  & [ si vi ] : & & 1800 & & & 1.92/1.82 & 1.7 +  & h@xmath45br@xmath46 : & & 1200 & & & 1.94/1.93 & 1.0 + iras 08344@xmath325105 & pa@xmath43 : & 3 jan 1996 & 1800 & @xmath44 & g0v & 1.09/1.10 & 1.7 +  & [ si vi ] : & & 1800 & & & 1.05/1.09 & 1.5 +  & h@xmath45br@xmath46 : & & 1800 & & & 1.07/1.09 & 1.5 + iras 08572@xmath323915 & pa@xmath43 : & 25 nov 1996 & 2400 & @xmath51 & g0v & 1.01/1.01 & 0.8 +  & [ si vi ] : & & 1200 & & & 1.03/1.02 & 0.7 +  & h@xmath45br@xmath46 : & & 1800 & & & 1.01/1.01 & 0.7 + iras 09061@xmath331248 & pa@xmath43 : & 28 nov 1996 & 1800 & @xmath52 & g0v & 1.49/1.51 & 1.5 +  & [ si vi ] : & & 1800 & & & 1.47/1.43 & 2.0 +  & h@xmath45br@xmath46 : & & 1800 & & & 1.45/1.46 & 1.8 + iras 09111@xmath331007 & pa@xmath43 : & 11 feb 1997 & 1800 & @xmath53 & g2v & 1.39/1.45 & 1.7 +  & [ si vi ] : & & 1200 & & & 1.42/1.39 & 1.5 +  & h@xmath45br@xmath46 : & & 1800 & & & 1.39/1.41 & 1.7 + iras 09583@xmath324714 & pa@xmath43 : & 3 jan 1996 & 1800 & 565 & g0v & 1.03/1.03 & 1.5 +  & [ si vi ] : & & 1800 & 565 & & 1.03/1.04 & 1.2 +  & h@xmath45br@xmath46 : & 31 dec 1995 & 1800 & @xmath54 & & 1.04/1.03 & 0.8 + iras 10035@xmath324852 & pa@xmath43 : & 22 feb 1997 & 1200 & @xmath55 & g0v & 1.04/1.05 & 1.0 +  & [ si vi ] : & 23 may 1997 & 1800 & & g1v & 1.21/1.15 & 0.8 +  & h@xmath45br@xmath46 : & & 1800 & & f9v & 1.09/1.06 & 0.7 + iras 10190@xmath321322 & pa@xmath43 : & 22 may 1997 & 1800 & @xmath56 & g0v & 1.15/1.16 & 1.0 +  & [ si vi ] : & & 1800 & & g1v & 1.26/1.23 & 1.2 +  & h@xmath45br@xmath46 : & & 1800 & & g0v & 1.44/1.42 & 1.0 + iras 10494@xmath324424 & pa@xmath43 : & 22 feb 1997 & 1800 & @xmath57 & g2v & 1.03/1.02 & 1.0 +  & [ si vi ] : & & 1200 & & g0vs & 1.15/1.10 & 0.8 +  & h@xmath45br@xmath46 : & & 1800 & & & 1.09/1.10 & 0.7 + iras 11095@xmath330238 & pa@xmath43 : & 6 may 1996 & 1800 & @xmath58 & g1v & 1.24/1.26 & 1.0 +  & [ si vi ] : & & 1800 & & & 1.29/1.31 & 1.0 +  & h@xmath45br@xmath46 : & & 1800 & & & 1.25/1.29 & 0.8 + iras 12112@xmath320305 & pa@xmath43 : & 5 may 1996 & 1800 & @xmath59 & & 1.09/ & 1.0 +  & [ si vi ] : & & 1800 & & & 1.05/ & 0.8 +  & h@xmath45br@xmath46 : & & 1800 & & & 1.07/ & 1.0 + iras 14348@xmath331447 & pa@xmath43 : & 5 may 1996 & 1800 & @xmath60 & g1v & 1.51/1.47 & 1.0 +  & [ si vi ] : & & 1800 & & g4v & 1.72/1.68 & 1.0 +  & h@xmath45br@xmath46 : & & 1800 & & & 1.59/1.62 & 1.0 + iras 14352@xmath331954 & pa@xmath43 : & 6 may 1996 & 1800 & @xmath44 & g3v & 1.70/1.73 & 1.2 +  & [ si vi ] : & & 1800 & & g0v & 1.71/1.79 & 0.8 +  & h@xmath45br@xmath46 : & & 1800 & & g3v & 1.68/1.72 & 0.8 + iras 14394@xmath325332 & pa@xmath43 : & 5 may 1996 & 1800 & @xmath61 & g1v & 1.34/1.30 & 0.8 +  & [ si vi ] : & 6 may 1996 & 1800 & & & 1.51/1.50 & 0.8 +  & h@xmath45br@xmath46 : & & 1800 & & & 1.36/1.32 & 0.7 + iras 15245@xmath321019 & pa@xmath43 : & 22 may 1997 & 1800 & @xmath62 & g8v & 1.11/1.12 & 1.2 +  & [ si vi ] : & & 1800 & & & 1.09/1.11 & 1.0 +  & h@xmath45br@xmath46 : & & 1800 & & & 1.10/1.10 & 1.0 + iras 15250@xmath323609 & pa@xmath43 : & 21 may 1997 & 1800 & @xmath63 & g2v & 1.03/1.01 & 1.0 +  & [ si vi ] : & & 2400 & & & 1.01/1.02 & 1.0 +  & h@xmath45br@xmath46 : & & 1800 & & & 1.00/1.01 & 0.7 + iras 15462@xmath330450 & pa@xmath43 : & 7 apr 1996 & 1800 & @xmath64 & g2.5v & 1.32/1.35 & 1.0 +  & [ si vi ] : & 8 apr 1996 & 1800 & & & 1.41/1.36 & 0.8 +  & h@xmath45br@xmath46 : & 7 apr 1996 & 1800 & & & 1.39/1.37 & 1.0 + iras 16487@xmath325447 & pa@xmath43 : & 21 may 1997 & 1800 & @xmath65 & g5v & 1.08/1.11 & 0.7 +  & [ si vi ] : & & 1800 & & & 1.11/1.12 & 0.8 +  & h@xmath45br@xmath46 : & & 1800 & & g0va & 1.17/1.20 & 0.7 + iras 17028@xmath325817 & pa@xmath43 : & 22 may 1997 & 1800 & @xmath66 & g2v & 1.14/1.13 & 0.8 +  & [ si vi ] : & & 1800 & & & 1.18/1.14 & 1.0 +  & h@xmath45br@xmath46 : & & 1800 & & g0va & 1.24/1.26 & 0.7 + iras 18470@xmath323233 & pa@xmath43 : & 31 jul 1996 & 1800 & @xmath67 & g8v & 1.05/1.00 & 0.8 +  & [ si vi ] : & & 1800 & & & 1.00/1.00 & 1.0 +  & h@xmath45br@xmath46 : & & 1800 & & & 1.01/1.00 & 1.0 + iras 19458@xmath320944 & pa@xmath43 : & 2 aug 1996 & 1800 & @xmath68 & f5v+g0v & 1.09/1.09 & 0.7 +  & [ si vi ] : & & 1200 & & & 1.09/1.12 & 0.7 +  & h@xmath45br@xmath46 : & & 1800 & & & 1.09/1.09 & 0.7 + iras 20046@xmath330623 & pa@xmath43 : & 12 aug 1997 & 1800 & @xmath69 & g2v & 1.37/1.40 & 1.0 +  & [ si vi ] : & & 1200 & & g1v & 1.31/1.27 & 1.0 +  & h@xmath45br@xmath46 : & & 1200 & & & 1.31/1.27 & 1.0 + iras 20087@xmath330308 & pa@xmath43 : & 11 aug 1997 & 1800 & @xmath70 & g1v & 1.36/1.35 & 1.0 +  & [ si vi ] : & & 1800 & & g0v & 1.51/1.43 & 0.8 +  & h@xmath45br@xmath46 : & 12 aug 1997 & 1800 & & g1v & 1.39/1.43 & 1.3 + iras 20414@xmath331651 & pa@xmath43 : & 1 aug 1996 & 1800 & @xmath71 & g2v & 1.56/1.56 & 1.0 +  & [ si vi ] : & & 1200 & & g1v & 1.71/1.70 & 0.8 +  & h@xmath45br@xmath46 : & & 1800 & & g2v & 1.62/1.56 & 0.8 + iras 21504@xmath330628 & pa@xmath43 : & 3 aug 1996 & 1800 & @xmath51 & g0v & 1.30/1.33 & 0.8 +  & [ si vi ] : & & 1200 & & g8v & 1.54/1.62 & 2.0 +  & h@xmath45br@xmath46 : & & 1800 & & g0v & 1.34/1.33 & 0.7 + iras 22491@xmath331808 & pa@xmath43 : & 2 aug 1996 & 1800 & @xmath72 & g8v & 1.62/1.61 & 1.0 +  & [ si vi ] : & & 1800 & & & 1.63/1.62 & 1.2 +  & h@xmath45br@xmath46 : & & 1800 & & & 1.60/1.61 & 1.0 + iras 23327@xmath322913 & pa@xmath43 : & 11 aug 1997 & 1800 & @xmath73 & g1.5v & 1.00/1.01 & 1.0 +  & [ si vi ] : & & 1200 & & & 1.03/1.01 & 0.7 +  & h@xmath45br@xmath46 : & 10 jul 1995 & 1200 & & g0v & 1.00/1.00 & 0.8 + iras 23365@xmath323604 & pa@xmath43 : & 26 nov 1996 & 1800 & @xmath74 & g1.5v & 1.01/1.01 & 0.8 +  & [ si vi ] : & 28 nov 1996 & 1800 & & g2v & 1.00/1.01 & 2.0 +  & h@xmath45br@xmath46 : & 26 nov 1996 & 1800 & & g1.5v & 1.03/1.02 & 0.8 +    the most significant stellar absorption feature present in the 2 @xmath1 m window for the atmospheric calibrator stars is the br@xmath3 line at 2.1655 @xmath1 m . while a template stellar spectrum may be adequate for removal of this line , we instead interpolated over the line in the two - dimensional g star calibration / flat - field spectrum before dividing the galaxy data by the calibrator data . multiplying the corrected galaxy spectrum by a planck blackbody function with a temperature matching that of the calibrator rectifies the spectrum to a proper representation in flux density . to remove the effects of the weaker absorption lines in the atmospheric calibrator , the galaxy spectrum was multiplied by a normalized solar spectrum , smoothed and re - binned to the same resolution . for the long wavelength grating setting covering the h@xmath40  10 s(1 ) and br@xmath3 lines , the co band structure becomes significant when using late - type g stars for the atmospheric calibration . when the atmospheric calibrator was of type g4g8 , a linear combination of g3 v and k0 v template spectra from @xcite was used instead of the solar spectrum . again , this template was smoothed and re - binned to match the individual galaxy spectra . the chief results from this spectroscopic survey  that agn appear to be rare among ulirgs  has been reported previously in @xcite . in the present paper we focus attention on the range of observed properties rather than on the collected sample as a whole . spectral extractions for the 33 primary ulirg nuclei . ] figure  [ fig : primary ] presents spectral extractions for the primary nuclei in the 33 sample galaxies , with aperture sizes indicated in table  [ tab : obs ] . each plot is fixed in the rest frame , with the observed wavelength scale indicated at bottom . annotations mark the locations of every common emission line , though in any individual spectrum only the lines with reasonable detections are labeled . the location of the tick marks is based on the observed redshift of the pa@xmath2 line , as is the rest wavelength scale at top . each spectrum is composed of three independent spectral datasets . for each of the three spectral ranges , a best linear fit to the line - free continuum is assessed for the purpose of determining an overall average slope for the spectrum . the individual spectra are then scaled to lie on a common linear `` backbone '' defined by the average slope , and combined in a noise - weighted manner across the overlap region around 1.92 @xmath1 m . the vertical scale is normalized such that the continuum is at one unit of flux density at 2.155 @xmath1 m , corresponding to the middle of the @xmath75 bandpass . spectral extractions for the 14 secondary ulirg nuclei . ] figure  [ fig : secondary ] contains spectra of the secondary nuclei in the 14 sample galaxies with a nearby galactic companion , employing the same wavelength scale used for figure  [ fig : primary ] . annotations and scalings are the same as for figure  [ fig : primary ] , though some of the weaker lines are not marked by default in this set . the tick marks denoting line positions are based on the redshift of the pa@xmath2 line in the _ primary _ nucleus , so that velocity displacements between the two nuclei are more easily seen . the numerical flux density scale for each is identical to that for the corresponding spectrum in figure  [ fig : primary ] , allowing direct comparison of the relative levels for each . the relative scaling of the three individual spectra determined for figure  [ fig : primary ] are used in figure  [ fig : secondary ] as well . the choice of primary versus secondary nucleus is based solely on the continuum level , with the brighter nucleus designated as the primary . the only exception is iras  22491@xmath311808 , for which the primary nucleus is identified with the dominant line - emitting , yet weaker continuum nucleus to the east . the western nucleus is 30% brighter , but lacks the strong h@xmath40 emission that is associated with _ every _ other primary ulirg nucleus . comparison of figures  [ fig : primary ] and [ fig : secondary ] indicate that the primary nuclei , besides being the brightest continuum sources , also exhibit apparently stronger line emission . yet