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intervening damped ly@xmath0 absorption - line systems ( dlas ) in quasar spectra are very rare , with an incidence of @xmath19 per unit redshift at @xmath1 ( rao , turnshek , & nestor 2004 , hereafter rtn2004 ) . consequently , unless dlas are correlated , the appearance of two dlas along any single quasar sightline ( `` double - damped '' ) represents a very unlikely event . as such , the discovery of any double - damped absorption warrants a closer investigation . here we report the discovery of double - damped absorption near @xmath1 in the sloan digital sky survey ( sdss ) quasar q1727 + 5302 during our most recent _ hubble space telescope _ uv spectroscopic survey for dlas ( rtn2004 ) . the purpose of the present paper is to report initial results pertaining to this discovery , and thereby encourage future studies of this region of the sky . the velocity separation of the absorption is 13,000 km s@xmath5 , which corresponds to a proper radial distance of @xmath20 mpc if interpreted as due to the hubble flow . , @xmath8 , and @xmath21 km s@xmath5 mpc@xmath5 ( @xmath22 ) . ] we speculate that this configuration may represent a neutral hydrogen gas filament with a large cosmological extent along our sightline . in fact , the comoving size of this putative filament would be larger than anything previously reported . as discussed in earlier contributions ( e.g. , rao & turnshek 2000 , hereafter rt2000 , and references therein ) , dlas are excellent tracers of the bulk of the neutral hydrogen gas in the universe , and the aim of our most recent dla uv survey has been to improve our knowledge of the incidence and cosmological mass density of dlas at redshifts @xmath23 . the new sample which led to the discovery of the double - damped absorption was derived from the sdss early data release ( edr ) ( schneider et al . we applied a strong - rest equivalent width ( rew ) selection criterion ( rt2000 ) to optical spectra in order to identify candidate dla absorption lines ( @xmath24 atoms @xmath4 ) , and then we obtained hst stis uv spectra to confirm or refute their presence . the current overall success rate for identifying dlas with this method is @xmath17% . since the @xmath252796,2803 absorption lines are saturated , the rew of the absorption is most closely tied to kinematic spread , not column density . recently , nestor et al . ( 2003 ) have discussed evidence for a correlation between kinematic spread and metallicity . the paper is organized as follows . in 2 we present the hst discovery spectrum for the double - damped absorption , a follow - up mmt spectrum used to determine neutral - gas - phase metal abundances , and irtf imaging data used to search for galaxies associated with the double - damped absorbers along the quasar sightline . in 3 we summarize evidence that exists for strong absorption systems near @xmath1 in other sdss quasars in the same region of the sky . a brief summary and discussion of the results is presented in  4 . q1727 + 5302 ( sdss j172739.03 + 530229.16 ) is the j2000 coordinate designation of the quasar which exhibits double - damped absorption near @xmath1 . the quasar has an emission redshift of @xmath26 and an sdss g - band magnitude of 18.3 . a search for the @xmath252796,2803 absorption doublet in bright ( @xmath27 ) sdss edr quasars resulted in the identification of two systems along this sightline at @xmath28 and @xmath29 . table 1 gives rews of some of the identified metal absorption lines . the spectrum shown in figure 1 was obtained during a 73 minute exposure with the hst on 1 january 2003 in the stis nuv g230l mode . see rtn2004 for details of the observing program . the dla lines at @xmath30 and @xmath31 are visible . the insert to the figure shows two voigt damping profiles that have been fitted simultaneously to the two ly@xmath0 lines . these have neutral hydrogen column densities of @xmath32 atoms @xmath4 and @xmath33 atoms @xmath4 , respectively . the @xmath34 errors were determined by assessing uncertainties in the continuum fit as described in rt2000 . we used the 6.5-m mmt on 2 july 2003 to obtain spectroscopic observations of both systems making up the double - damped absorption in order to determine their neutral gas phase metal abundances . we used the and lines ( figure 2 ) , as well as , , and lines ( not shown ) . the method employed to measure the metal abundances is the same as that used by nestor et al . relative to the solar measurements of grevesse & sauval ( 1998 ) , we find zn abundances of [ zn / h ] = @xmath35 ( 26.5% solar ) and @xmath36 ( 4.7% solar ) for the @xmath28 and @xmath29 systems , respectively . the corresponding cr abundances are [ cr / h ] = @xmath37 ( 5.5% solar ) and @xmath38 ( 1.7% solar ) , so there is evidence for depletion onto grains . these determinations fall within the range of dla metallicities that have been reported near @xmath1 ( e.g. , see prochaska et al . also , for the @xmath28 system we find [ si / h ] = @xmath39 ( 15.6% solar ) , [ fe / h ] = @xmath40 ( 4.3% solar)hiresdla by jason prochaska . ] , and [ mn / h ] = @xmath41 ( 6.6% solar ) ; for the @xmath29 system we find [ si / h ] = @xmath42 ( 3.5% solar ) , [ fe / h ] = @xmath43 ( 0.8% solar)@xmath44 , and [ mn / h ] = @xmath45 ( 1.3% solar ) . some of our recent imaging results on low - redshift dlas have been reported by rao et al . ( 2003 ) . on 2 april 2003 we made similar ir ( jhk bands ) observations of the double - damped sightline with the 3.0-m irtf . figure 3 shows a 14  x 14section of the h - band image centered on the quasar . the limiting 1@xmath46 surface brightness is 22.0 h magnitudes per square arcsec . objects labeled g1 and g2 are reasonable candidates for the dla galaxies based on their proximity to the sightline . if at @xmath1 , their impact parameters of @xmath47 and @xmath48 correspond to proper transverse distances of @xmath49 kpc and @xmath50 kpc , respectively . g1 and g2 have h - band magnitudes of @xmath51 and @xmath52 . they are also visible in our j - band and k - band images ( not shown ) , with magnitudes @xmath53 , @xmath54 , @xmath55 , and @xmath56 .    adopting @xmath57 from local galaxies studies ( e.g. , bell et al . 2003 ) , a @xmath58-correction appropriate for an sa type galaxy from poggianti ( 1997 ) , and assuming no evolution , a local @xmath59 galaxy redshifted to @xmath1 would have @xmath60 for our adopted cosmology . we note that at @xmath1 @xmath58-corrections for all galaxy types are relatively small in the k - band , and the magnitudes of g1 and g2 would correspond to luminosities of @xmath61 and @xmath62 , respectively , if they were at @xmath1 . however , recently ellis & jones ( 2004 ) have presented k - band observations of the galaxy population residing in three x - ray selected massive clusters at @xmath63 . fits to their data indicate that at redshifts near @xmath1 the galaxy population in massive clusters has @xmath64 lying in the range @xmath65 mag . thus , @xmath66 is @xmath67 mag brighter than @xmath68 , which is what might be expected from passive evolution . therefore , g1 and g2 are @xmath69 and @xmath70 , respectively , and it should be emphasized that there are no good candidate dla galaxies with luminosities near @xmath66 . consequently , if g1 and g2 are passively evolving dla galaxies , they can simply be interpreted as the progenitors of local dwarfs . we note that if g1 was at the quasar redshift ( @xmath26 ) , it would be @xmath71 . however , g1 is offset from the quasar , so if it were related to the quasar it would likely be associated with an interaction . low - redshift quasar host galaxies are generally several times @xmath72 ( e.g. , hamilton , casertano , & turnshek 2002 ; jahnke & wisotzki 2003 ) . one way to test the hypothesis that the double - damped absorption represents an extended filament along our sightline is to search for absorption near @xmath1 in other quasars with @xmath73 that lie approximately in the same direction . this possibility can be considered since the sdss has searched this region of the sky for quasars . five other @xmath73 quasars are known that meet the criteria of having spectra of high enough quality to detect strong - systems and lying within 30 arcmin of the double - damped sightline . an angular size of 30 arcmin ( sdss data presently cut off beyond this on one edge ) is considerably smaller than the putative @xmath74 mpc ( proper distance ) long filament at @xmath1 , but this is reasonable if we are viewing cosmic structure along a filament . all five of these other quasars fall below the brightness limits used for inclusion in our current hst uv survey for low - redshift dlas ( rtn2004 ) . three of the quasars ( sdss q1727 + 5301 at z@xmath75 , sdss q1727 + 5306 at z@xmath76 , and sdss q1729 + 5312 at z@xmath77 ) do not show evidence for strong absorption near @xmath1 , so there is little chance that they have dla absorption near @xmath1 . however , the remaining two do show strong - absorption which meet the criterion for selecting dla candidates ( rt2000 ) . the sightline locations and presence of absorption may constrain