You are an expert at summarizing long articles. Proceed to summarize the following text: 
more than three decades ago partridge and peebles ( 1967 ) predicted that galaxies in their first throes of star - formation should be strong emitters in the lyman-@xmath1  line . their predictions were optimistic , based on converting roughly 2% of gas into stars in @xmath19 years in milky way sized galaxies , which translates into a luminosity of @xmath20 . these objects are also expected to be common - if all the @xmath21 galaxies have undergone a phase of rapid star - formation one should see a surface density of about @xmath22 ( pritchet 1994 ) . searches based on these expectations did not detect lyman-@xmath1   emitters ( laes ) . ( see review by pritchet 1994 ; koo & kron 1980 ; pritchet & hartwick 1987 , 1990 ; cowie 1988 ; rhea et al 1989 ; smith et al 1989 ; lowenthal et al 1990 ; wolfe et al 1992 ; de propris et al 1993 ; macchetto et al 1993 ; mller & warren 1993 ; djorgovski & thompson 1992 ; djorgovski , thompson , & smith 1993 ; thompson , djorgovski , & trauger 1992 ; thompson et al 1993 ; thompson , djorgovski , & beckwith 1994 ; thommes et al 1998 . )    only recently have lyman-@xmath1  emitters been observed , albeit at luminosity levels roughly a hundred times lower than the original prediction . these lyman-@xmath1  emitters have been found from both deep narrow band imaging surveys ( cowie & hu 1998 ; hu , cowie & mcmahon 1998 ; hu , mcmahon , & cowie 1999 ; kudritzki et al 2000 ) , and from deep spectroscopic surveys ( dey et al 1998 ; manning et al 2000 ; but see stern et al 2000 ) . weak lyman-@xmath1  emitters have also been found through targeted spectroscopy of lyman break objects ( e.g. , steidel et al 1996 , lowenthal et al 1997 ) . the lower luminosity in the lyman-@xmath1  line may be because of attenuation by dust if chemical enrichment is prompt ; or because the star - forming phase is more protracted ; or because the star - formation happens in smaller units which later merge . the first two scenarios will give a smaller equivalent width than early predictions , while the last scenario results in low luminosities but high equivalent width . dust effects are expected to be severe even a small amount of dust can greatly attenuate this line , because it is resonantly scattered . however , two factors can help the lyman-@xmath1  photons escape . if lyman-@xmath1  photons are produced in diffuse regions of a clumpy interstellar medium , they can simply scatter off the dense clumps and escape ( neufeld 1991 ) , and some geometries can even lead to an increase in the equivalent width of the line . secondly , energetic winds are seen in low-@xmath23 lyman-@xmath1  emitters ( kunth et al 1998 ) . these can displace the neutral gas and doppler - shift the peak wavelength of the resonant scattering , thereby reducing the amount of scattering and the path length for interaction with dust . detailed predictions for luminosities and surface densities of laes using a press - schechter formalism and exploring a range of dust obscuration and star - formation time scales have been explored by haiman and spaans ( 1999 ) , who are able to reproduce the surface densities of laes reported by hu et al ( 1998 ) with a wide range of models . in order to narrow down the range of possibilities and characterize the high redshift lyman-@xmath1  population , better statistics over a wide range of flux and source density are needed . an efficient search for lyman-@xmath1  emitters ( and other emission line galaxies ) was started in 1998 using the ccd mosaic camera at the kitt peak national observatory s 4 m mayall telescope . the mosaic camera has eight @xmath24 chips in a @xmath25 array comprising a @xmath26 field of view . the final area covered by the lala survey is @xmath27 square - degrees in two mosaic fields centered at 14:25:57 + 35:32 ( 2000.0 ) and 02:05:20 -04:55 ( 2000.0 ) . five overlapping narrow band filters of width fwhm@xmath28are used . the central wavelengths are @xmath16 , @xmath29 , @xmath30 , @xmath31 , and @xmath32 , giving a total redshift coverage @xmath33 . this translates into surveyed comoving volume of @xmath34 comoving @xmath35 per field for @xmath36 , @xmath37 , @xmath38 . about 70% of the imaging at @xmath39 is complete , and an extension of the survey to @xmath40 is planned . in about 6 hours per filter per field we are able to achieve line detections of about @xmath41 . the survey sensitivity varies with seeing . broadband images of these fields in a custom @xmath42 filter ( @xmath43 , @xmath44 ) and the johnson - cousins @xmath45 , @xmath46 , and @xmath47 bands are being taken as part of the noao deep widefield survey ( jannuzi & dey 1999 ) . the images were reduced using the mscred package ( valdes & tody 1998 ; valdes 