despite the weaker line strengths , the secondary nuclei often do show appreciable line emission activity , appearing relatively similar to the primary nuclei . if the observed pa@xmath2 line strength is proportional to star formation rates  and therefore total luminosity  then even though the dominant luminosity can be associated with one nucleus , the lesser nucleus still contributes a substantial fraction of the total luminosity . it must be realized , however , that the high levels of extinction present in ulirgs could easily lead one to misjudge the identity of the primary source of infrared luminosity based simply on apparent line strengths . table  [ tab : mofoprim ] presents the line measurements from the primary nuclear data shown in figure  [ fig : primary ] and table  [ tab : mofosec ] presents the same data for the secondary nuclear spectra of figure  [ fig : secondary ] . for each galaxy , the continuum slope and measured pa@xmath2 fwhm are given , as well as equivalent width , fwhm , and velocity centroid of each significant line . in this context , weaker hydrogen recombination and molecular hydrogen vibration - rotation lines are excluded from the table as their properties follow from those of the stronger lines . the continuum slope , @xmath2 , is reported as the power - law index , where @xmath76 . the equivalent linear slope , with @xmath77   $ ] , where @xmath78 m , is @xmath79 . the measured line properties reported in tables  [ tab : mofoprim ] and [ tab : mofosec ] are obtained by the procedures detailed in appendix  [ procedures ] . lcclccccccc + iras 00262@xmath324251 & @xmath80 & 331 & & & & & & & & + & & & ew & @xmath810.1 & @xmath820.15 & @xmath830.14 & & @xmath840.088 & @xmath850.24 & + & & & fwhm & 420@xmath86240 & 366@xmath8627 & 480@xmath8642 & & 260@xmath86160 & 380@xmath8656 & + & & & @xmath87 & @xmath8878 & @xmath8918 & @xmath9029 & & @xmath9163 & @xmath9227 & + iras 01521@xmath325224s & @xmath93 & 394 & & & & & & & & + & & & ew & & @xmath940.44 & @xmath950.14 & & & @xmath960.36 & @xmath970.49 + & & & fwhm & & 356@xmath8623 & 170@xmath8696 & & & 290@xmath86120 & 0@xmath86110 + & & & @xmath87 & & @xmath9816 & @xmath9946 & & & @xmath10034 & @xmath10176 + iras 04232@xmath321436 & @xmath102 & 183 & & & & & & & & + & & & ew & & @xmath1030.25 & @xmath1040.15 & & & @xmath1050.16 & @xmath1060.26 + & & & fwhm & & 218@xmath8618 & 195@xmath8670 & & & 203@xmath8651 & 220@xmath86100 + & & & @xmath87 & & @xmath10715 & @xmath10833 & & & @xmath10926 & @xmath11046 + iras 05246@xmath320103e & @xmath111 & 110 & & & & & & & & + & & & ew & @xmath1120.13 & @xmath1130.19 & @xmath1140.26 & @xmath1150.2 & & @xmath1160.14 & @xmath1170.24 + & & & fwhm & 497@xmath8678 & 190.1@xmath869.6 & 575@xmath8677 & 310@xmath86260 & & 161@xmath8640 & 210@xmath86210 + & & & @xmath87 & @xmath11851 & @xmath11922 & @xmath12047 & @xmath12155 & & @xmath12238 & @xmath123110 + iras 08030@xmath325243 & @xmath124 & 322 & & & & & & & & + & & & ew & @xmath1250.13 & @xmath1260.16 & @xmath1270.11 & & @xmath1280.1 & @xmath1290.22 & @xmath1300.28 + & & & fwhm & 340@xmath86160 & 326.2@xmath866.8 & 331@xmath8677 & & 0@xmath8691 & 270@xmath86130 & 380@xmath86110 + & & & @xmath87 & @xmath13171 & @xmath13212 & @xmath13334 & & @xmath13487 & @xmath13531 & @xmath13644 + iras 08311@xmath332459 & @xmath137 & 314 & & & & & & & & + & & & ew & & @xmath1380.29 & @xmath1390.15 & @xmath1400.088 & @xmath1410.078 & @xmath1420.092 & @xmath1430.25 + & & & fwhm & & 477@xmath8610 & 501@xmath8641 & 708@xmath8652 & 360@xmath86130 & 351@xmath8627 & 470@xmath86170 + & & & @xmath87 & & @xmath14416 & @xmath14526 & @xmath14620 & @xmath14745 & @xmath14821 & @xmath14934 + iras 08344@xmath325105 & @xmath150 & 287 & & & & & & & & + & & & ew & @xmath1510.13 & @xmath1520.26 & @xmath1530.13 & & & @xmath1540.27 & @xmath1300.36 + & & & fwhm & 60@xmath86110 & 300@xmath8613 & 196@xmath8640 & & & 310@xmath86100 & 310@xmath86140 + & & & @xmath87 & @xmath15539 & @xmath15612 & @xmath15728 & & & @xmath15830 & @xmath15935 + iras 08572@xmath323915n & @xmath160 & 340 & & & & & & & & + & & & ew & @xmath1610.13 & @xmath1620.1 & @xmath1630.062 & & & @xmath1640.027 & @xmath1650.072 + & & & fwhm & 440@xmath86110 & 334.8@xmath868 & 139@xmath8645 & & & 135@xmath8630 & 271@xmath8662 + & & & @xmath87 & @xmath16653 & @xmath16719 & @xmath16837 & & & @xmath16931 & @xmath17037 + iras 09061@xmath331248s & @xmath171 & 371 & & & & & & & & + & & & ew & @xmath1720.5 & @xmath1730.25 & @xmath1740.092 & & @xmath1750.19 & @xmath1160.13 & @xmath1760.23 + & & & fwhm & 240@xmath86280 & 320@xmath8624 & 421@xmath8699 & & 100@xmath86240 & 266@xmath8658 & 260@xmath86110 + & & & @xmath87 & @xmath177120 & @xmath17813 & @xmath17926 & & @xmath18096 & @xmath18123 & @xmath18245 + iras 09111@xmath331007 & @xmath183 & 223 & & & & & & & & + & & & ew & @xmath810.11 & @xmath1840.1 & @xmath1850.033 & & @xmath1860.052 & @xmath1870.035 & @xmath1880.062 + & & & fwhm & 290@xmath86210 & 246@xmath8613 & 189@xmath8627 & & 310@xmath8691 & 164@xmath8625 & 168@xmath8666 + & & & @xmath87 & @xmath18999 & @xmath19012 & @xmath19117 & & @xmath19251 & @xmath14620 & @xmath19326 + iras 09583@xmath324714ne & @xmath194 & 247 & & & & & & & & + & & & ew & @xmath1950.11 & @xmath1960.2 & @xmath1970.22 & & & @xmath1980.25 & @xmath1990.25 + & & & fwhm & 0@xmath8663 & 263.6@xmath866.1 & 411@xmath8674 & & & 384@xmath8659 & 256@xmath8698 + & & & @xmath87 & @xmath16924 & @xmath20012 & @xmath10825 & & & @xmath20128 & @xmath20238 + iras 10035@xmath324852ne & @xmath203 & 237 & & & & & & & & + & & & ew & @xmath2040.049 & @xmath2050.19 & @xmath2060.13 & & @xmath2070.11 & @xmath2080.089 & @xmath2090.15 + & & & fwhm & 0@xmath8661 & 254@xmath8612 & 315@xmath8643 & & 0@xmath86240 & 256@xmath8618 & 201@xmath8671 + & & & @xmath87 & @xmath21029 & @xmath21117 & @xmath21235 & & @xmath213170 & @xmath21428 & @xmath21544 + iras 10190@xmath321322e & @xmath216 & 445 & & & & & & & & + & & & ew & & @xmath2170.13 & @xmath2180.081 & & @xmath2190.072 & @xmath2200.096 & @xmath2210.11 + & & & fwhm & & 437@xmath8616 & 412@xmath8672 & & 0@xmath86130 & 336@xmath8665 & 320@xmath8691 + & & & @xmath87 & & @xmath22219 & @xmath22337 & & @xmath224140 & @xmath18232 & @xmath22534 + iras 10494@xmath324424 & @xmath226 & 430 & & & & & & & & + & & & ew & @xmath2270.11 & @xmath2280.12 & @xmath2290.072 & & & @xmath2300.11 & @xmath2310.16 + & & & fwhm & 510@xmath86160 & 404.2@xmath867.1 & 377@xmath8638 & & & 427@xmath8665 & 364@xmath8675 + & & & @xmath87 & @xmath16634 & @xmath23216 & @xmath21429 & & & @xmath23329 & @xmath16631 + iras 11095@xmath330238 & @xmath234 & 269 & & & & & & & & + & & & ew & @xmath2350.3 & @xmath2360.39 & @xmath2370.25 & & & @xmath2380.61 & @xmath2391 + & & & fwhm & 360@xmath86180 & 286@xmath8612 & 350@xmath8649 & & & 241@xmath8643 & 0@xmath86100 + & & & @xmath87 & @xmath24064 & @xmath24118 & @xmath24226 & & & @xmath24335 & @xmath244130 + iras 12112@xmath320305sw & @xmath245 & 159 & & & & & & & & + & & & ew & @xmath2460.61 & @xmath2470.39 & @xmath2480.086 & & @xmath2490.14 & @xmath2500.071 & @xmath2510.15 + & & & fwhm & 380@xmath86340 & 303@xmath8621 & 295@xmath8663 & & 220@xmath86290 & 0@xmath8631 & 185@xmath8653 + & & & @xmath87 & @xmath244140 & @xmath25221 & @xmath25342 & & @xmath254190 & @xmath25530 & @xmath16633 + iras 14348@xmath331447sw & @xmath256 & 227 & & & & & & & & + & & & ew & @xmath1760.15 & @xmath2570.19 & @xmath1140.12 & & @xmath2580.083 & @xmath2590.14 & @xmath2600.2 + & & & fwhm & 550@xmath86120 & 236.8@xmath868.4 & 450@xmath8635 & & 120@xmath86210 & 334@xmath8636 & 127@xmath8680 + & & & @xmath87 & @xmath26143 & @xmath26222 & @xmath25335 & & @xmath263110 & @xmath26430 & @xmath26540 + iras 14352@xmath331954 & @xmath266 & 192 & & & & & & & & + & & & ew & @xmath2670.089 & @xmath2680.23 & @xmath2690.11 & & & @xmath2700.15 & @xmath2710.26 + & & & fwhm & 258@xmath8655 & 205.3@xmath864.9 & 193@xmath8658 & & & 243@xmath8648 & 166@xmath8646 + & & & @xmath87 & @xmath27233 & @xmath27317 & @xmath27437 & & & @xmath26534 & @xmath27532 + iras 14394@xmath325332 & @xmath276 & 363 & & & & & & & & + & & & ew & @xmath2770.064 & @xmath2780.13 & @xmath2790.097 & & @xmath2800.08 & @xmath2810.16 & @xmath2820.27 + & & & fwhm & 558@xmath8677 & 381.8@xmath868.7 & 490@xmath8622 & & 160@xmath86280 & 426@xmath8633 & 370@xmath86110 + & & & @xmath87 & @xmath28333 & @xmath28421 & @xmath28531 & & @xmath286170 & @xmath28733 & @xmath28847 + iras 15245@xmath321019e & @xmath289 & 283 & & & & & & & & + & & & ew & @xmath2900.069 & @xmath2910.14 & @xmath2920.067 & & @xmath2930.078 & @xmath2940.088 & @xmath2950.13 + & & & fwhm & 0@xmath8675 & 294.6@xmath867.3 & 307@xmath8646 & & 300@xmath86210 & 247@xmath8650 & 304@xmath8651 + & & & @xmath87 & @xmath10839 & @xmath29616 & @xmath25530 & & @xmath29767 & @xmath10827 & @xmath29825 + iras 15250@xmath323609 & @xmath299 & 57 & & & & & & & & + & & & ew & @xmath3000.23 & @xmath3010.16 & @xmath3020.14 & & @xmath3030.1 & @xmath3040.059 & @xmath3050.077 + & & & fwhm & 0@xmath86180 & 147.8@xmath869.9 & 323@xmath8688 & & 400@xmath86180 & 223@xmath8619 & 204@xmath8634 + & & & @xmath87 & @xmath306200 & @xmath30717 & @xmath27535 & & @xmath29860 & @xmath30832 & @xmath30933 + iras 15462@xmath330450 & @xmath310 & 285 & & & & & & & & + & & & ew & & @xmath3110.085 & @xmath3120.029 & & & @xmath3130.024 & @xmath3140.06 + & & & fwhm & & 535@xmath8615 & 297@xmath8653 & & & 102@xmath8643 & 360@xmath86110 + & & & @xmath87 & & @xmath31524 & @xmath31640 & & & @xmath31732 & @xmath31844 + iras 16487@xmath325447w & @xmath319 & 347 & & & & & & & & + & & & ew & @xmath3200.18 & @xmath3210.34 & @xmath3220.29 & & & @xmath3230.39 & @xmath3240.89 + & & & fwhm & 500@xmath86140 & 332.3@xmath869.1 & 374@xmath8636 & & & 265@xmath8654 & 230@xmath86140 + & & & @xmath87 & @xmath32540 & @xmath32623 & @xmath32733 & & & @xmath20230 & @xmath32878 + iras 17028@xmath325817w & @xmath329 & 339 & & & & & & & & + & & & ew & @xmath3300.097 & @xmath3310.13 & @xmath3320.078 & & @xmath3330.13 & @xmath1160.14 & @xmath3340.25 + & & & fwhm & 