the geometry of any filament . the two new absorption systems ( rews are reported in table 1 ) are at @xmath78 ( in sdss q1725 + 5254 at @xmath79 ) and @xmath80 ( in sdss q1727 + 5311 at @xmath81 ) , separated from the double - damped sightline by @xmath82 arcmin and @xmath83 arcmin , respectively . thus , there are four strong - absorption systems that lie along similar sightlines , within a redshift spread of @xmath84 , corresponding to a velocity separation of @xmath85 km s@xmath5 . given our strong - selection criterion , if these new systems are not dlas , they are almost certainly sub - dlas with column densities @xmath86 atoms @xmath4 . figure 4 shows continuum - normalized regions of the sdss spectra for all three quasars with strong - absorption . investigations of the possibility of clustering between dlas and lyman break galaxies ( lbg ) have been made at high redshift ( e.g. , gawiser et al . 2001 , aldelberger et al . 2003 , bouch & lowenthal 2003 ) . gawiser et al . ( 2001 ) and aldelberger et al . ( 2003 ) find no significant evidence for clustering at @xmath87 and @xmath88 , respectively , while bouch & lowenthal ( 2003 ) present weak evidence ( @xmath89 significance ) for clustering at @xmath90 on a size scale up to @xmath91 mpc ( @xmath92 mpc comoving ) .    here we have reported the discovery of an apparently non - random ( see below ) structure on the sky near redshift @xmath1 (  2.1 ) which is much larger than the scales explored in high - redshift dla - lbg clustering studies and much larger than normal galaxy clustering . the structure consists of two extremely high-@xmath15 ( even by dla standards ) dla absorbers separated by 13,000 km s@xmath5 along a single quasar sightline . if interpreted as a cosmological redshift separation , this double - damped structure has a radial proper distance of @xmath74 mpc . the incidence of dlas above a survey threshold of @xmath24 atoms @xmath4 is @xmath19 per unit redshift at @xmath1 ( rtn2004 ) . given this incidence , the probability of observing a second dla within @xmath93 of another dla is @xmath943% . we quote this probability as an upper limit because the incidence of dlas is significantly smaller for higher column density systems , and the column densities of the two systems included in the double - damped absorption are factors of @xmath95 and @xmath96 times larger than the survey threshold.@xmath97 therefore , the double - damped absorption may be the result of correlated dlas and be caused by a cosmologically extended filament of neutral gas along our sightline . in addition to the identification of this remarkable structure , we have made metal abundance determinations for the two systems which make up the double - damped absorption . we find them to have [ zn / h ] = @xmath9 ( 26.5% solar ) and @xmath98 ( 4.7% solar ) , with evidence for some depletion onto grains in both cases (  2.1 ) . infrared imaging indicates that the two most likely dla candidate galaxies are relatively faint in relation to the galaxy population at @xmath1 , with @xmath99-band luminosities that are @xmath69 and @xmath70 (  2.2 ) . these add to the list of underluminous galaxies that have been identified as being responsible for dla absorption ( rao et al . the results indicate that the presence of luminous galaxies relative to the local population evidently is not a requirement for the presence of large concentrations of neutral gas . we have also identified two new candidate dla systems in this same region of the sky , separated from the original sightline by 10 arcmin and 25 arcmin (  3 ) . the discovery of these two additional systems increases the probability that this is a non - random structure . if the two new redshift systems are included , the structure stretches @xmath85 km s@xmath5 along the sightline , corresponding to a radial proper distance of @xmath100 mpc .    filaments nearly as large as that implied by the double - damped absorption have been seen in mock redshift surveys and cold dark matter simulations of structure formation . for example , faltenbacher et al . ( 2002 ) report correlations in cluster orientations with respect to one another and find alignments of galaxy clusters major axes on comoving scales of @xmath101 mpc , corresponding to a proper distance of @xmath102 mpc at @xmath1 . the large scale structure simulations of eisenstein , loeb , & turner ( 1997 ) show similar alignments . recently , palunas et al . ( 2004 ) have reported evidence for a structure with a proper size of @xmath103 mpc ( @xmath104 mpc comoving ) , which they found during a search for ly@xmath0-emitting galaxies at @xmath105 . miller et al . ( 2004 ) have reported evidence for structures on even larger scales based on an analysis of the qso distribution in the 2df redshift survey . for this case of double - damped absorption , the size of the putative neutral hydrogen gas filament would be larger than any claimed so far . therefore , our interpretation should be considered speculative pending future studies . nevertheless , the properties of the double - damped absorption are of course relevant to studies of dlas in general .    