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rrrccc crrcc helms 0015@xmath60404 & 3.9286&@xmath64.0715 & 61.7@xmath76.0 & 67.8@xmath75.7 & 54.3@xmath77.2&26.5 & 3.93038&@xmath64.07262 & 69.4@xmath76.7 & 72.4@xmath713.4 + helms 0041@xmath80410 & 10.3541&@xmath84.1679 & 80.8@xmath76.2 & 83.0@xmath76.5 & 43.6@xmath77.0&15.8 & & & @xmath935.1 & + l6-xmm 0223@xmath80605 & 35.8056&@xmath86.0860 & 33.8@xmath72.3 & 40.3@xmath72.5 & 24.2@xmath73.5&69.7 & & & @xmath914.4 & + g09 0918@xmath80039 & 139.6159&@xmath80.6644 & 41.1@xmath76.9 & 49.7@xmath78.1 & 31.2@xmath79.1&18.9 & & & @xmath917.7 & + g09 0920@xmath60024 & 140.2475&@xmath60.4049 & 35.0@xmath77.0 & 51.6@xmath78.1 & 32.0@xmath78.9&22.9 & & & @xmath918.6 & + ngp 1313@xmath62924 & 198.4530&@xmath629.4126 & 59.7@xmath75.6 & 78.5@xmath76.6 & 53.6@xmath77.8&17.2 & & & @xmath942.3 & + ngp 1330@xmath62540 & 202.5866&@xmath625.6749 & 49.2@xmath75.8 & 54.3@xmath76.4 & 29.3@xmath77.8&18.0 & & & @xmath915.3 & + ngp 1333@xmath62357 & 203.3743&@xmath623.9592 & 30.4@xmath75.4 & 31.8@xmath76.4 & 29.1@xmath77.5&55.2&203.37514&@xmath623.95909 & 20.0@xmath73.0 & 31.0@xmath78.1 + ngp 1335@xmath62805 & 203.9409&@xmath628.0986 & 41.7@xmath75.5 & 49.8@xmath76.4 & 38.2@xmath77.7&27.6&203.94249&@xmath628.09750 & 38.3@xmath74.5 & 51.4@xmath711.0 + g15 1413@xmath60058 & 213.4580&@xmath60.9725 & 46.6@xmath76.4 & 52.8@xmath77.7 & 36.1@xmath78.5&16.2&213.45743&@xmath60.97321 & 37.3@xmath75.8 & 37.3@xmath711.6 + g15 1435@xmath60110 & 218.9043&@xmath61.1682 & 63.0@xmath76.7 & 63.8@xmath78.0 & 56.6@xmath78.8 & 9.2&218.90494&@xmath61.16958 & 75.6@xmath77.3 & 89.1@xmath716.0 + g15 1450@xmath60026 & 222.6773&@xmath60.4351 & 47.5@xmath76.9 & 47.9@xmath78.1 & 29.1@xmath78.9&16.3 & & & @xmath918.0 & + l6-fls 1712@xmath66001 & 258.0352&@xmath660.0281 & 32.3@xmath72.2 & 34.0@xmath72.4 & 22.4@xmath73.6&30.1&258.03111&@xmath660.02722 & 10.8@xmath72.1 & 10.8@xmath74.2 + & & & & & & & 258.04010 & @xmath660.02625 & 13.9@xmath72.1 & 15.6@xmath74.5    the first milli - jansky - level submillimeter surveys discovered a population of distant submillimeter - bright galaxies ( smgs ) , namely , unresolved sources with 850  @xmath10 m  flux density ( @xmath11 ) greater than 3@xmath85  mjy @xcite . the smgs selected at wavelengths between 850  @xmath10 m  and 1  mm are intense starbursts ( sfr @xmath12  @xmath13 ) at a median redshift of @xmath14 @xcite . the intense star formation is dust - enshrouded so that the smgs radiate most of their bolometric luminosity in the far - infrared ( ir ) . the observed molecular and stellar emission indicates that they are massive gas - rich galaxies ( @xmath15@xmath2 ; e.g. , * ? ? ? * ; * ? ? ? * ; * ? ? ? * ) , but the typical halo mass of smgs remains uncertain , with estimates ranging from @xmath16 to @xmath17@xmath2 . two lines of evidence suggest that smgs may inhabit dark matter halos as massive as @xmath1  @xmath2 : ( 1 ) their strong clustering strength estimated from either the angular two - point correlation function ( e.g. , * ? ? ? * ; * ? ? ? * ) or the cross - correlation function between smgs and other high - redshift galaxies ( e.g. , * ? ? ? * ) , and ( 2 ) their high stellar mass and the @xmath18@xmath8@xmath19  relation from abundance matching ( @xmath19  @xmath20  @xmath2  for @xmath18  @xmath21  @xmath2  at @xmath22 ; e.g. , * ? ? ? however , because source blending due to the large beams of single - dish ( sub)millimeter telescopes may have significantly elevated clustering strength @xcite and the stellar mass estimates remain uncertain within an order of magnitude ( e.g. , * ? ? ? * ; * ? ? ? * ; * ? ? ? * ) , it is possible that a typical smg may inhabit much less massive halos ( @xmath010@xmath23@xmath2 ) . smgs are absent in the local universe and it is commonly thought that they have evolved into the massive ellipticals today ( e.g. , * ? ? ? * ; * ? ? ? * ) . to understand the evolution of smgs , it is imperative to know how long the observed intense star formation would last . for @xmath17  @xmath2  halos at @xmath24 , the average baryonic accretion rate from the mass growth rate of dark matter haloes is @xmath25  @xmath13  @xcite . in such massive halos , it is expected that most of the baryons will be shock - heated to the virial temperature of the halo ( @xmath26  k ) so that only a small fraction of the accreted gas can actually cool and accrete onto galaxies ( e.g. , * ? ? ? * ; * ? ? ? therefore , the ongoing gas accretion is unlikely to sustain the extreme sfrs . without a comparable gas supply rate , the sfr would decline with an @xmath27-folding timescale of only @xmath0200  myr ( 2@xmath28/sfr ; e.g. , * ? ? ? * ; * ? ? ? * ; * ? ? ? * ; * ? ? ? * ; * ? ? ? at such a rate , the smgs would become red sequence galaxies in only a gyr or 5 @xmath27-folding times ) from @xmath29  gyr@xmath30 for the smgs at the observed epoch to @xmath31  gyr@xmath30 for the red sequence at @xmath5 @xcite . ] . such a short transitional time of a significant high - redshift star - forming population might help explain the rapid build - up of the massive end of the red sequence at @xmath32 ( e.g. , * ? ? ? starbursts thus provide an alternative mechanism to the qso - mode feedback ( e.g. * ? ? ? * ; * ? ? ? * ) to form red and dead galaxies . note that both mechanisms still require feedback from radio jets ( i.e. , the maintenance mode ; e.g. , * ? ? ? * ; * ? ? ? * ) to prevent the hot gaseous halo from cooling . on the other hand , if smgs were in @xmath16  @xmath2  halos , the intense star formation is also unsustainable because the gas accretion rate is only @xmath0110  @xmath13  at @xmath24 .    