320@xmath86130 & 315.6@xmath868.7 & 250@xmath8648 & & 140@xmath86170 & 301@xmath8674 & 336@xmath8694 + & & & @xmath87 & @xmath21546 & @xmath33524 & @xmath33633 & & @xmath337130 & @xmath27539 & @xmath22545 + iras 18470@xmath323233 & @xmath338 & 228 & & & & & & & & + & & & ew & @xmath3390.4 & @xmath3400.44 & @xmath3410.16 & & @xmath3420.16 & @xmath2060.17 & @xmath820.25 + & & & fwhm & 200@xmath86120 & 258.2@xmath866.9 & 305@xmath8668 & & 200@xmath86300 & 319@xmath8649 & 249@xmath8650 + & & & @xmath87 & @xmath34351 & @xmath34421 & @xmath34544 & & @xmath346120 & @xmath34534 & @xmath10030 + iras 19458@xmath320944 & @xmath347 & 269 & & & & & & & & + & & & ew & @xmath3480.086 & @xmath3490.075 & @xmath3500.13 & & & @xmath3510.089 & @xmath3520.29 + & & & fwhm & 390@xmath86180 & 272.9@xmath865.1 & 324@xmath8684 & & & 237@xmath8650 & 150@xmath86110 + & & & @xmath87 & @xmath35358 & @xmath35435 & @xmath11859 & & & @xmath33647 & @xmath35553 + iras 20046@xmath330623 & @xmath356 & 244 & & & & & & & & + & & & ew & & @xmath3570.21 & @xmath1720.2 & & @xmath3580.2 & @xmath3590.44 & @xmath3601 + & & & fwhm & & 247@xmath8627 & 190@xmath86100 & & 0@xmath86140 & 0@xmath8688 & 310@xmath86170 + & & & @xmath87 & & @xmath36114 & @xmath36243 & & @xmath363140 & @xmath27560 & @xmath30980 + iras 20087@xmath330308 & @xmath364 & 595 & & & & & & & & + & & & ew & @xmath3650.093 & @xmath3660.14 & @xmath3670.067 & & & @xmath3680.15 & @xmath3690.24 + & & & fwhm & 270@xmath86150 & 574@xmath8623 & 475@xmath8649 & & & 570@xmath86110 & 280@xmath86280 + & & & @xmath87 & @xmath37048 & @xmath37120 & @xmath37233 & & & @xmath37330 & @xmath37497 + iras 20414@xmath331651 & @xmath375 & 515 & & & & & & & & + & & & ew & @xmath3760.16 & @xmath3770.21 & @xmath1530.16 & & & @xmath3780.17 & @xmath3790.63 + & & & fwhm & 400@xmath86190 & 488@xmath8620 & 287@xmath8682 & & & 460@xmath86120 & 170@xmath86180 + & & & @xmath87 & @xmath38071 & @xmath38125 & @xmath38249 & & & @xmath26546 & @xmath28392 + iras 21504@xmath330628 & @xmath383 & 256 & & & & & & & & + & & & ew & @xmath3840.14 & @xmath3850.14 & @xmath3860.14 & & & @xmath3870.099 & @xmath3520.13 + & & & fwhm & 230@xmath86100 & 260.8@xmath865.6 & 250@xmath86130 & & & 310@xmath8643 & 144@xmath8640 + & & & @xmath87 & @xmath38852 & @xmath38926 & @xmath21544 & & & @xmath39043 & @xmath39141 + iras 22491@xmath331808e & @xmath392 & 128 & & & & & & & & + & & & ew & & @xmath3930.38 & @xmath3940.23 & & & @xmath3950.37 & @xmath3960.83 + & & & fwhm & & 131@xmath8618 & 0@xmath8652 & & & 302@xmath8660 & 430@xmath86350 + & & & @xmath87 & & @xmath39724 & @xmath39841 & & & @xmath21433 & @xmath39977 + iras 23327@xmath322913s & @xmath400 & 151 & & & & & & & & + & & & ew & @xmath3000.11 & @xmath4010.12 & @xmath3860.26 & & & @xmath4020.36 & + & & & fwhm & 170@xmath86130 & 193@xmath869 & 110@xmath86120 & & & 210@xmath86120 & + & & & @xmath87 & @xmath40376 & @xmath40418 & @xmath40577 & & & @xmath40651 & + iras 23365@xmath323604 & @xmath407 & 186 & & & & & & & & + & & & ew & & @xmath4080.083 & @xmath4090.079 & & & @xmath4100.057 & @xmath4110.094 + & & & fwhm & & 191.1@xmath865 & 245@xmath8645 & & & 219@xmath8622 & 216@xmath8639 + & & & @xmath87 & & @xmath41217 & @xmath15724 & & & @xmath41325 & @xmath34328 +    lcclcccccc + iras 01521@xmath325224n & @xmath414 & 403 & & & & & & & + & & & ew & & @xmath2210.6 & & & & + & & & fwhm & & 190@xmath86230 & & & & + & & & @xmath87 & & @xmath415170 & & & & + iras 05246@xmath320103w & @xmath416 & 54 & & & & & & & + & & & ew & @xmath4170.16 & @xmath4180.31 & & & @xmath4190.29 & + & & & fwhm & 0@xmath86140 & 72@xmath8619 & & & 60@xmath86100 & + & & & @xmath87 & @xmath420120 & @xmath42123 & & & @xmath42277 & + iras 08572@xmath323915s & @xmath423 & & & & & & & & + & & & ew & & @xmath4240.21 & & & & + & & & fwhm & & 0@xmath8619 & & & & + & & & @xmath87 & & @xmath42522 & & & & + iras 09061@xmath331248n & @xmath426 & & & & & & & & + & & & ew & & @xmath4270.59 & @xmath1610.29 & & @xmath3340.39 & + & & & fwhm & & 252@xmath8676 & 0@xmath86170 & & 253@xmath8695 & + & & & @xmath87 & & @xmath42830 & @xmath420140 & & @xmath42961 & + iras 09583@xmath324714sw & @xmath430 & 186 & & & & & & & + & & & ew & & @xmath4310.16 & @xmath4190.19 & & & + & & & fwhm & & 246@xmath8641 & 320@xmath86180 & & & + & & & @xmath87 & & @xmath43231 & @xmath43376 & & & + iras 10035@xmath324852sw & @xmath434 & 424 & & & & & & & + & & & ew & @xmath1880.23 & @xmath1730.54 & @xmath4350.13 & & @xmath2700.14 & @xmath1290.17 + & & & fwhm & 50@xmath86160 & 430@xmath8631 & 320@xmath86110 & & 292@xmath8647 & 212@xmath8667 + & & & @xmath87 & @xmath420130 & @xmath43631 & @xmath43754 & & @xmath43842 & @xmath38848 + iras 10190@xmath321322w & @xmath439 & 265 & & & & & & & + & & & ew & & @xmath4400.14 & @xmath3420.092 & @xmath4410.11 & @xmath3690.13 & @xmath4420.18 + & & & fwhm & & 271@xmath8626 & 40@xmath86110 & 0@xmath86150 & 63@xmath8689 & 150@xmath86120 + & & & @xmath87 & & @xmath44321 & @xmath30664 & @xmath444170 & @xmath44556 & @xmath44668 + iras 12112@xmath320305ne & @xmath447 & 196 & & & & & & & + & & & ew & & @xmath4480.52 & @xmath4490.13 & & @xmath4500.2 & @xmath3600.39 + & & & fwhm & & 271@xmath8619 & 401@xmath8627 & & 366@xmath8649 & 278@xmath8678 + & & & @xmath87 & & @xmath45120 & @xmath45236 & & @xmath13530 & @xmath19136 + iras 14348@xmath331447ne & @xmath453 & 283 & & & & & & & + & & & ew & @xmath4540.3 & @xmath4550.35 & @xmath4560.15 & & @xmath2390.25 & @xmath1530.5 + & & & fwhm & 450@xmath86160 & 272@xmath8614 & 218@xmath8663 & & 274@xmath8676 & 0@xmath86120 + & & & @xmath87 & @xmath45764 & @xmath45823 & @xmath45935 & & @xmath46036 & @xmath461100 + iras 15245@xmath321019w & @xmath462 & 426 & & & & & & & + & & & ew & & @xmath4630.38 & @xmath4640.19 & & @xmath4650.24 & + & & & fwhm & & 310@xmath8670 & 406@xmath8674 & & 78@xmath8654 & + & & & @xmath87 & & @xmath46631 & @xmath18042 & & @xmath25441 & + iras 17028@xmath325817e & @xmath467 & 0 & & & & & & & + & & & ew & & @xmath4680.76 & & & & + & & & fwhm & & 97@xmath8633 & & & & + & & & @xmath87 & & @xmath46932 & & & & + iras 22491@xmath331808w & @xmath470 & 86 & & & & & & & + & & & ew & @xmath4710.38 & @xmath4720.2 & @xmath4730.21 & & & + & & & fwhm & 0@xmath86140 & 0@xmath8623 & 0@xmath8699 & & & + & & & @xmath87 & @xmath244100 & @xmath47423 & @xmath121160 & & & +    figure  [ fig : twod ] presents the two - dimensional spectra around the pa@xmath2 line for each of the sample galaxies . in each , the horizontal axis origin is located at the center of the primary nucleus . the apertures corresponding to the extractions displayed in figures  [ fig : primary ] and [ fig : secondary ] are indicated at the top and bottom of each panel . annotations to the left and right of each panel denote the sense of the slit orientation , the exact position angles for which are listed in table  [ tab : obs ] . the two - dimensional spectrum for iras  09583@xmath274714 has been split into two panels , though the horizontal scale reflects the real separation between the two nuclear components . the @xmath18 band spectra of the galaxies in this sample are dominated by the pa@xmath2 line , which is the strongest line accessible to search for signs of broad emission . only in two of the sample galaxies , iras  08311@xmath312459 and iras  15462@xmath310450 , is any such emission apparent . both of these galaxies are optically classified as seyfert galaxies , so it seems that obscured agn in ulirgs are either extremely rare , or suffer a few magnitudes of extinction even at near - infrared wavelengths , as was also suggested by @xcite . a few galaxies , such as iras  14348@xmath311447 and iras  14394@xmath275332 have excess emission on the blue side of the pa@xmath2 line , but in both cases there appears to be emission from contributing significantly to this flux , and no appreciable emission on the red side of the narrow pa@xmath2 profile . furthermore , the h@xmath40 lines in iras  14394@xmath275332 have an asymmetric blue shape more typical of wind or outflow phenomena than of velocity - broadening due to an agn . iras  20087@xmath310308 has some hint of broad emission at the base of the pa@xmath2 line , though not significantly elevated out of the noise , and again confused by . as seen in figure  [ fig : twod ] , the moderate 600 km  s@xmath6 width of the narrow portion of the pa@xmath2 profile in iras  20087@xmath310308 can be attributed to a steep rotation gradient on the nucleus . the apparent broad base on the pa@xmath2 line in the southwestern nucleus of iras  12112@xmath270305 may be spurious , as this is the one galaxy in the sample whose spectra were not calibrated by a g star observation , but rather by a template atmospheric spectrum . the excess blue emission happens to be coincident with the significant co@xmath40 atmospheric absorption at 2.002.02 @xmath1 m , so that a slight error in estimated column depth results in a large localized continuum offset in the corrected spectrum . two dimensional spectra around the pa@xmath475 line for each galaxy . the horizontal scale is in arcseconds , with the primary nucleus at the origin . the spectral direction spans about 11 resolution elements , or about 3300 km  s@xmath476 . extraction apertures are indicated on the top and bottom of each frame . ] the two - dimensional spectra of pa@xmath2 in figure  [ fig : twod ] demonstrate that the atomic recombination line emission in ulirgs is almost always dominated by a nuclear component . several ulirgs show off - nuclear emission at lower levels , and a couple of sources appear to be primarily characterized by diffuse line emission  namely iras  04232@xmath271436 and iras  20046@xmath310623 . when two nuclei are present , the secondary nucleus generally shows emission in pa@xmath2 as well . exceptions to this are iras  16487@xmath275447 and iras  23327@xmath272913 . the extra continuum object seen in the panel with iras  08030@xmath275243 is a nearby star . the fraction of the total line emission that is unresolved can be estimated for each ulirg by computing