1998 ) in the iraf environment ( tody 1986 , 1993 ) , together with assorted custom iraf scripts . in addition to standard ccd data reduction steps ( overscan subtraction , bias frame subtraction , and flatfielding ) , it was necessary to remove crosstalk between pairs of ccds sharing readout electronics and to remove a ghost image of the telescope pupil . astrometry of usno - a catalog stars was used to interpolate all chips and exposures onto a single coordinate grid prior to combining final images . cosmic ray rejection is of particular importance in a narrowband search for emission line objects . we therefore identified cosmic rays in individual images using a digital filtering method ( rhoads 2000 ) and additionally applied a sigma clipping algorithm at the image stacking stage . weights for each exposure were determined using the method of fischer & kochanski ( 1994 ) , which accounts for sky level , transparency , and seeing to optimize the signal to noise level for compact sources in the final image . catalogs were generated using the sextractor package ( bertin & arnouts 1996 ) . fluxes were measured in @xmath48 ( 9 pixel ) diameter apertures , and colors were obtained using matched @xmath48 apertures in registered images that had been convolved to yield matched point spread functions . spectroscopic followup of a cross - section of emission line candidates was obtained with the lris instrument ( oke et al 1995 ) at the keck 10 m telescope on 1999 june 13 ( ut ) . two dithered exposures of @xmath49 seconds each were obtained through a single multislit mask in good weather . these data were reduced using a combination of standard iraf onedspec and twodspec tasks together with the homegrown slitmask reduction iraf task `` bogus '' ( stern , bunker , & stanford , private communication ) for reducing lris data . our imaging data yield numbers of sources as a function of their fluxes in several filters , from which we can construct number densities as a function of magnitudes , colors , and equivalent widths . in order to gracefully handle sources that are not detected in all filters , we have chosen to use `` asinh magnitudes '' ( lupton , gunn , & szalay 1999 ) , which are a logarithmic function of flux for well detected sources but approach a linear function of flux for weak or undetected sources . figure  [ cmdfig ] shows the color - magnitude diagram for the 0  ( @xmath50 ) and @xmath45 filters . the color scatter achieved for bright sources ( @xmath51 ) is @xmath52 magnitudes ( semi - interquartile range ) . this includes the true scatter in object colors , and is therefore a firm upper limit on the scatter introduced by any residual systematic error sources , which we expect to be a few percent at worst .    to sharpen our focus on the high redshift lyman-@xmath1  population , we identify the range of parameter space occupied by known @xmath53 lyman-@xmath1emitters . these sources have typical observed equivalent widths @xmath54  and line+continuum fluxes @xmath55 ( adjusted to @xmath7 and measured in an @xmath56  filter ) . we further want to restrict attention to sources with sufficiently reliable detections that the number of false emission line candidates is a small fraction of the total candidate sample . the data set includes many continuum sources ( @xmath57 in the flux range above ) . it is expected to contain a few hundred emission line sources , based on earlier source counts from smaller samples ( hu et al 1998 ) and on our own findings ( below ) . the fraction of continuum sources with measured equivalent widths above some threshold @xmath58 can be calculated as a function of signal to noise level @xmath59 and filter width @xmath60 : the measured equivalent width for a source with no line emission or absorption in the narrowband filter will be @xmath61 . thus , a false positive becomes an @xmath62 event , where @xmath63 . our sample would be reasonably safe from contamination with @xmath64 , which would correspond to about 20 false positives in a sample of 14000 sources ; and very safe for @xmath65 , corresponding to @xmath66 false positive in 14000 sources . we have conservatively chosen detection thresholds @xmath67 and @xmath68 , which gives @xmath69 . this keeps the number of false positives small even when the foregoing analysis is expanded to include errors in the continuum flux ( which increase photometric error in the color by a factor @xmath70 ) and a realistic distribution of equivalent widths for the low-@xmath23 galaxy populations . as a further check on our sample , we use the photometric color error estimates from sextractor to demand that the source be an emission line source at the @xmath71 level .    