however , could there be enough cool gas in the circumgalactic medium ( cgm ) around smgs to fuel a prolonged starburst phase ( e.g. see the simulation of * ? ? ? the cgm of co - eval qsos may give us a hint , because they inhabit comparably massive ( @xmath33 ) halos @xcite . contrary to the expected dominance of virialized x - ray plasma , absorption line spectroscopy of a statistical sample of @xmath5 projected qso pairs reveals the prevalence of cool ( @xmath34  k ) , metal - enriched ( @xmath35 ) , and optically thick ly@xmath3  absorbers ( @xmath36  @xmath37 ) extending to at least the expected virial radius of 160  kpc ( the `` qso probing qso '' [ qpq ] project : @xcite ) . the high observed covering factor of the cool cgm gas ( @xmath38 ) in @xmath33 halos has been compared to predictions from numerical simulations . while several studies found that they can not reproduce the high covering factor around qsos ( @xcite , but see @xcite ) , it has been argued that efficient star - formation - driven winds from accreted satellite galaxies that interact with cosmological filaments are required to increase the hi  covering factor to the observed level , which is only resolved in the highest resolution cosmological zoom simulations @xcite . these high - resolution simulations predict that the covering factor is roughly independent of sfr , decreases with redshift , and has relatively large halo - halo variations . in the relevant halo mass range , @xmath39@xmath2 , simulations show hi  covering factors of @xmath40 with little mass dependence at @xmath5 . given the estimated halo masses for qsos and smgs , one could expect similar covering factors if these are determined primarily by the interplay between gas infall and star - formation - driven outflows . to test this , we exploit qso absorption line spectroscopy to probe the cgm of smgs . we first present the data sets and the method we used to select projected smg@xmath8qso pairs  in   [ sec : sample ] . we then describe our followup observations in   [ sec : obs ] , including radio interferometer imaging , near - infrared spectroscopy , and optical spectroscopy . we present our analysis and results in   [ sec : result ] , including a comparison between the covering fraction of optically thick gas around smgs and that of @xmath5 qsos . we summarize the results and conclude in   [ sec : summary ] . throughout we adopt a @xmath41cdm cosmology with @xmath42 , @xmath43 and @xmath44 = 70 km  s@xmath30  mpc@xmath30 . because both high - redshift qsos and smgs have low number counts , we need large samples of both to come up with a sizable sample of projected smg@xmath8qso pairs  with small angular separations . we compiled 464,866 spectroscopically confirmed qsos from various surveys : primarily , the sloan digital sky survey ( sdss ; * ? ? ? * ) , the 2df qso redshift survey ( 2qz ; * ? ? ? * ) , the agn and galaxy evolution survey ( ages ; * ? ? ? * ) , and the mmt hectospec redshift survey of 24 @xmath10 m sources in the spitzer first look survey @xcite . since we select the foreground galaxies that are likely at @xmath45 ( see the next paragraph ) , we keep only the 102,472 qsos at @xmath46 . the average surface density of these background qsos is @xmath010deg@xmath47 .    to select the foreground smgs , we combined source catalogs from a number of wide - area extragalactic surveys carried out by the _ space observatory @xcite : the _ herschel _  multi - tiered extragalactic survey ( hermes , 95 deg@xmath48 ; * ? ? ? * ; * ? ? ? * ) , the _ herschel _  astrophysical terahertz large area survey ( h - atlas , 600 deg@xmath48 ; * ? ? ? * ; * ? ? ? * ) , the _ herschel _  large mode survey ( helms , 301 deg@xmath48 ; * ? ? ? * ; * ? ? ? * ; * ? ? ? * ; clarke et al . in prep . ) , the _ herschel _  stripe 82 survey ( hers , 79 deg@xmath48 ; * ? ? all of these surveys used spire ( spectral and photometric imaging receiver ; * ? ? ? * ) to image the sky at 250 , 350 , and 500  @xmath10 m  and the combined xid250 m  fluxes were all extracted at source positions detected on the 250  @xmath10 m  map ( e.g. , * ? ? ? * ; * ? ? ? ] catalog contain 1,586,047 sources covering a total of 767 deg@xmath48  ( hers and helms fields overlap by 10deg@xmath48 ) .    however , most _ herschel _  sources are not smgs ; they are , instead , less luminous dusty star - forming galaxies at lower redshifts ( @xmath49 ; * ? ? ? * ; * ? ? ? * ) . to select _ herschel _  sources that are likely _ bona fide _ smgs , we chose only the subsample that satisfies the following criteria : ( 1 ) flux densities peak at 350  @xmath10 m  ( @xmath50 and @xmath51 ; i.e. , `` 350@xmath10 m  peakers '' ) , ( 2 ) @xmath52  mjy , and ( 3 ) @xmath533@xmath54 detections in all three spire bands . criterion 1 is essentially a photometric redshift selection because emission from dusts at @xmath55  k would peak at 350  @xmath10 m  if redshifted to @xmath14 . in addition , the blind carbon monoxide ( co @xmath56 ) survey of the brightest 350  @xmath10 m  peakers ( @xmath57  mjy ) has shown a strikingly similar redshift distribution as 850  @xmath10m - selected smgs ( @xmath58 ; * ? ? ? * ) , even though most of these bright sources are strongly lensed . criterion 2 is introduced to ensure that the rayleigh - jeans extrapolation would give @xmath59  mjy , the classic definition of an smg , given a typical power - law slope of 3.5 for a modified blackbody with a frequency - dependent absorption cross section ( @xmath60 ) . criterion 3 ensures that all of the sources we considered are statistically significant . this is necessary because the image depth varies substantially from field to field , ranging from @xmath61mjybeam@xmath30 for the large helms and hers fields ( confusion noise included ; * ? ? ? * ; * ? ? ? * ) to confusion limited with @xmath62mjybeam@xmath30 for the deeper hermes fields @xcite . of course , our crude color plus flux selection is not completely reliable , but it has weeded out most of the dusty star - forming galaxies that are not smgs . only 70,823 _ herschel _  sources remained after this selection . the average surface density of 92deg@xmath47 is five times lower than the observed 870  @xmath10 m  source count above @xmath63  mjy ( @xmath0500deg@xmath47 ; * ? ? ? * ; * ? ? ? this is not surprising given that almost half of the total _ herschel _ area is only 10 - 20% complete at @xmath64  mjy . note that this incompleteness is not a concern for our purpose . we identified 230 projected smg@xmath8qso pairs  with angular separations ( @xmath65 ) is measured between the _ herschel _ 250@xmath10 m  position and the optical position of the qso . ] between 5  @xmath66  36  by cross - matching