the flux contribution from a gaussian spatial profile having the width of the estimated seeing and sharing the maximum amplitude of the spatial line emission distribution . comparing this flux with the measured flux provides an upper limit to the fraction of light contained within an unresolved core . most of the primary ulirg nuclei are consistent with being unresolved , with a median seeing of 10 and a median physical resolution of 1.4  kpc . the scatter in the estimated fraction of unresolved flux is at the @xmath47720% level , mostly due to uncertainty in the seeing estimate . only 8 of the 33 ulirgs have estimated unresolved contributions lower than 80% , such that the majority of ulirgs contain most  if not all  of their pa@xmath2 emission within the central 1  kpc radius . only three ulirgs in the sample ( iras  04232@xmath271436 , iras  18470@xmath273233 , and iras  20046@xmath310623 ) have less than 50% of their pa@xmath2 flux in an unresolved core . the sample was chosen to allow coverage of both the pa@xmath2 and br@xmath3 emission lines so that a comparison between these lines could be made in order to assess extinction to the line emitting source . table  [ tab : props ] lists the inferred visual extinction values for the sample galaxies . these estimates assume an intrinsic ratio for pa@xmath4br@xmath3 of 12.1 ( * ? ? ? * case b with @xmath478 @xmath479 and @xmath480 k ) , and @xmath481 and @xmath482 ( interpolated from the extinction law of * ? ? ? though there exists very little leverage in wavelength between pa@xmath2 and br@xmath3 , some ulirgs are apparently obscured enough to show definite reddening across this short range . typical 1@xmath483 errors on the extinction estimates are around 5 magnitudes , though the actual errors could be larger if disparate aperture sizes were used in the spectral extractions . indeed , each of the four ulirgs with extinctions more negative than 1@xmath483 have aperture differences between the pa@xmath2 and br@xmath3 spectra of at least 50% . almost half of the ulirgs have estimated extinctions greater than the 1@xmath483 limit , and 11 have at least 2@xmath483 extinction estimates  typically corresponding to @xmath484 mag , with many in the range of @xmath485 mag . histogram of estimated visual extinctions to the pa@xmath475 line emission for the 33 primary ulirg nuclei . different shadings represent varying levels of reliability , with most of the negative estimates owing to mismatched seeing conditions and therefore significant extraction aperture differences . the mean and median points are marked for the reliable set of measurements . one point , corresponding to @xmath486 , is beyond the plot boundary . ] figure  [ fig : extinct ] presents the extinction measurements in histogram form . different shading represents varying levels of confidence in the measurement . higher extinction estimates are allowed greater variance in classifying their reliability , provided they are significantly different from zero extinction . counting only the 22 ulirgs with reliable extinction measures , the median visual extinction of @xmath487 mag corresponds to an extinction at pa@xmath2 of approximately one magnitude . this is a lower limit to the nuclear extinction because we can only estimate extinction to the gas we actually detect at 2 @xmath1 m , which very likely is peripheral to the nuclear region . nonetheless , one magnitude of extinction at pa@xmath2 would not readily hide the presence of a central agn . the very large visual extinctions of 25 mag seen in some ulirgs translate to almost 4 mag of extinction at pa@xmath2 , which almost certainly would hide an agn from view with the currently achieved signal - to - noise ratio in the ulirg spectra . lcccc    iras 00262@xmath324251 & @xmath488 & @xmath489 & @xmath490 & mixture + iras 01521@xmath325224 & @xmath491 & @xmath492 & 5000 & thermal@xmath493 + iras 04232@xmath321436 & @xmath494 & @xmath495 & & thermal + iras 05246@xmath320103 & @xmath496 & @xmath497 & & thermal + iras 08030@xmath325243 & @xmath498 & @xmath499 & & thermal + iras 08311@xmath332459 & @xmath500 & @xmath501 & & thermal + iras 08344@xmath325105 & @xmath502 & @xmath503 & @xmath504 & mixture + iras 08572@xmath323915 & @xmath505 & @xmath506 & @xmath507 & mixture + iras 09061@xmath331248 & @xmath508 & @xmath509 & 3500 & thermal@xmath493 + iras 09111@xmath331007 & @xmath510 & @xmath511 & 2700 & thermal@xmath493 + iras 09583@xmath324714 & @xmath512 & @xmath513 & & thermal + iras 10035@xmath324852 & @xmath514 & @xmath515 & & thermal + iras 10190@xmath321322 & @xmath516 & @xmath517 & & thermal + iras 10494@xmath324424 & @xmath518 & @xmath519 & 3500 & thermal@xmath493 + iras 11095@xmath330238 & @xmath520 & @xmath521 & @xmath522 & mixture + iras 12112@xmath320305 & @xmath523 & @xmath506 & 5000 & thermal@xmath493 + iras 14348@xmath331447 & @xmath524 & @xmath525 & & thermal + iras 14352@xmath331954 & @xmath526 & @xmath527 & & thermal + iras 14394@xmath325332 & @xmath528 & @xmath529 & 2800 & thermal@xmath493 + iras 15245@xmath321019 & @xmath530 & @xmath531 & & thermal + iras 15250@xmath323609 & @xmath532 & @xmath533 & @xmath534 & mixture + iras 15462@xmath330450 & @xmath535 & @xmath536 & & thermal + iras 16487@xmath325447 & @xmath537 & @xmath521 & & thermal + iras 17028@xmath325817 & @xmath538 & @xmath539 & & thermal + iras 18470@xmath323233 & @xmath540 & @xmath541 & & thermal + iras 19458@xmath320944 & @xmath542 & @xmath543 & & thermal + iras 20046@xmath330623 & @xmath544 & @xmath545 & & thermal + iras 20087@xmath330308 & @xmath546 & @xmath547 & & thermal + iras 20414@xmath331651 & @xmath548 & @xmath549 & & thermal + iras 21504@xmath330628 & @xmath550 & @xmath551 & & thermal + iras 22491@xmath331808 & @xmath552 & @xmath553 & & thermal + iras 23327@xmath322913 & @xmath554 & @xmath555 & & thermal + iras 23365@xmath323604 & @xmath556 & @xmath557 & @xmath558 & mixture +    because an active galactic nucleus is believed to indicate the presence of a qso ( quasi - stellar object ) in the galaxy nucleus , one may quantify the amount of obscuration necessary to hide a central agn by estimating the extinction required to hide the line flux expected from the high - velocity wings of pa@xmath2 in a typical qso of the appropriate bolometric luminosity . for this purpose , we use the median qso spectrum presented in @xcite to represent the appearance of an unobscured quasar . identifying the region around 1.881 @xmath1 m ( @xmath559 km  s@xmath6 redward of the pa@xmath2 line center ) as being well away from the narrow pa@xmath2 emission , we measure the median qso flux density to be 22% higher than the qso continuum level . it is found that the broad pa@xmath2 emission from an exposed qso contributing half the bolometric luminosity of a ulirg would be comparable at 1.881 @xmath1 m to the typical ulirg continuum emission ( neglecting the contribution from the qso continuum ) . therefore , hiding the presence of the broad emission requires an attenuation factor that is comparable to the signal - to - noise ratio of the ulirg continuum . in the case of the median ulirg spectrum presented in @xcite , this translates to an extinction at pa@xmath2 of 4.4 mag , and therefore a visual extinction , @xmath560 mag . the spectra of the individual ulirgs presented here typically have continuum signal - to - noise ratios around 20 , so that @xmath561 mag , or @xmath56225 mag of extinction is required to hide a qso at half the ulirg luminosity . details on this calculation appear in appendix  [ qsoextinct ] . the estimates of extinction made in this way are conservative in two regards . first , the aperture photometry of ulirgs used to establish typical continuum flux densities are based on 5@xmath563 apertures , such that extra flux is included from diffuse emission away from the nucleus . in smaller apertures corresponding to the spectral extractions employed in this paper , the ulirg continuum is diminished , while the spatially unresolved qso component would be hardly affected , thus increasing the qso - to - ulirg contrast considerably , making the qso more difficult to conceal . the second conservative aspect is that the criterion of forcing the broad pa@xmath2 from the qso to lie at the 1@xmath483 level on the ulirg continuum would not hide the broad emission very effectively , the reason being that the 1@xmath483 level is computed for a single pixel ( one - quarter of a resolution element ) . when summed over a few resolution elements ( i.e. , up to the narrow shoulder of the pa@xmath2 line ) , the broad signal would stand out at more than the 3@xmath483 level . the result that , given the current level of sensitivity , at least 25 magnitudes of visual extinction are required to obscure a qso in a typical ulirg , together with the paucity of detections of low - level broad line emission  especially evident in the median spectrum of @xcite  leads us to conclude that agn are either rare in ulirgs , or very deeply buried . there are five ulirgs in the present sample with measured extinctions based on the pa@xmath2/br@xmath3 ratio that are high enough to hide agn without requiring additional attenuation . we are unable to determine the likelihood that the rest of the ulirgs have the requisite nuclear extinctions to hide the presence of agn . the technique of comparing ulirg spectra to a typical qso spectrum can also be used to estimate the extinction to the broad - line region seen in iras  15462@xmath310405 . based on the flux density at 1.881 @xmath1 m , the pa@xmath2 emission contributes an additional 9% to the continuum level , some of which is likely continuum emission from the qso itself . correcting for this , one finds an excess over the assumed ulirg continuum ( 60% of the total continuum ) measuring 15% , suggesting an extinction @xmath564 mag , or @xmath565 mag . the most abundant species of line emission in the ulirg spectra arise from the vibration - rotation spectrum of molecular hydrogen . the molecular