the present - day cosmological model ( approximately 73% dark energy , 24% dark matter and 5% ordinary matter , e.g. , spergel et al . 2003 ) is one in which large - scale structures can form early in time . however , a specific set of cosmological parameters may also indicate that it is highly improbable for certain structures to grow from initial gaussian perturbations . thus , surveys to find evidence for extreme large - scale structure at high redshift have the potential to result in important cosmological constraints . based on the numbers of dlas discovered at low redshift so far and the dla column density distribution , the existence of this double - damped absorption along one sightline represents evidence for a non - random distribution of dlas which should be further investigated . we thank members of the sdss collaboration who made the sdss project a success and who made the edr spectra available . we acknowledge support from nasa - stsci , nasa - ltsa , and nsf . hst - uv spectroscopy made the @xmath15 determinations possible , while follow - up metal abundance measurements ( mmt ) and imaging ( irtf ) were among the aims of our ltsa and nsf programs . funding for creation and distribution of the sdss archive has been provided by the alfred p. sloan foundation , participating institutions , nasa , nsf , doe , the japanese monbukagakusho , and the max planck society . the sdss web site is www.sdss.org . the sdss is managed by the astrophysical research consortium for the participating institutions : university of chicago , fermilab , institute for advanced study , the japan participation group , johns hopkins university , los alamos national laboratory , the max - planck - institute for astronomy ( mpia ) , the max - planck - institute for astrophysics ( mpa ) , new mexico state university , university of pittsburgh , princeton university , the united states naval observatory , and university of washington . lcccccc q1727 + 5302 & 1.44 & 0.9448 & 2.19(0.13 ) & 2.83(0.07 ) & 2.51(0.07 ) & 0.99(0.07 ) & & 1.0312 & 0.76(0.11 ) & 0.92(0.06 ) & 1.18(0.08 ) & 0.33(0.10 ) q1725 + 5254 & 1.36 & 0.9706 & 0.76(0.13 ) & 1.20(0.12 ) & 1.15(0.12 ) & q1727 + 5311 & 1.81 & 1.1536 & 1.26(0.39 ) & 2.32(0.45 ) & 2.38(0.41 ) &
we report the discovery of two damped ly@xmath0 absorption - line systems ( dlas ) near redshift @xmath1 along a single quasar sightline ( q1727 + 5302 ) with neutral hydrogen column densities of @xmath2 and @xmath3 atoms @xmath4 . 
 their sightline velocity difference of 13,000 km s@xmath5 corresponds to a proper separation of 106@xmath6 mpc if interpreted as the hubble flow ( @xmath7 , @xmath8 ) . 
 the random probability of such an occurrence is significantly less than 3% . 
 follow - up spectroscopy reveals neutral gas - phase zn abundances of [ zn / h ] = @xmath9 ( 26.5% solar ) and @xmath10 ( 4.7% solar ) , respectively . 
 the corresponding cr abundances are [ cr / h ] = @xmath11 ( 5.5% solar ) and @xmath12 ( 1.7% solar ) , respectively , which is evidence for depletion onto grains . 
 follow - up ir images show the two most likely dla galaxy candidates to have impact parameters of @xmath13 kpc and @xmath14 kpc if near @xmath1 . 
 they are significantly underluminous relative to the galaxy population at @xmath1 . to investigate the possibility of additional high-@xmath15 absorbers we have searched the sdss database for @xmath16 quasars within 30 arcmin of the original sightline . 
 five were found , and two show strong - absorption near @xmath1 , consistent with classical dla absorption @xmath17% of the time , but almost always @xmath18 atoms @xmath4 . 
 consequently , this rare configuration of four high-@xmath15 absorbers with a total sightline velocity extent of 30,600 km s@xmath5 may represent a large filament - like structure stretching over a proper distance of 241@xmath6 mpc along our sightline , and a region in space capable of harboring excessive amounts of neutral gas . 
 future studies of this region of the sky are encouraged .