the qso and smg subsamples described above . the corresponding impact parameters ( @xmath67 ) are between @xmath68  300  kpc for @xmath69 . the impact parameter is defined as the transverse proper distance at the redshift of the foreground smg , which equals the angular diameter distance of the smg multiplied by the angular separation on sky ( @xmath70 ) . these qso sightlines thus probe out to @xmath01.5 virial radii of @xmath17  @xmath2  halos ( @xmath71  kpc at @xmath24 ) . given the positional uncertainty of the _ herschel _  250  @xmath10 m  sources , the 5  lower limit on the angular separation is imposed as an attempt to avoid far - ir - luminous qsos , i.e. , the qsos are the smgs themselves . through visual inspection of the qso spectra , we further excluded 31 pairs whose qsos exhibit strong broad absorption lines ( bals ; this makes them unsuitable for absorption line work ) , have wrong redshifts , or are misclassified . therefore , our final sample includes 199 pairs , among which 90/163 qsos have sdss @xmath72 ( bright enough for absorption line spectroscopy ) . one _ herschel _  source is probed by two qsos at @xmath738.0  and 18.2 . no qso probes multiple _ herschel _  sources within 36 , but a `` single '' _ herschel _  source may consists of multiple smgs due to the large beam size ; so it is possible that a single qso could probe multiple smgs at different impact parameters ( this is indeed the case for the last source shown in fig .  [ fig : detections ] ) . extensive followup observations are needed to perform the absorption - line analysis of smgs . identifying the absorption features in the spectrum of the background qso requires a precise redshift for the foreground smg . but the full - width - at - half - maximum ( fwhm ) angular resolution of _ herschel _   18/25/36  at 250/350/500@xmath10 m   precludes longslit spectroscopic observations with typical slit width of 1 . to obtain more accurate positions and to identify blended sources , we exploit interferometer observations with the karl g. jansky very large array ( vla ) . furthermore , once the positions are determined with sub - arcsec accuracy , we need to determine the spectroscopic redshifts of the smgs with near - ir spectrographs by targeting rest - frame optical lines that suffer less dust extinction than rest - frame uv lines . finally , a high s / n optical / near - uv spectrum of the background qso is needed to detect the uv absorption lines imprinted by the diffuse medium around the smgs . below we describe these observations in more details . far - ir - luminous galaxies like our _ herschel _  sources are expected to be luminous in the radio wavelengths , according to the ir - radio correlation @xcite . we can thus obtain better positions for the _ herschel _  sources by identifying the radio counterparts with interferometers . we observed 15 smg@xmath8qso pairs  with the vla in the b configuration with the c - band ( 6  ghz ) receivers ( program i d : 15a-266 ) . the sample was selected randomly from the smg@xmath8qso pairs  with qsos brighter than @xmath74 , excluding those with @xmath7530 . we later realized that two of the vla targets in the helms field are likely spurious detections because of galactic cirrus ( clarke et al . in prep . ) , so we excluded them in this discussion . table  [ tab : vlaphoto ] lists the _ herschel _  positions and photometry of the final vla sample . the receivers have a total bandwidth of 4ghz at a central frequency of 5.9985ghz . the targets were selected from six different extragalactic fields . to maximize the observing efficiency , we grouped the targets with their r.a . into five scheduling blocks ( sbs ) of 0.8 to 3.1hours . a nearby unresolved calibrator was observed every @xmath010  min . depending on the r.a . of the targets , 3c48 , 3c286 , or 3c295 was observed for bandpass and flux - density calibration . the entire program took 8.3 hrs of vla time . the on - source integration time ranges from 9 to 70 minutes , allocated based on the 6  ghz flux density estimated from fitting the _ herschel _  photometry with the sed template of a well - studied , strongly lensed smg at @xmath76 ( smm  j2135 - 0102 , aka . the  eyelash  ; * ? ? ? * ) . the observations were calibrated using the common astronomical software applications ( casa ) package @xcite . we used the vla pipeline to perform basic flagging and calibration . additional flagging was performed whenever necessary by inspecting the visibility data . we used the self - calibration technique with bright sources within the primary beam to reduce the gain errors for four fields : helms0015@xmath60404 , helms0041@xmath80410 , l6-xmm0223@xmath80605 , and ngp1313@xmath62924 . for imaging and deconvolution , we used the standard casa task clean with `` natural '' weighting to achieve the best sensitivity . the resulted restoring beams are on average 15@xmath7712 fwhm . the rms image noise level ranges from 2.1 to 14.1  @xmath10jybeam@xmath30 , with a mean of 6  @xmath10jybeam@xmath30 . this measured noise is consistent with thermal noise using natural weighting of the visibility data , except for maps contaminated by the sidelobes of the strong sources lying far outside the cleaned image . we will present the results in   [ sec : radiodetections ] , but here is a summary : the vla detected six of the 13 smgs ; for one of those , ngp  1335 + 2805 , the qso at @xmath78 itself is responsible for the ir emission , so we excluded it from subsequent spectroscopic observations .      