hydrogen emission shows up strongly in _ all _ of the primary nuclei of this sample . three or four lines are typically present in each spectrum , most notably the odd rotation state 10 transitions . the h@xmath40 line strengths vary by more than one order - of - magnitude relative to pa@xmath2 . figure  [ fig : pah2 ] shows the range of observed line ratios , comparing the h@xmath40  10 s(1 ) line to pa@xmath2 . a typical ulirg has a h@xmath40  10 s(1)@xmath566pa@xmath2 ratio around 0.15 , varying from @xmath567 to 0.40 . one galaxy , iras  00262@xmath274251 exhibits h@xmath40 emission even stronger than the pa@xmath2 line . the bins in figure  [ fig : pah2 ] are shaded to reflect the classification of the h@xmath40 excitation mechanisms , as will be discussed below . ratio of the h@xmath568  10 s(1 ) line strength to that of pa@xmath475 for the 33 primary ulirg nuclei . a linear scale appears at top . typical 1@xmath569 errors are comparable to the bin width , though perhaps a little smaller . the mean is performed on the linear scale , rather than on the logarithmic scale . shadings indicating excitation mechanisms are included for reference . ] the apparent weakness of the even rotation transitions is an indication that the emission is thermally excited , as the thermal condition produces three times as many odd ( ortho ) states as even ( para ) states . this so - called ortho - to - para ratio is more typically in the range of 1.01.8 for fluorescent excitation conditions @xcite . in addition , the lack of strong emission from the 21 vibration transition indicates a thermal origin . molecular hydrogen population diagrams for six of the ulirgs , showing the range of properties seen in the h@xmath568 excitation . the top two show examples of mixed thermal and fluorescent excitation , while the bottom four are consistent with a single temperature characterization . iras 10494@xmath5704244 and iras 14394@xmath5705332 may have some slight component of fluorescent excitation , judging by the elevated 21 lines . for even ( para ) rotation states , two points are plotted , corresponding to ortho - to - para ratios of 3:1 and 1:1 , with boxes corresponding to the 3:1 case , and asterisks for 1:1 . the population diagram for iras 18470@xmath5703233 is fairly typical of most of the ulirgs in this sample in terms of excitation information and signal - to - noise ratio . line fits are accompanied by temperatures corresponding to the slopes . ] population diagrams were constructed for each of the primary nucleus extractions in the sample , and each characterized in terms of rotation temperature , vibration temperature ( where appropriate ) and classified in terms of thermal or mixed thermal / fluorescent excitation . the construction of population diagrams is discussed in appendix  [ append ] . examples of these diagrams are shown in figure  [ fig : h2plots ] . the even rotation state transitions are plotted as two points representing ortho - to - para ratios of 3:1 and 1:1 . the 3:1 points , represented by boxes in figure  [ fig : h2plots ] , are almost always preferred over the 1:1 points ( asterisks ) appearing below them . the great majority of ulirgs in this sample have no detections in the 21 transition lines , but with error bars consistent with purely thermal excitation . solid lines in figure  [ fig : h2plots ] represent rotation temperature fits , and dashed lines , where present , indicate the vibration temperature separating 10 and 21 points . a few ulirgs appear to be definite mixtures of thermal and fluorescent excitation , with the population diagrams showing the 21 data points well above the rotation temperature line passing through the 10 points . a number of other ulirgs have somewhat elevated 21 measures , with both the s(2 ) and s(4 ) lines above the 10 rotation temperature line , but with error bars that are still consistent with pure thermal emission . the excitation characterizations are summarized in table  [ tab : props ] . for the mixed cases , a vibration temperature estimate is given , assuming that the high energy states exist with a lower ortho - to - para ratio , likely around 2:1 . this has been observed in other mixed excitation cases ( e.g. , * ? ? ? * ) , and is explained by the fact that the enhanced 21 transition emission arises from molecules that are primarily excited by fluorescent processes , and thus reflect an ortho - to - para ratio closer to that expected for pure fluorescence . one point that should be mentioned in the context of the line population diagrams is the possible blend of a 2.0024 @xmath1 m [ line with the h@xmath40  21 s(4 ) line at 2.0035 @xmath1 m . the expected flux ratio between the [ line at 1.9670 @xmath1 m and that at 2.0024 @xmath1 m is 23:1 . with the 1.9670 @xmath1 m line appearing to be present in a number of ulirgs , it is to be expected that the h@xmath40  21 s(4 ) line will suffer some contamination by the [ line . indeed , the population diagrams often show an anomalously high 21 s(4 ) line flux , with an incompatibly low 21 s(2 ) flux . there is a good correlation between such cases and galaxies for which the 1.9670 @xmath1 m [ line is believed to be present . rotation temperatures determined for the 33 primary ulirg nuclei . the majority are consistent with thermal emission , for which the rotation temperature represents the ambient equilibrium temperature . a few are definitely mixtures of thermal and fluorescent processes , with vibration temperatures in the range of 31005000 k. ]    figure  [ fig : h2temp ] illustrates the distribution of h@xmath40 rotation temperatures , indicating a strong clustering around 2200 k. two of the three ulirgs with estimated rotation temperatures in excess of 4000 k are classified as having agn . it is possible in these cases that a significant fraction of the h@xmath40 emission arises from molecular gas excited by x - ray radiation from the central source . @xcite found this mechanism to be the most viable source of h@xmath40 excitation in ngc  6240 , with the x - rays in this instance most probably arising from frequent supernovae or from merger - induced shock activity . table  [ tab : h2 ] of appendix  [ append ] provides expected h@xmath40 line ratios in the thermal regime across the 1.82.4 @xmath1 m wavelength range . referring to figure  [ fig : pah2 ] , there does not appear to be an overwhelming trend between h@xmath40 line strength relative to pa@xmath2and excitation mechanism , except for a possible slight tendency for mixtures to be seen among the brighter lines . this could simply be a consequence of higher signal - to - noise levels for the weak 21 lines in those galaxies with bright h@xmath40 lines . more mixed states may be revealed with greater sensitivity , most likely in those cases identified here as possible mixtures . the lineis detected in the majority ( 27/33 ) of the primary nuclear spectra , and in a number of the secondary spectra . the noise - weighted mean of detected @xmath566pa@xmath2 is 0.058 , with a relatively small dispersion of @xmath571 about this value . the median @xmath566pa@xmath2 ratio is 0.063 among detected lines , and 0.056 including non - detections as zero - valued points . the full range of detected ratios goes from 0.02 ( iras  15245@xmath273609 ) to about 0.11 ( iras  14348@xmath311447 ; iras  14394@xmath275332 ) . lower resolution near - infrared spectra could easily mistake this line for a blue wing on the pa@xmath2 line . the [ line at 1.9670 @xmath1 m appears in several spectra , most notably the primary nuclei of iras  00262@xmath274251 , iras  09111@xmath311007 , iras  15245@xmath271019 , and iras  15250@xmath273609 , and the secondary nucleus of iras  10190@xmath271322 . these stronger [ emitters have [ @xmath566pa@xmath2 ratios up to 0.05 , though iras  00262@xmath274251 , with its weak pa@xmath2 , has a ratio around 0.12 . there are many possible line detections near the noise limit that have typical [ @xmath566pa@xmath2 ratios around 0.010.02 . lines weaker than 1% of pa@xmath2 are generally too weak to be detected with confidence . the [ line reported for iras  08311@xmath312459 is possibly confused with the wing of [ . a more thorough analysis of this line complex at slightly higher resolution does not find conclusive evidence for the [ line in this galaxy @xcite . searching for the 1.9629 @xmath1 m [ line was a primary motivation for the present spectroscopic survey . with a 167 ev excitation potential , this line indicates a high probability of the presence of an active nucleus . while it is possible to produce si@xmath572 atoms via powerful shocks , @xcite find that photoionization is the more likely excitation mechanism among a sample of nearby seyfert galaxies . as stars are incapable of producing significant quantities of photons at this high energy , the natural photoionization source is an active nucleus with its power - law spectrum extending into the far - ultraviolet wavelength range . only one galaxy in the sample of 33 has prominent [ emission . iras  08311@xmath312459 has a very strong [ line , comparable in strength to the adjacent h@xmath40  10 s(3 ) line , and appearing broader than the other lines in the spectrum . a detailed discussion of the [ line in this galaxy can be found in @xcite and @xcite .    