combining all of these requirements , the final criteria for good candidates in our survey become @xmath72 , @xmath73 , and @xmath74 , where @xmath75 . there are @xmath76 such sources detected in the 0  filter in a solid angle of @xmath77 square degree and redshift range @xmath78 . this corresponds to @xmath79 good candidate lyman-@xmath1 emitters per square degree per unit redshift . the precise upper flux cutoff used is somewhat arbitrary , but including sources with larger fluxes in our list of good candidates has little effect on the total source counts . the final 16  image had a broader psf than the 0  image , and consequently a reduced effective sensitivity . in addition , it samples a slightly higher redshift , resulting in an @xmath80 magnitude increase in distance modulus ( @xmath81 , @xmath38 ) . accounting for both effects , the matched flux range is @xmath82 at 0  and @xmath83 at 16 . the corresponding counts are @xmath84 candidates at 16 , and 104 candidates at 0 . thus , the source density in directly comparable luminosity bins varies by a factor of @xmath85 , a difference that is significant at about the @xmath86 level . we interpret this as a likely signature of large scale structure in the lyman-@xmath1  emitter distribution at @xmath0 . the comoving distance between the centers of the two redshift slices is @xmath87 , while the thickness of each slice is @xmath88 and the transverse comoving size @xmath89 ( again assuming @xmath90 , @xmath37 , @xmath38 ) . comparable variations have been observed in the density of lyman break galaxies at @xmath91 ( steidel et al 1998 ) . our spectroscopic followup sampled a wide range of flux and equivalent width in order to characterize the different populations of sources with extreme narrowband colors , and to thereby tune candidate selection criteria for future spectroscopic observations . those sources with @xmath92 narrowband detections and photometrically measured @xmath93  were all confirmed as emission line objects at the narrowband wavelength . in total , we detected two emitters at @xmath94 , three  emitters at @xmath95 and @xmath12 , one confirmed lyman-@xmath1  ( @xmath96 ) emitter at @xmath97 , and one source that could either be lyman-@xmath1  at @xmath11 or at @xmath12 . we also found a @xmath98 galaxy with emission lines of , @xmath991640 , and @xmath991663 . this source may have been included in the narrowband candidate list ( with @xmath100 ) because of a weak spectral break around @xmath101 . finally , we found a few serendipitous emission line sources in the slit spectra . one of these is a single - line source with large equivalent width , possibly lyman-@xmath1  at @xmath102 . spectra of two interesting sources are shown in figure  [ specfig ] . by combining our imaging survey with these spectroscopic results , we can estimate the source density of lyman-@xmath1  emitters passing our selection cut . our spectra included three sources fulfilling the criteria given above for good candidates . of these , one was confirmed as a @xmath10 lyman-@xmath1  source . a second remains a candidate @xmath11 source , but is more conservatively interpreted as a @xmath12 [ o ii ] emitter on the basis of a rather strong continuum on the blue side of the line . the third is a clear @xmath94 [ o iii ] emitter . we therefore estimate that roughly @xmath103 to @xmath104 of the good candidates will be confirmed as lyman-@xmath1  sources , yielding @xmath105 emitters per square degree per unit redshift . this is compatible with earlier measurements from smaller volumes ( hu et al 1998 ) after accounting for differences in flux threshold . our measurement is distinct from previous efforts in the field for its basis in a large number of candidate emitters . poisson errors in our source counts are of order @xmath106 . this is smaller than the variations observed in the comparison of two filters ( of order @xmath107 ) . by combining observations in multiple fields , we will be able to average over local fluctuations in number densities effectively . when completed , the lala survey will yield comoving volume of @xmath108 (  [ obsdesc ] ) and a sample of several hundred laes , and will allow the luminosity function , equivalent width distribution , and correlation function of this population to be determined for the first time . we thank andy bunker and steve dawson for help with the spectroscopic observations and frank valdes for writing and helping with the mscred package in iraf . jer s research is supported by a kitt peak postdoctoral fellowship and by an stsci institute fellowship . sm s research is supported by nasa through hubble fellowship grant # hf-01111.01 - 98a from the space telescope science institute , which is operated by the association of universities for research in astronomy , inc . , under nasa contract nas5 - 26555 . bertin , e. and arnouts , s. , 1996 ,  117,393 cowie , l. l. 1988 , in `` the