with the vla positions that are accurate to @xmath790.1 , we carried out a redshift survey for the five vla - detected smgs with near - ir spectrographs . the vla resolved the _ herschel _  source of l6-fls1712@xmath66001 into two sources at similar brightness ( fig . [ fig : detections ] _ last panel _ ) . we chose the one closer to the _ herschel _  position as the nominal counterpart and have obtained a deep near - ir spectrum at that location . it is unclear whether the two sources are physically related because the redshift of the other source remains to be determined . we observed g151435@xmath60110 , l6-fls1712@xmath66001 , and ngp1333@xmath62357 with the luci-1 spectrograph ( lbt nir - spectroscopic utility with camera and integral - field unit ; * ? ? ? * ) on the large binocular telescope ( lbt ) on 2015 apr 14 . we used the 200 l / mm @xmath80+@xmath81 grating at @xmath82  @xmath10 m  and the n1.8 camera ( 025 per pixel ) to obtain a spectral range between 1.5 and 2.3  @xmath10 m . the 1-wide 4-long slit is centered on the vla - determined smg position , and it is aligned with the qso in each pair to obtain the qso spectrum simultaneously . the qso spectrum is bright enough to serve as a useful reference for spatial alignment . the spectral resolution ( @xmath83 ) is @xmath0940 in @xmath80-band and @xmath01290 in @xmath81-band . we obtained 32@xmath77 120  s exposures for each target . between exposures , we dithered along the slit among six dithering positions distributed within 40 . atmospheric transparency varied dramatically during the night and no telluric star was observed . so we used the telluric star observation from the previous night for an approximate telluric and flux calibration . we observed helms0015@xmath60404 and g151413@xmath60058 with the gemini near - infrared spectrograph ( gnirs ; * ? ? ? * ) in the queue mode ( program ids : gn-2015b - q-46 , gn-2016a - q-41 ) . we used the cross - dispersing prism with the 31.7 l / mm grating and the short camera to obtain a complete spectral coverage between 0.85 - 2.5  @xmath10 m . with the 068 short slit , the spectral resolution is @xmath84 across all orders . we obtained 14@xmath77 300  s exposures for helms0015@xmath60404 on 2015 aug 9 , aug 12 , and oct 18 , and 24@xmath77 115  s exposures for g151413@xmath60058 on 2016 may 20 . we dithered by 3  along the 7-slit between exposures . data reduction was carried out with a modified version of ` longslit_reduce ` @xcite for luci-1 by fuyan bian @xcite and a modified version of spextool @xcite for gnirs by katelyn allers ( private communication ) . these idl packages carry out the standard data reduction steps for near - ir spectroscopy : flat - fielding , wavelength calibration , pairwise sky subtraction , residual sky removal , shifting and coadding , spectral extraction , telluric correction , and flux calibration . although previous optical spectra exist for all of the qsos in our sample , most of them do not have the necessary wavelength coverage or sufficient s / n for absorption line analysis . hence we obtained new optical spectra for the three background qsos that are associated with the spectroscopically identified smgs . we observed helms0015@xmath60404 and ngp1333@xmath62357 on 2016 jan 9 and l6-fls1712@xmath66001 on 2015 jun 13 with the low resolution imaging spectrometer ( lris ; * ? ? ? * ) on the keck i telescope . we used the 1  longslit and the 560 dichroic for both runs . for the former run , we used the 600/4000 grism on the blue side ( @xmath85 at 4,000   , fwhm = 300 km s@xmath30 ) and the 600/7500 grating at @xmath86    on the red side to cover from 3,300 to 8,700 . for the latter run , we used the 400/3400 grism on the blue side ( @xmath87 at 4,000   , fwhm = 500 km s@xmath30 ) and the 400/8500 grating tilted to a central wavelength of @xmath88    on the red side to cover from 3,300 to 10,200 . the total integration time for each source ranged between 30 min and 40 min . conditions were non - photometric for both nights . we reduced the raw data with ` xidl ` , an idl data reduction package for a number of spectrographs written by two of us ( jxp and jfh ) . the pipeline follows the standard data reduction steps and reduces the blue and red channels separately . it begins by subtracting a super bias from the raw ccd frames , tracing the slit profiles using flat fields , and deriving the 2d wavelength solution for each slit using the arcs . then it flat - fields each slit and rejects cosmic - rays , identifies objects automatically in the slit , and builds the bspline super sky model without rectification @xcite . after subtracting the super sky model , it performs optimal 1d extraction based on the spatial profile of the qso @xcite . finally , it removes instrument flexure using isolated sky lines , performs heliocentric correction , and does flux calibration . we targeted the _ herschel _  sources in 13 smg@xmath8qso pairs  with the vla and made six clear detections ( 46% detection rate ) . figure  [ fig : detections ] shows the vla detections and table  [ tab : vlaphoto ] lists the 6  ghz positions and flux density measurements for the detections and 3@xmath54 upper limits on the non - detections . the vla revealed five genuine smg@xmath8qso pairs  ( helms0015@xmath60404 , ngp1333@xmath62357 , g151413@xmath60058 , g151435@xmath60110 , l6-fls1712@xmath66001 ) and one far - ir - luminous qso ( ngp1335@xmath62805 ) . the vla - detected smgs have 6  ghz peak flux densities between 11 and 76  @xmath10jy  with a mean of @xmath040  @xmath10jy . for these detections , there is a clear positive correlation between the observed flux densities at 6  ghz and 500  @xmath10 m , as expected from the ir - radio correlation convolved with a redshift distribution . the 54% non - detection rate is not surprising given the uncertainties in the predicted 6  ghz flux density and the fact that the sensitivity of radio data may deviate substantially from the theoretical prediction because of confusion sources and weather .    the offsets between the _ herschel _  positions and the vla positions range between 2.7  and 7.9 . the maximum offset is comparable to the estimated 2@xmath54 positional uncertainty of _ herschel _  250  @xmath10m - selected catalogs : @xmath89 for fwhm = 18.1  and snr = 5 @xcite . this indicates that a fraction of the `` pairs '' that have small angular separations ( @xmath90 ) could be single sources , i.e. , qsos that are far - ir - luminous . in our vla - detected sample , ngp1335@xmath62805 , with @xmath91 , is the only such case ( see the third panel of fig .  [ fig : detections ] ) . the vla position agrees with the optical qso position within 01 , which is well within the cross - band astrometry accuracy of @xmath00.3  ( sdss@xmath6vla ; e.g. , * ? ? ? but on the other hand , the smg in l6-fls 1712@xmath66001 is a separate source from the qso , despite its even smaller 250  @xmath10 m  separation ( @xmath92 ) . we thus chose not to exclude pairs with @xmath93 . the far - ir - luminous qsos in our sample are interesting on their own right and they will be discussed elsewhere .      rccccc ccccc helms 0015@xmath60404 & j001543.29 + 040421.4 & 2.515&13.1 & 2.1&j001542.31 + 040433.5 & 3.256 & 19.0 & 157&@xmath94 & ambiguous + ngp 1333@xmath62357 & j133330.03 + 235732.7 & 2.184&12.6 & 2.3&j133330.36 + 235709.9 & 3.108 & 23.3 & 198&@xmath95 & ambiguous + l6-fls 1712@xmath66001 & j171207.47 + 600138.0 & 2.043&12.6 & 2.6&j171209.00 + 600144.4 & 2.821 & 13.1 & 112&@xmath96 & optically thin    estimating the intrinsic properties of the vla - detected _ herschel _  sources require knowledge of their spectroscopic redshifts . we obtained deep near - ir spectra for the five vla counterparts whose spectroscopic redshifts are previously unknown . we detected h@xmath3  and [ nii ]  lines from three of the five sources , enabling accurate determination of the spectroscopic redshifts ( fig . [ fig : smgspec ] and table  [ tab : nhi ] ) . the [ nii]/h@xmath3  line ratios are consistent with the range observed in typical smgs ( @xmath97  [ nii]/h@xmath98 ; @xcite ) . the two remaining sources , g151413 + 0058 and g151435 + 0110 , show near - ir continuum emission without detectable emission lines or stellar absorption features . it is possible that the emission lines either fall into one of the telluric absorption bands or are simply outside of the spectral range . our redshift success rate is thus 60% , comparable to previous redshift surveys of smgs in the optical ( e.g. , * ? ? ? * ; * ? ? ? * ) .    with the photometry from _ herschel _  and the vla and spectroscopic redshifts , we can estimate the total rest - frame 8@xmath81000  @xmath10 m  luminosity ( @xmath99 ) and the ir - to - radio luminosity ratio ( @xmath100 ) . the results are listed in table  [ tab : nhi ] . we fit the _ herschel _  photometry with a modified blackbody at the spectroscopic redshift , with @xmath101 fixed to 1.5 . we find that they have @xmath102@xmath103  and sfr@xmath104 = 470 - 1500  @xmath13 . the sfr is estimated from @xmath99 using the calibration of @xcite for a @xcite initial mass function : @xmath105 the extrapolated 850  @xmath10 m  flux densities range between @xmath106  mjy , confirming that they are _ bona fide _ smgs . the ir - to - radio luminosity / flux ratio is estimated using the following equation @xcite : @xmath107 where @xmath108 is rest - frame integrated 8 - 1000  @xmath10 m  flux and @xmath109 is the rest - frame 1.4  ghz flux density . the former is from the modified blackbody fit to the ir sed , and the latter is converted from the observed 6  ghz flux densities assuming a typical synchrotron slope ( @xmath110 ) . the three smgs show @xmath111 , consistent with the observed radio - ir correlation for _ herschel _  sources ( @xmath112 ; * ? ? ? this indicates that our sample is not biased to radio - loud agns despite being radio selected .      to compare with previous work on the cgm of other high - redshift galaxies , we need to characterize the covering fraction of optically thick gas clouds around smgs ( i.e. , lyman limit systems [ llss ] with column densities of neutral hydrogen @xmath113 @xmath37 ) . we search for ly@xmath3  absorbers near the spectroscopic redshifts of the foreground smgs and classify the ly@xmath3  absorbers in the qso spectra following the procedure used in the qpq study ( e.g. , * ? ? ? the classification is based on the ly@xmath3  equivalent widths ( ews ) and the presence of associated metal transitions . we first identify hi  ly@xmath3  absorption due to the smgs in the qso spectra within @xmath7600  km s@xmath30  of the systemic redshifts of the smgs . the search window is chosen because the escape velocity is 610  km s@xmath30  at the virial radius ( @xmath114  kpc ) of a @xmath17  @xmath2  dark matter halo at @xmath22 @xcite . strong ly@xmath3  absorption lines are detected in all three systems ( fig . [ fig : abs ] ) . we report their rest - frame ews ( @xmath115 ) , updated angular separations based on the vla positions ( @xmath116 ) , and the impact parameters ( @xmath67 ) in table  [ tab : nhi ] . as we have found in our earlier qso absorption line studies , systematic error associated with continuum placement and line blending generally dominates the statistical error of the qso spectra . so we estimate the error of @xmath115 assuming 10% error in the continuum placement . from our best - fit gaussians to the absorption profiles ( shown in fig . [ fig : abs ] ) , we find that all three systems show strong ly@xmath3  absorption lines with @xmath117   . note that when @xmath118 , the hi  ly@xmath3  absorption from the smg may lie within the qso ly@xmath101 forest . unfortunately , all three pairs fall in this category , so contamination from ly@xmath101  lines from systems in the ly@xmath3  forest may be a concern . however , it turned out that ly@xmath101  contamination is not a serious issue for these systems . because assuming the detected ly@xmath3  absorption to be ly@xmath101 , we searched for the corresponding ly@xmath3  lines in each spectrum but did not find any . given the absorption profiles of the ly@xmath3  line , we then searched for the associated metal transitions commonly observed in optically thick absorption systems ( e.g. , * ? ? ? * ) : siii@xmath1191260,1304,1527 , oi@xmath1191302 , cii@xmath1191335 , siiv@xmath1191394,1403 , civ@xmath1191548,1551 , feii@xmath1191608,2383,2600 , alii@xmath1191671 , and mgii@xmath1192796,2804 . however , none of these metal transitions is convincingly detected in any of our systems . in the following , we discuss the systems individually and provide our classifications :    * _ helms0015@xmath60404_. this system shows an hi  ly@xmath3  absorption at @xmath6500  km s@xmath30  with @xmath120    and _ intrinsic _ fwhm = 430  km s@xmath30  ( fig . [ fig : abs ] _ top panels _ ) . given this measurement , one may have concluded that this is an optically thick