the only other galaxy in the sample with possible [ emission is iras  05246@xmath270103 . the spectrum of this galaxy shows emission at the expected position of [ , with a signal - to - noise ratio of @xmath375 . the adjacent h@xmath40 line appears broad on the short - wavelength side as well , calling into question the nature of the emission on the long - wavelength side of this line . no emission appears at this position in the secondary nucleus spectrum of figure  [ fig : secondary ] , ruling out a problem with atmospheric calibration or related issues . the h@xmath40 10 s(1 ) line appears unresolved in the primary nucleus , strongly suggesting that the [ emission is real . while iras  05246@xmath270103 has no evidence of broad line emission , it does have a peculiar radio spectrum . @xcite find this galaxy to be a gigahertz - peaked - spectrum ( gps ) radio source , with a 6 cm flux density much higher than that measured at 20 cm . this condition is not found in many ulirgs , with the parks ( pks ) galaxy iras  13451@xmath271232 being the only other 2 jy ulirg identified as a gps source . this galaxy may also have [ emission , as reported by @xcite , though the blend with the h@xmath40 line at their lower resolution makes it difficult to estimate the relative contribution of [ . the rest of the h@xmath40 lines in iras  13451@xmath271232 , as presented in @xcite , indicate a h@xmath40 rotation temperature around 2500 k , in which case the [ line would account for @xmath3730% of the total combined line flux . iras  13451@xmath271232 does show impressive broadening of the pa@xmath2 line , clearly standing out as an agn - dominated source . while iras  05246@xmath270103 has a similar radio spectrum to iras  13451@xmath271232 , it is almost three orders of magnitude less luminous at 20 cm , according to @xcite perhaps there is a low - luminosity active nucleus component to iras  05246@xmath270103 , with the total infrared luminosity dominated by star formation . the double nucleus ulirgs often exhibit strong pa@xmath2 emission from both nuclei , which can be used to study orbital dynamics of the galactic pair . table  [ tab : vels ] lists the relative radial velocities for the 12 ulirgs with measured pa@xmath2 velocities on both nuclei . the reported errors are considerably smaller than would be obtained by simply subtracting the velocities presented in table  [ tab : mofosec ] from those in table  [ tab : mofoprim ] , reflecting the fact that the relative velocities are not affected by systematic errors associated with wavelength calibration or slit illumination .    lcc    iras 01521@xmath325224 & @xmath573 & 7.6 + iras 05246@xmath320103 & @xmath574 & 9.8 + iras 08572@xmath323915 & @xmath575 & 5.8 + iras 09061@xmath331248 & @xmath576 & 7.2 + iras 09583@xmath324714 & @xmath577 & 37.5 + iras 10035@xmath324852 & @xmath578 & 11.2 + iras 10190@xmath321322 & @xmath579 & 5.3 + iras 12112@xmath320305 & @xmath580 & 3.7 + iras 14348@xmath331447 & @xmath581 & 4.7 + iras 15245@xmath321019 & @xmath582 & 3.4 + iras 17028@xmath325817 & @xmath583 & 23.0 + iras 22491@xmath331808 & @xmath584 & 2.2 +    surprisingly , the velocity differences are very small , with the largest difference at 120 km  s@xmath6 . random projection effects diminish the observed relative velocities statistically by a factor of two , as the most probable observing configuration involves pure angular motion on the plane of the sky . if all ulirgs have the same relative nuclear velocity , random orientations would produce a distribution of observed velocities described by a cosine curve , with the peak at zero observed relative velocity . binning the data from table  [ tab : vels ] into 50 km  s@xmath6 bins , the best fit to a cosine function has a maximum velocity of about 190 km  s@xmath6 , shown in figure  [ fig : velhist ] . using 40 km  s@xmath6 bins results in a 165 km  s@xmath6 maximum relative velocity . histogram of the absolute values of projected relative nuclear velocities . if all ulirgs had the same relative velocity , projection effects would result in our observing a cosine distribution . the best fit cosine to the data has a 190 km  s@xmath476 velocity amplitude . ] the lack of high relative velocities among ulirgs contrasts with the predictions obtained either from simple newtonian calculations or from more sophisticated @xmath585-body models of galaxy interactions which include gas dynamics and dynamical friction . below we discuss these two approaches , with attention to how these models might be modified to match the observations . first , we will treat the galaxies as point masses approaching each other from far away . these calculations assume conservative dynamics , that all loss of potential energy is converted into kinetic energy of the galaxies . in reality some fraction of the energy is lost to dynamical friction , which is why the galaxies do eventually merge . the foregoing discussion will not explicitly treat the frictional component , but will concentrate on the more basic elements of merger dynamics that offer much greater latitude in the ultimate relative velocities calculated . in the point mass scenario , the galaxies would reach a relative velocity @xmath586 in excess of 1300 km  s@xmath6 at a separation distance of 10 kpc , assuming total system masses of @xmath587 per galaxy . even ten times less mass per galaxy results in over 400 km  s@xmath6 of relative motion at 10 kpc . changing the initial separation from infinity to 100 kpc or 50 kpc reduces these speeds by 5% and 10% , respectively . the high velocities computed above are obviously not consistent with the observations , but perhaps understandably given that galaxies are not well represented by point masses at close range . galaxy masses are dominated by very large dark halos that act to soften the potential gradients  i.e . , accelerations  experienced during a close encounter . dark halos are thought to exist based on the anomalously flat rotation curves of galaxies to very large radii . such phenomena suggest the enclosed spherical mass is proportional to radius , such that the circular rotation velocity , @xmath588 is constant . rotation curves of large samples of galaxies via optical emission lines ( e.g. , * ? ? ? * ) indicate that this scaling persists at least out to 1015  kpc . observations in find a continuation of the flat rotation curves at radii 23 times the optical radii , often to 2030 kpc ( e.g. , * ? ? ? * ; * ? ? ? assuming a spherical halo distribution , the total halo mass following the @xmath589 scaling is given by@xmath590 where @xmath591 is the circular velocity , and @xmath592 is the outer dimension of the halo distribution . under this prescription , two galaxies falling from infinity are moving with a relative velocity of @xmath593 by the time the halos touch , or when the galaxies are @xmath594 apart . the galaxies will clearly accelerate even more as the separation decreases to @xmath595 kpc , even though the acceleration is somewhat moderated by the effect of overlapping halo masses . this already presents a problem in that the observed relative velocities are less than the typical rotation velocities observed in ulirgs . simulations of galaxy interactions , courtesy of j. c. mihos . at left is the orbital track , at center is nuclear separation as a function of time , and at right is the relative velocity as a function of time . the model galaxies each have a total mass of @xmath596 , and possess milky - way - like disk rotation velocities . ]    the simulations by @xcite provide a means of evaluating encounter velocities via models employing realistic halo potentials and dynamical friction . output from these models is shown in figure  [ fig : mihos ] . the model defines a halo that yields a total galaxy mass of @xmath597 @xmath598 when scaled to the milky way s rotation velocity . the halo has a relatively small physical extent , containing most of its mass within 25 kpc , and with an effective cutoff radius at 40 kpc . the simulations start with identical galaxies approaching from infinity ( parabolic orbits ) , resulting in time - averaged velocities of @xmath599 km  s@xmath6 when the nuclei are between 425 kpc apart . this average sums over first , second , and third encounters . the peak velocities are in the neighborhood of 500 km  s@xmath6 , as seen in figure  [ fig : mihos ] . starting the galaxies from rest at a 100 kpc separation rather than infinity reduces the peak velocities by only @xmath600% . many ulirgs are found at times well after the first major encounter , judging by the fact that their tidal features , formed at the epoch of the first significant encounter , are often vastly more extended than the nuclei are separated . however , some ulirgs are believed to be experiencing their first major encounter @xcite . looking at the first - encounter and late - encounter model predictions separately , the simulation discussed above predicts first - encounter velocities averaging 475 km  s@xmath6 , after which the galaxies separate by 50 kpc , followed by a final merger sequence with an average velocity around 275 km  s@xmath6 , ranging from 0500 km  s@xmath6 during this time . models with larger - scale halos maintaining the same internal rotation structure demand higher total masses , resulting in increased net velocity differences between the merging galaxies . on the other hand , if ulirgs lacked any halo material past 10 kpc , corresponding to total masses around @xmath601 @xmath602 , the galaxies would behave much more like point masses , and dynamical friction would be less important . in this scenario , the 10 kpc velocities would be approximately 400 km  s@xmath6 . it is very difficult to reproduce the small observed relative velocities among ulirgs with simple physical models  with or without large halos . ulirgs are massive , gas rich galaxies with rotation velocities and near - infrared absolute magnitudes characteristic of galaxies at least as massive as the milky way . the highest observed relative velocity in this sample is a mere 120 km  s@xmath6 , though the observed distribution is consistent with a typical deprojected velocity difference of 200 km  s@xmath6 . some ulirgs have been observed with velocity differences very near this value ( e.g. , iras  20046@xmath330623 ; iras  19252@xmath337245 : * ? ? ? * ; * ? ? ? * ) , but none are seen near the 400 km  s@xmath6 value as expected from a variety of physical predictions . one possibility deserves mention , which is that during the late - merger state , eccentric orbits may conspire with the delay in coordinating the accumulation of fuel for a nuclear starburst in