post - recombination universe '' , ed . n. kaiser & a. n. lasenby ( dordrecht , kluwer ) p.  1 cowie , l. l. , & hu , e. m. 1998 ,  115 , 1319 de propris , r. pritchet , c. j. , hartwick , f. d. a. , & hickson , p. 1993 ,  105 , 1243 dey , a. , spinrad , h. , stern , d. , graham , j. r. , & chaffee , f. h. 1998 ,  498 , 93 djorgovski , s. , & thompson , d. j. 1992 , in `` the stellar populations of galaxies , '' iau symposium 149 , ed . b. barbuy & a. renzini ( dordrecht , kluwer ) , p. 337 djorgovski , s. , thompson , d. j. , & smith , j. d. 1993 , in `` first light in the universe , '' ed . b. rocca - volmerange , m. dennefeld , b. guiderdoni , & j. tran thanh van ( gif sur yvette , editions frontires ) , p. 67 fischer , p. , & kochanski , g. p. 1994 ,  107 , 802 hu , e. m. , cowie , l. l. , & mcmahon , r. g. 1998 ,  502 , l99 jannuzi , b. t. , & dey , a. , 1999 , in `` photometric redshifts and high redshift galaxies '' , asp conference series , vol . 191 , editors r. j. weymann , l. j. storrie - lombardi , m. sawicki , and r. j. brunner , p.111 koo , d. c. , & kron , r. g. 1980 ,  92 , 537 kudritzki , r .- p . , et al 2000 , in preparation landolt , a. u. 1992 ,  104 , 340 lowenthal , j. d. , hogan , c. j. , leach , r. w. , schmidt , g. d. , & foltz , c. b. 1990 ,  357 , 3 lowenthal , j. d. , koo , d. c. , guzman , r. , gallego , j. , phillips , a. c. , faber , s. m. , vogt , n. p. , illingworth , g. d. , & gronwall , c. 1997 ,  481 , 673 lupton , r. h. , gunn , j. e. , & szalay , a. s. 1999 , aj 118 , 406 . manning , xx , et al 2000 , astro - ph/0002239 kunth , d. , mas - hesse , j. m. , terlevich , e. , terlevich , r. , lequeux , j. , & fall , s. m. 1998 ,  334 , 11 macchetto , f. , lipari , s. , giavalisco , m. , turnshek , d. a. , & sparks , w. b. 1993 ,  404 , 511 mller , p. , & warren , s. j. 1993 ,  270 , 43 oke , j.b . et al 1995 , pasp , 107 , 375 pritchet , c. j. 1994 ,  106 , 1052 pritchet , c. j. , & hartwick , f. d. a. 1987 ,  320 , 464 pritchet , c. j. , & hartwick , f. d. a. 1990 ,  355 , l11 rhee , g. f. r. n. , webb , j. k. , & katgert , p. 1989 ,  217 , 1 rhoads , j. e. 2000 , in press ( may 2000 issue ) smith , h. e. , cohen , r. d. , burns , j. e. , moore , d. j. , & uchida , b. a. 1989 ,  347 , 87 steidel , c.c . , giavalisco , m. , pettini , m. , dickinson , m. , & anderson , k. l. 1996 ,  462 , l17 steidel , c. c. , adelberger , k. l. , dickinson , m. , giavalisco , m. , pettini , m. , & kellogg , m. 1998 ,  492 , 428 . stern , d. , & spinrad , h. 1999 ,  111 , 1475 stern , d. , et al 2000 , astro - ph/0002241 thompson , d. , djorgovski , s. , & trauger , j. 1992 , in `` cosmology and large - scale structure in the universe , '' asp conference series , vol . 24 ( san francisco , asp ) , p. 147 thompson , d. , djorgovski , s. , trauger , j. , & beckwith , s. 1993 , in `` sky surveys : protostars to protogalaxies , '' asp conference series , vol . b. t. soifer ( san francisco , asp ) , p. 185 thommes , e. , meisenheimer , k. , fockenbrock , r. , hippelein , h. , rser , h .- j . , & beckwith , s. 1998 ,  293 , l6 thompson , d. , djorgovski , s. , & beckwith , s. 1994 ,  107 , 1 tody , d. 1986 , proc . spie instrumentation in astronomy vi , ed . crawford , 627 , 733 tody , d. 1993 , adass ii , a.s.p . conference ser . , vol 52 , eds . hanisch , r.j.v . brissenden , & j. barnes , 173 p valdes , f. , & tody , g. 1998 , proceedings of spie vol . 3355 valdes , f. 1998 , adass vii , asp conf . series 145 , ed . r. albrect wolfe , a. m. , turnshek , d. a. , lanzetta , k. m. , & oke , j. b. 1992 ,   385 , 151
we report on a new survey for @xmath0 lyman-@xmath1  sources , the large area lyman alpha ( lala ) survey . 
 our survey achieves an unprecedented combination of volume and sensitivity by using narrow - band filters on the new @xmath2 pixel ccd mosaic camera at the 4 meter mayall telescope of kitt peak national observatory . 
 well - detected sources with flux and equivalent width matching known high redshift lyman-@xmath1  galaxies ( i.e. , observed equivalent width @xmath3 , @xmath4 , and @xmath5 ) have an observed surface density corresponding to @xmath6 per square degree per unit redshift at @xmath7 . 
 spatial variation in this surface density is apparent on comparison between counts in @xmath8  and @xmath9  filters . 
 early spectroscopic followup results from the keck telescope included three sources meeting our criteria for good lyman-@xmath1  candidates . 
 of these , one is confirmed as a @xmath10 source , while another remains consistent with either @xmath11 or @xmath12 . 
 we infer that @xmath13 to @xmath14 of our good candidates are _ bona fide _ 
 lyman-@xmath1 
 emitters , implying a net density of @xmath15 lyman-@xmath1  emitters per square degree per unit redshift . 
 0@xmath16 16@xmath17 17f_17    2s @xmath18