absorber with @xmath121  @xmath37 if the absorption is dominated by a single component . the column density is estimated from the theoretical curve of growth where @xmath122    for @xmath123  @xmath37 , the regime where the relation is insensitive to the @xmath124-parameter @xcite . there is putative cii  absorption at @xmath6400  km s@xmath30  ( @xmath125   ) ; but it is likely a false identification of a ly@xmath3  forest line , because it is misaligned in velocity with the ly@xmath3  absorption and none of the strong metal lines in the `` clean '' region outside of the lyman forests is detected ( e.g. , siii , civ , feii , alii ) . without detections of strong metal lines and/or the damping wings in ly@xmath3 , we can not conclude that the absorber is optically thick based on current data because the high @xmath115 could be due to line blending . we conservatively classify this system as ambiguous , even though the non - detection of strong metal lines strongly suggests that it is an optically thin absorber , because fewer than 5% of optically thick absorbers have no detection of a metal transition @xcite . * _ ngp1333@xmath62357_. this system shows an hi  ly@xmath3  absorption at @xmath126  km s@xmath30  with @xmath127    and intrinsic fwhm = 260  km s@xmath30  ( fig . [ fig : abs ] _ middle panels _ ) . we observe putative absorption around the expected locations of cii , siiv , siii , and civ . but all of these lines lie within the ly@xmath3  forest , and their velocity profiles are distinctly different from that of the ly@xmath3  line , indicating that these putative metal absorption are spurious . the feii  and alii  lines are outside of the lyman forests , but the expected absorption lines are not detected . like the previous case , we conservatively classify this system as ambiguous . * _ l6-fls1712@xmath66001_. there is a broad but shallow hi  ly@xmath3  absorption line at @xmath128  km s@xmath30  with @xmath129    and intrinsic fwhm = 950  km s@xmath30  ( fig . [ fig : abs ] _ bottom panels _ ) . despite the high ew , this system is unlikely optically thick because the line center drops only to 40% of the continuum intensity ( i.e. , it does not go line black ) . like the previous cases , we observe putative metal absorption inside the lyman forests ( e.g. , oi , cii , siii , and siiv ) but no metal absorption outside of the forests ( e.g. , civ , feii , alii , and mgii2796,2804 lines . ] ) . note that the weak civ  absorption is blueshifted by more than 200 km s@xmath30  from the ly@xmath3  line center . hence , we classify this system as optically thin .    in summary , we have identified one clearly optically thin case and two ambiguous cases with three qso sightlines covering impact parameters between @xmath130  kpc around smgs at @xmath131 . the two ambiguous cases are also likely to be optically thin , given the non - detection of any metal transitions . for background qso sightlines probing the cgm of @xmath132 foreground qsos , one observes hi  absorbers optically thick at the lyman limit @xmath13360% of the time . the high hi  covering fraction extends to at least the expected virial radius of @xmath0160  kpc @xcite . in figure [ fig : covfrac ] , we compare ( 1 ) the smg sample distribution with the qsos from the qpq project , and ( 2 ) the covering fraction of optically thick hi  gas around smgs with that around qsos . the two samples overlap in the plane of foreground redshift vs. impact parameter : the smgs cover the same redshift range as the qsos in the intermediate impact parameter range between 100 and 200  kpc . we calculate the 1@xmath54 binomial confidence intervals of the optically thick fraction using the quantiles of the beta distribution @xcite . because there is no clearly optically thick absorber among the three systems we analyzed , our 1@xmath54 confidence interval of the covering fraction is 4.2@xmath836.9% for a non - detection in a sample of three . for comparison , we consider all of the qso sightlines with @xmath130  kpc from the qpq project @xcite and we find the optically thick covering fraction is @xmath134% for 21 clearly optically thick systems among 33 systems . therefore , despite our small sample , the upper bound of our 1@xmath54 confidence interval is 3@xmath54 below the best - estimated covering fraction around qsos at the same ranges of redshifts and impact parameters .    note that although our analysis may appear to have limitations based on the classification of two of the smg absorbers as ambiguous , these same limitations also apply to the qpq analysis , and are inherent to any attempt to classify absorbers using low or moderate resolution spectra without coverage of the hi  lyman limit ( at 912  in the rest - frame ) . as such , we have followed exactly the same procedure for absorber classification as in the qpq studies , enabling a direct comparison to that work . note , however , there are some minor differences between our analysis and the qpq analysis which are compared in fig . [ fig : covfrac ] . first , the bulk of the qpq dataset @xcite utilized background qso spectra with moderate resolution ( @xmath135 ) . this is a factor of two higher than the resolution we used for the qso spectra probing two of our smgs ( @xmath85 ) , whereas in l6-fls1712@xmath66001 our spectrum has @xmath87 . but the lower resolution of our spectra does not play a significant role in the resulting classification of our absorbers , since the typical strength of the metal lines seen in the optically thick systems would still be easily detectable at @xmath85 , and the one system that we observed at @xmath87 is unambiguously optically thin , since it does not go line black . another significant difference , is whereas @xcite restricted analysis to only those foreground qsos with redshifts lying within the ly@xmath3  forest of the background qso , we have included foreground smgs landing in the ly@xmath101  forest , owing to the larger redshift separations between the smgs and the qsos in our sample . this is unlikely to impact our deduced covering factor in fig . [ fig : covfrac ] though . for example , earlier qpq studies @xcite also considered