such a way as to select ulirgs at the apocenters of their orbits , when velocities are low . referring to figure  [ fig : mihos ] , during the final merger process ( well after the initial encounter ) , the nuclear orbits are very eccentric , owing to the removal of angular momentum , largely by tidal debris . the very high velocity peaks near 500 km  s@xmath6 are associated with the close passages . if the ultraluminous activity is episodic in nature , with relatively short duration bursts occurring in reaction to transitory tidal disruptions , then the ulirgs we see may represent stages shortly after these close encounters . a finite time is required to organize gas motions in response to the close encounter , with the ultraluminous activity turning on only after a significant gas concentration has been established . it is not difficult to imagine in this scenario that the gas  moving more slowly than the galaxy , but with less distance to travel  will reach the nucleus at approximately the same time that the galaxies reach apocenter . keeping in mind that we are discussing the events occurring after the second close encounter , the orbital timescale is @xmath603 yr , such that apocenter is reached a few@xmath604 yr after close approach . this timescale is in rough agreement with expectations of the dynamical timescale within a few kpc of the nucleus  the region from which the fuel for the nuclear event is gathered . during the final merger process , it is possible that several ultraluminous bursts occur following close interactions of the nuclei . the idea of repetitive starbursts in the final stages of merging was first explored by @xcite , though in the context of cloud - cloud collisions in this case . the scheme proposed here of delayed  and potentially episodic  bursting would bias late - merger ulirgs to larger separations and therefore lower velocities , quite possibly consistent with the observed distribution . the problem with first encounter ulirgs remains , since the average first encounter velocity is around 475 km  s@xmath6 . unfortunately the current sample is not large enough to probe possible differences in the velocity distributions of early and late ulirg encounters . near - infrared spectra in the 2 @xmath1 m window have been obtained for a nearly complete , volume - limited sample of 33 ulirgs . lines of atomic hydrogen and helium recombination , molecular hydrogen vibration - rotation , [ , and [ are seen in the spectra . the pa@xmath2 is predominantly seen as a nuclear event , with the majority of the emission arising in the central kpc of most ulirgs . initiated as a search for optically hidden active galactic nuclei , this survey reveals only two such galaxies , both of which are characterized as seyfert galaxies based on visible - light spectra . it is found via comparison of the pa@xmath2 and br@xmath3 lines that significant extinction of several magnitudes persists into the near - infrared bands , such that buried active nuclei can not be ruled out in many objects . it is estimated that with the sensitivity of the current survey , at least 25 mag of visual extinction would be needed to hide a qso with half the bolometric luminosity of the ulirg . strong molecular hydrogen emission is seen in all of the primary nuclear spectra . the molecular hydrogen spectrum indicates that most h@xmath40 emission in ulirgs stems from thermal excitation rather than fluorescent processes . some ulirgs appear to have a small fraction of their molecular hydrogen emission owing to fluorescence , though none approach pure fluorescence . the relative radial velocities found in this sample of ulirgs does not exceed @xmath605 km  s@xmath6 , with an inferred maximum deprojected velocity differential of around 200 km  s@xmath6 . this relatively low velocity differential is difficult to produce from interactions between galaxies with masses comparable to that of the milky way using simple physical assumptions . it is suggested that ulirgs in the late stages of merging may be selectively found near the time of apocenter , leading to a lower average relative velocity between nuclei . we thank michael strauss for his role in defining the sample of ulirgs , and for participating in the early stages of the caltech effort in studying the 2 jy ulirgs . we also thank chris mihos , gerry neugebauer , nick scoville , and sterl phinney for helpful discussions . james graham provided information on [ transition strengths and wavelengths . many persons accompanied us on the observing runs to palomar , most notably rob knop and james larkin , both of whom provided expert tutelage on the use of the spectrograph , and on methods of data reduction . we thank the night assistants at palomar , juan carasco , rick burruss , skip staples , and karl dunscombe for their assistance in the observations . this research has made use of the nasa / ipac extragalactic database ( ned ) , which is operated by the jet propulsion laboratory , caltech under contract with nasa . is supported by the nasa graduate student researchers program , and the lewis kingsley foundation . this research is supported by a grant from the national science foundation . the line properties reported in tables  [ tab : mofoprim ] and [ tab : mofosec ] represent measured values from the spectra , rather than gaussian fit parameters . especially for the weaker lines , the quality of the continuum subtraction is vital to proper line characterization . this is done with a simultaneous multiple gaussian line fit and quadratic continuum fit . in all but a few cases , this results in a very close fit to the continuum . for those few cases where the continuum fit is questionable , some of the weaker lines are excluded from tables  [ tab : mofoprim ] and [ tab : mofosec ] . all of the fitting procedures make use of the noise estimate computed on a pixel - by - pixel basis , as described earlier . in the end , the noise is scaled slightly ( typically by factors of less than 30% ) in order to make the final reduced chi - squared value equal to one . the scaled noise is used in estimating errors on the values in tables  [ tab : mofoprim ] and [ tab : mofosec ] . each of the line parameters is computed based on the following line definition procedure . each potential line is searched for by summing two resolution elements ( 7 pixels ) of information around the line center , and determining if there is emission above the continuum exceeding 2.5@xmath483 . for lines satisfying this condition , cutoffs are established on the red and blue sides of the line center by the following rules :    1 . if the spectrum extends below the continuum level , establish a cutoff , including only positive valued points . if there exists a local minimum , followed by at least three higher points , assume that the following points belong to another line and exclude them , keeping only the minimum point . 3 .   if the next two pixels are between 01@xmath483 above the continuum , include them and quit . these procedures obviously can not practice the same discrimination that a diligent scientist might , but we find that a reasonable job is performed  especially on lines whose boundaries are obvious to the eye . nonetheless , lines with a final computed equivalent width measuring less than 2.5@xmath483 are typically excluded , though those that appear to be real in the one - dimensional spectra are preserved . conversely , a few lines with reported detections greater than 2.5@xmath483 are excluded on the basis of either poor local continuum fit or simply less - than - believable results .    because the weaker lines do not have a well defined full - width at half - maximum , the fwhm values in tables  [ tab : mofoprim ] and [ tab : mofosec ] are based on a combination of line flux and peak amplitude . if a gaussian shape is assumed , the fwhm of the line would be equal to the line flux divided by the peak amplitude , times the proportionality constant of @xmath606 . performed in this manner , it is relatively simple to compute the error of such an estimate . a strict measure of the fwhm is provided in tables  [ tab : mofoprim ] and [ tab : mofosec ] for the pa@xmath2 line in each galaxy for comparison to the value estimated by this technique . all of the reported fwhm values have been reduced to intrinsic line widths by subtracting the instrumental resolution in quadrature . each spectrum uses the measured line widths in the wavelength calibration spectrum as a basis for the decomposition . values in tables  [ tab : mofoprim ] and [ tab : mofosec ] for which the measured line width is exceeded by the instrumental width are listed as unresolved lines of zero width . almost all of the lines in this survey are resolved , typically around 200300 km  s@xmath6 in width . the velocity measurements in tables  [ tab : mofoprim ] and [ tab : mofosec ] are indicated relative to pa@xmath2 , though the pa@xmath2 velocity itself is expressed as a direct quantity in km  s@xmath6 . these values are simply computed as first - moment measures in the spectral lines , with error estimates computed from the noise estimates in a straightforward way . added to this error estimate is a wavelength calibration error , assumed to be 1015 km  s@xmath6 , and a slit placement error . the latter uncertainty attempts to estimate the wavelength offset induced by a non - uniform slit illumination . when the line source is spatially compact enough to produce a discernible peak somewhere in the slit , the location of this peak within the slit influences the ultimate wavelength associated with this emission . for these purposes , it is assumed that the line source has a real physical size roughly half that of the continuum , which is almost always spatially resolved . further , it is assumed that the source is centered in the slit with a gaussian 1@xmath483 placement error of one - quarter of a slit width . only cases of exceptional seeing result in an appreciable error from this source , with a median error estimate of 15 km  s@xmath6 and a maximum of 38 km  s@xmath6 . despite the accounting of uncertainties , the velocities indicate a dispersion outside of the error budgets that is not correlated with line species or grating setting . it may well be that the variable spectral range over which the line is summed is largely responsible for determining the offset , as the range is biased to contain only positive points , and will extend as far as possible from the line center to include such points . with the large moments generated from distant points , the estimated velocity may wander far from its true center . thus the values of velocity in tables  [ tab : mofoprim ] and [ tab : mofosec ] , especially for weaker lines , are perhaps of little use . gaussian fit centers may do a better job of appropriately representing the line velocities . the median qso spectrum is constructed from nine optically identified qsos in the redshift range @xmath607 , seven of which have available @xmath18 band photometry . comparing the absolute @xmath18 band magnitudes to bolometric luminosities of the quasars , one finds the approximate relation @xmath608 . the median infrared luminosity of the 31 non - agn ulirgs in the present sample is @xmath609 . a qso within a ulirg that contributes half of the bolometric luminosity  most of which is reprocessed into the infrared  would then have @xmath610 , or @xmath611 mag . this establishes the @xmath18 band flux density of the qso continuum . at a rest wavelength of 1.881 @xmath1 m , corresponding to @xmath612 km  s@xmath6 in velocity , it is found that the median qso has an excess flux density due to pa@xmath2 measuring 22% of the baseline continuum level @xcite . thus the absolute magnitude of the line emission at this velocity is @xmath613 mag . typical ulirg continuum levels can be approximated in a similar way as that above , using 5@xmath563 aperture photometry of the sample galaxies where available @xcite . eight of the 31 non - agn ulirgs have @xmath75 measurements , and comparing to the infrared luminosity ( 81000 @xmath1 m ) leads to the relation @xmath614 . therefore the typical non - agn ulirg in this sample , with a median luminosity @xmath615 , has @xmath616 mag . the expected ratio of broad pa@xmath2 to the ulirg continuum at 1.881 @xmath1 m is then 0.92 , with the 1@xmath483 range extending from 0.44 to 1.91 . if a given ulirg has a spectrum with a signal - to - noise ratio @xmath617 , then diminishing the qso pa@xmath2 to the 1@xmath483 level requires@xmath618 where @xmath619 represents qso contributions to continuum and line flux , @xmath620 is the ulirg continuum contribution , and @xmath621 is the attenuating factor . with the additional relations that @xmath622 and @xmath623 , we find that the required attenuation is @xmath624 , or @xmath625 magnitudes at pa@xmath2 . the visual extinction is roughly 7 times this value . the specific mechanism responsible for the h@xmath40 emission can be investigated via population diagrams . by accounting for various degeneracies , transition probabilities , and photon energy , one may compute the relative populations of the upper states of each transition based on the observed line flux . in pure thermal equilibrium , the population levels will follow an exponential profile with respect to upper state energy . more formally , the intensity of a given transition is described by@xmath626 where the upper and lower states are denoted by the 1 and 2 subscripts , @xmath627 represents vibration state , @xmath628 the rotation state , @xmath629 the ortho / para degeneracy , @xmath630 the transition probability , @xmath631 the emitted photon frequency , @xmath632 the upper state energy , and @xmath633 the equilibrium temperature . therefore a logarithmic plot of the line intensities divided by the terms multiplying the exponential above produces a linear relation against the upper energy scale with a slope proportional to the inverse of the temperature . the physical parameters for selected @xmath18 band transitions are given in table  [ tab : h2phys ] . 10 s(0 ) & 2.22268 & 2 & 2.53 & 6471 + 10 s(1 ) & 2.12125 & 3 & 3.47 & 6951 + 10 s(2 ) & 2.03320 & 4 & 3.98 & 7584 + 10 s(3 ) & 1.95702 & 5 & 4.21 & 8365 + 10 s(4 ) & 1.8914 & 6 & 4.19 & 9286 + 10 s(5 ) & 1.8353 & 7 & 3.96 & 10341 + 21 s(0 ) & 2.3550 & 2 & 3.68 & 12095 + 21 s(1 ) & 2.2471 & 3 & 4.98 & 12550 + 21 s(2 ) & 2.1536 & 4 & 5.60 & 13150 + 21 s(3 ) & 2.0729 & 5 & 5.77 & 13890 + 21 s(4 ) & 2.0035 & 6 & 5.57 & 14764 + 21 s(5 ) & 1.9443 & 7 & 5.05 & 15763 + 21 s(6 ) & 1.8942 & 8 & 4.30 & 16880 + 21 s(7 ) & 1.8523 & 9 & 3.38 & 18107 + 32 s(3 ) & 2.2008 & 5 & 5.63 & 19085 +    when the molecular hydrogen is not in thermal equilibrium  such as may be the case in fluorescent excitation by ultraviolet photons  a diagram such as that described above will not show all points lying along a common line , but rather disconnected linear segments , with common vibration states each defining a line connecting the different rotation states . the characteristic temperature of this line is referred to as the rotation temperature , @xmath634 . ideally , each vibration state has the same characteristic rotation temperature , and these segments can be connected with another line characterized by a vibration temperature , @xmath635 . the thermal case simply has @xmath636 . typically , pure fluorescent conditions are characterized by @xmath6371400 k , and @xmath6386000 k @xcite . excitation conditions associated with thermal processes are typically found to have @xmath6392400 k , with a slight difference between rotation and vibration temperatures characterized by @xmath640 @xcite .      because ulirgs tend to show relatively uniform h@xmath40 emission properties  namely those corresponding to thermal emission at 15002500 k  it may be useful for other researchers to have a table of expected h@xmath40 line strengths in the @xmath18 band for a range of typical temperatures . table  [ tab : h2 ] provides such a reference , with line intensities normalized to the h@xmath40  10 s(1 ) line . these estimates assume an ortho - to - para ratio of 3:1 , and @xmath642 , as is found empirically @xcite . in order to regain the pure thermal regime characterized by a single temperature , the 21 entries in table  [ tab : h2 ] should be multiplied by @xmath643 , and the 31 transition multiplied by @xmath644 , lowering these values considerably . 10 s(0 ) & 2.22268 & 0.228 & 0.211 & 0.201 & 0.187 + 10 s(1 ) & 2.12125 & 1.000 & 1.000 & 1.000 & 1.000 + 10 s(2 ) & 2.03320 & 0.336 & 0.374 & 0.398 & 0.438 + 10 s(3 ) & 1.95702 & 0.805 & 1.019 & 1.174 & 1.450 + 10 s(4 ) & 1.8914 & 0.177 & 0.261 & 0.329 & 0.468 + 10 s(5 ) & 1.8353 & 0.295 & 0.519 & 0.728 & 1.211 + 21 s(0 ) & 2.3550 & 0.015 & 0.030 & 0.045 & 0.083 + 21 s(1 ) & 2.2471 & 0.068 & 0.143 & 0.225 & 0.441 + 21 s(2 ) & 2.1536 & 0.023 & 0.053 & 0.089 & 0.191 + 21 s(3 ) & 2.0729 & 0.055 & 0.145 & 0.259 & 0.622 + 21 s(4 ) & 2.0035 & 0.012 & 0.037 & 0.072 & 0.197 + 21 s(5 ) & 1.9443 & 0.020 & 0.072 & 0.156 & 0.496 + 21 s(6 ) & 1.8942 & 0.003 & 0.014 & 0.033 & 0.124 + 21 s(7 ) & 1.8523 & 0.004 & 0.020 & 0.054 & 0.246 + 32 s(3 ) & 2.2008 & 0.003 & 0.016 & 0.045 & 0.202 +    turner , kirby - docken , & dalgarno.(1977 ) beckwith , s. , evans , n. j. , gatley , i. , gull , g. , & russel , r. w. 1983 , , 264 , 152 black , j. h. , & van dishoeck , e. f. 1987 , , 322 , 412 carico , d. p. , graham , j. r. , matthews , k. , wilson , t. d. , soifer , b. t. , neugebauer , g. , & sanders , d. b. 1990 , , 349 , l39 casertano , s. & van albada , t. s. 1990 , in _ baryonic dark matter _ , ed . d. lynden - bell & g. gilmore ( dordrecht : kluwer ) p. 298 clements , d. l. , sutherland , w. j. , mcmahon , r. g. , & saunders , w. 1996 , , 279 , 477 crawford , t. , marr , j. , partridge , b. , & strauss , 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we present near - infrared spectroscopy for a complete sample of 33 ultraluminous infrared galaxies at a resolution of @xmath0 . 
 most of the wavelength range from 1.802.20 @xmath1 m in the rest frame is covered , including the pa@xmath2 and br@xmath3 hydrogen recombination lines , and the molecular hydrogen vibration - rotation 10 s(1 ) and s(3 ) lines . 
 other species , such as he  i , [ fe  ii ] , and [ si  vi ] appear in the spectra as well , in addition to a number of weaker molecular hydrogen lines . 
 nuclear extractions for each of the individual galaxies are presented here , along with spectra of secondary nuclei , where available . 
 the pa@xmath2 emission is seen to be highly concentrated on the nuclei , typically with very little emision extending beyond a radius of 1  kpc . 
 this survey was carried out primarily to search for signatures of active nuclei via velocity - broadened hydrogen recombination or the presence of the [ si  vi ] coronal line . 
 these signatures are rare in the present sample , occurring in only two of the 33 galaxies . 
 the extinction to the hydrogen recombination lines is investigated via the pa@xmath4br@xmath3 line ratio . 
 it is found that visual extinctions to the nuclei in excess of 10 magnitudes are relatively common among ulirgs , and that visual extinctions greater than 25 mag are necessary to conceal a qso emitting half the total bolometric luminosity . 
 the ionized hydrogen regions in many ulirgs are sufficiently obscured that dust - enshrouded active galactic nuclei would remain hidden at 2 @xmath1 m at the current level of sensitivity 
 . the vibration - rotation lines of molecular hydrogen appear to be predominantly thermal in origin , with effective temperatures generally around 2200 k. the relative nuclear velocities between double nucleus ulirgs are investigated , through which it is inferred that the maximum deprojected velocity difference is @xmath5 km  s@xmath6 . this figure is lower than the velocities predicted by physical models of strong interactions / mergers of large , gas - rich galaxies .