absorbers lying in the ly@xmath101  forest , and the covering factor deduced there was consistent with that derived in @xcite . furthermore , in all of the three smg@xmath8qso pairs  shown in fig . [ fig : abs ] , multiple metal lines that are strong in optically thick absorbers ( i.e. , siii@xmath1191527 , civ@xmath1361548,1551 , aliii@xmath1191671 , and mgii@xmath1362796,2804 ) land in the clean regions outside of the lyman forest but remain undetected . finally , whereas qpq searched for absorbers within a velocity window in a @xmath71500 km s@xmath30  window owing to the large errors in qso redshifts , we adopted a @xmath7600 km s@xmath30  search window . we were able to consider a smaller velocity interval because our smg redshifts derived from narrow rest - frame optical lines are much more accurate , and this interval was instead chosen to encompass the typical virial motions in a @xmath17@xmath2  dark matter halo . the larger velocity interval used in qpq formally implies a higher level of contamination from physically unassociated absorbers . but the contamination is estimated to be at only @xmath03% level for a @xmath71500 km s@xmath30  window ( * ? ? ? * see their figure 10 and table 7 ) . this is too small to be responsible for the difference in the optically thick covering factor that we observe between qsos and smgs . motivated by the unique properties of smgs and their purported evolutionary link to high - redshift qsos and today s massive ellipticals , we have started a project to use qso absorption line spectroscopy to probe the diffuse cool hi  gas in the cgm of smgs . this work requires a sample of projected smg@xmath8qso pairs , which are extremely rare . wide - area sub - millimeter surveys are needed to compile such a sample . thanks to the advent of _ herschel _ , we have identified 163 smg@xmath8qso pairs  with bright @xmath137 qsos ( @xmath138 ) and angular separations between 5  and 36  from a suite of wide - area _ herschel _  surveys and spectroscopic qso surveys . extensive followup observations are required to carry out the absorption line study . to allow slit spectroscopy , the first stage is to use an interferometer to pin down the positions of the _ herschel _  sources . this paper focuses on a subsample of 13 smg@xmath8qso pairs  that were observed with the vla in c - band . with an average integration time of 25  min per source , the vla detected sources within the _ herschel _  beam in six fields . one of the six smg@xmath8qso pair  turns out to be a far - ir - luminous qso at @xmath78 , while the _ herschel _  source in another pair turns out to be a pair of smgs . hence , we effectively identified six smg@xmath8qso pairs  from observations of 13 fields ( 46% ) . the second stage is to measure the spectroscopic redshifts of the smgs . we observed five of the six vla - identified smg@xmath8qso pairs  with near - ir spectroscopy . we were able to determine the redshifts for three of the five smgs ( 60% ) from the redshifted h@xmath3 and [ nii ]  lines . the remaining two sources show only featureless continuum in the near - ir windows , making it impossible to determine an accurate redshift . the last stage is to obtain optical spectroscopy of the qsos once it is confirmed that the qsos are in the background of the smgs . because we have selected only the brightest qsos , the success rate of this step is approaching 100% once bad qsos are removed from the sample . because of the low success rates of the first two followup stages , only @xmath023% of the initial sample ( i.e. , three smg@xmath8qso pairs  out of 13 ) are spectroscopically confirmed and are suitable for final absorption line study . our main findings are as follows :    1 . the near - ir spectra of the five vla - detected sources are all detected in hour - long integrations with 8-meter telescopes , although only three of those show emission lines that yielded accurate spectroscopic redshifts . since we positioned the slits on the vla positions , this result shows that the spatial offset , if any , between the near - ir and the radio counterparts is less than an arcsec ( the slit width ) , consistent with the finding from differential lensing in strongly lensed sources ( e.g. , * ? ? ? the vla - identified _ herschel _  `` 350  @xmath10mpeakers '' at @xmath131 are similar to smgs selected at longer wavelengths ( i.e. , 850  @xmath10 m  to 1  mm ) , in terms of the [ nii]/h@xmath3  ratio ( i.e. , gas metallicity ) , the ir luminosity ( i.e. , sfr ) , and the ir - to - radio luminosity ratio ( i.e. , radio excess due to agn ) . the vla - identified smgs are optically faint and unbiased to radio - loud agns , so they indeed represent a galaxy population distinct from the optical selected qsos and the lyman break galaxies ( lbgs ) in the same redshift range . strong hi  ly@xmath3  absorption is found in the background qso spectra for all of the three spectroscopically confirmed smg@xmath8qso pairs  with impact parameters between @xmath130  kpc . here we have adopted a much narrower search window ( @xmath7600  km s@xmath30 ) than the qpq study , further reducing the level of contamination from physically unrelated clouds . however , none of the three absorption line systems seems optically thick at the lyman limit ( i.e. , llss with @xmath113  @xmath37 ) , in contrast to the @xmath060% covering fraction of llss around qsos from the qpq study despite similar data quality , foreground redshifts , and impact parameters . our comparison thus suggests _ either _ that smgs do not have a substantial neutral gas reservoir in their halos that could potentially fuel a prolonged star formation phase _ or _ that smgs inhabit @xmath139@xmath2  halos so that our sightlines are yet to probe inside their virial radii . if the latter , their halos are comparable to those of lbgs . @xcite found an optically thick covering fraction of @xmath140% around lbgs at @xmath141 and @xmath142  kpc . note that this is @xmath02@xmath77 lower than that of coeval qsos and is consistent with the 1@xmath54 confidence interval that we were able to place for the smgs . on the other hand , the difference in the optically thick hi  covering fraction between smgs and qsos casts doubt on the evolutionary link between the two populations , unless agn outflows can somehow affect the physical state of gas at hundreds of kpc scales within its short lifetime . our final conclusion is limited by the small sample size . to enable a more robust comparison with previous absorption - line studies , we badly need to increase the effective yield of our survey from the current level of @xmath023% . in a future publication , we will present observations with the atacama large millimeter / submillimeter array ( alma ) to pinpoint the positions of the _ herschel _  sources in the smg@xmath8qso pairs . observing at a wavelength ( 870  @xmath10 m ) much closer to the selection wavelengths ( 250 to 500  @xmath10 m ) , we expect doubling the detection rate with integration times of just several minutes per source herschel _ sources in our sample are too faint to allow a quick co redshift search with alma ( e.g. , the survey of strongly lensed smgs by * ? ? ? * ) , but spectrographs on large optical telescopes covering the full optical@xmath6ir range at a moderate spectral resolution ( @xmath143 ) will greatly increase the redshift search range and decrease the areas blinded by strong airglow lines . we thank f.  bian and k.  allers for providing the data reduction packages , and d.  ludovici for helping with the vla data calibration of l6-xmm0223@xmath80605 . the national radio astronomy observatory is a facility of the national science foundation ( nsf ) operated under cooperative agreement by associated universities , inc . support for this work was provided by the nsf through award gssp sospa3 - 016 from the nrao . acknowledges support from the nsf grant ast-1614326 , the nasa jpl award 1495624 , and funds from the university of iowa . j.x.p . acknowledges support from the nsf grants ast-1010004 , ast-1109452 , ast-1109447 and ast-1412981 . a.c . acknowledges support from the nsf grant ast-1313319 and the nasa grants nnx16af39 g and nnx16af38 g . d.k . acknowledges support from the nsf grant ast-1412153 and funds from the university of california san diego . z.y.z . acknowledges support from the european research council in the form of the advanced investigator programme , 321302 , cosmicism . herschel_-atlas is a project with _ herschel _ , which is an esa space observatory with science instruments provided by european - led principal investigator consortia and with important participation from nasa . the h - atlas website is http://www.h - atlas.org/. the us participants acknowledge support from the nasa _ _  science center / jpl . the lbt is an international collaboration among institutions in the united states , italy and germany . lbt corporation partners are : the university of arizona on behalf of the arizona university system ; istituto nazionale di astrofisica , italy ; lbt beteiligungsgesellschaft , germany , representing the max - planck society , the astrophysical institute potsdam , and heidelberg university ; the ohio state university , and the research corporation , on behalf of the university of notre dame , university of minnesota and university of virginia . some of the data presented herein were obtained at the w.m . keck observatory , which is operated as a scientific partnership among the california institute of technology , the university of california and the national aeronautics and space administration . the observatory was made possible by the generous financial support of the w.m . keck foundation . based on observations obtained at the gemini observatory , which is operated by the association of universities for research in astronomy , inc . , under a cooperative agreement with the nsf on behalf of the gemini partnership : the national science foundation ( united states ) , the national research council ( canada ) , conicyt ( chile ) , ministerio de ciencia , tecnologa e innovacin productiva ( argentina ) , and ministrio da cincia , tecnologia e inovao ( brazil ) . the authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of mauna kea has always had within the indigenous hawaiian community . we are most fortunate to have the opportunity to conduct observations from this mountain .                                                                                                                        , j.  p. , waters , b. , schiebel , d. , young , w. , & golap , k. 2007 , in astronomical society of the pacific conference series , vol . 376 , astronomical data analysis software and systems xvi , ed . r.  a. shaw , f.  hill , & d.  j. bell , 127
we present the first results from an on - going survey to characterize the circumgalactic medium ( cgm ) of the massive high - redshift galaxies detected as submillimeter galaxies ( smgs ) . 
 we constructed a parent sample of 163 smg - qso pairs with separations less than @xmath036  by cross - matching far - infrared - selected galaxies from _ herschel _ with spectroscopically confirmed qsos . 
 the _ 
 herschel _ sources were selected to match the properties of smgs . 
 we determined the sub - arcsecond positions of six _ herschel _ sources with the very large array and obtained secure redshift identification for three of those with near - infrared spectroscopy . the qso sightlines probe transverse proper distances of 112 , 157 , and 198  kpc at foreground redshifts of 2.043 , 2.515 , and 2.184 , respectively , which are comparable to the virial radius of the @xmath1  @xmath2  halos expected to host smgs . 
 high - quality absorption - line spectroscopy of the qsos reveals systematically strong hi  ly@xmath3  absorption around all three smgs , with rest - frame equivalent widths of @xmath4   . 
 however , none of the three absorbers exhibits compelling evidence for optically thick hi  gas or metal absorption , in contrast to the dominance of strong neutral absorbers in the cgm of luminous @xmath5 qsos . 
 the low covering factor of optically thick hi  gas around smgs tentatively indicates that smgs may not have as prominent cool gas reservoirs in their halos as the co - eval qsos and that they may inhabit less massive halos than previously thought .