In order to study the size and shape of the absorbers that lie in front of the QSOs , in particular the Ly \alpha forest , we present an analysis of 785 absorption lines in the spectra of five QSOs in close groupings : a pair ( LB9605 : 1517+2357 at z = 1.834 and LB9612 : 1517+2356 at z = 1.903 , with a separation of 102 arcsec between them ) and a triplet ( KP 76 : 1623+2651A at z = 2.467 , KP 77 : 1623+2653 at z = 2.526 , and KP 78 : 1623+2651B at z = 2.605 , with separations of 127 , 147 and 177 arcsec between pairs 76:78 , 76:77 and 77:78 , respectively ) .
Both of these QSO groups have been observed before , but these data represent a drastic increase in signal-to-noise ratio and/or wavelength coverage over earlier data , and provide a qualitatively different view of the nature of the absorbers .
The pair samples a scale critical in determining the size upper bound of Ly \alpha absorbers , with significant leverage in redshift compared to previous studies .
In the case of the triplet , this represents the spatially densest sample of Ly \alpha forest absorbers ever studied , and an almost ideally-suited probe of the shape of absorbers .
We observe a significant number of Ly \alpha lines in common between the triplet sightlines , for lines stronger than rest equivalent width W _ { o } > 0.4 Å ( and no detected metal lines ) and velocity differences up to 200 km s ^ { -1 } , corresponding to a two-point correlation function \xi = 1.88 ^ { +0.78 } _ { -0.50 } on scales 0.5 to 0.8 h ^ { -1 } Mpc with \langle z \rangle = 2.14 , and inconsistent at the 99.999 % level with the absence of any clustering .
These data also show that a significant fraction of the W _ { o } > 0.4 Å Ly \alpha forest absorbers span all three sightlines to the KP triplet , indicating that the strong-lined absorbers are consistent with nearly round shapes , chosen from a range of possible cylinders of different elongations .
This may be inconsistent with results from hydrodynamic/gravitational simulations of H I in the early Universe indicating that the theoretical counterparts of Ly \alpha forest clouds are long and filamentary .
Furthermore , there is a probable correlation of W _ { o } with \Delta v suggestive of the clouds being flattened and expanding with the Hubble flow in their long dimension , as would be indicative of sheets or filaments .
This is supported by the uniformity of linestrengths between the three sightlines , for W _ { o } > 0.4 Å .
We conclude , tentatively , that the W _ { o } > 0.4 Å Ly \alpha forest objects are sheetlike .
In contrast , the weaker lines , 0.2Å > W _ { o } > 0.4 Å show no evidence of spanning the sightlines of these groups , but have sizes significantly larger than the luminous portions of galaxies , and C IV absorbers as revealed by closer-separation QSO pairs .
When the LB sightline pair is included with other pairs at different redshifts and sightline separations , one finds no evidence for evolution of Ly \alpha absorber size with redshift .
We also show that there is no evidence of large-scale structure in the Ly \alpha forest consistent with ionization of H II by foreground QSOs as seen in the spectrum of background QSOs ( the “ foreground proximity effect ” ) .
Finally , we see a marginal detection of the sightline two-point cross-correlation function for C IV lines \xi = 2.05 ^ { +1.82 } _ { -1.21 } over scales of 0.5 to 1 h ^ { -1 } Mpc .
This is significantly weaker than \xi measured by auto-correlation along single sightlines for 200 km s ^ { -1 } < \Delta v < 600 km s ^ { -1 } , suggesting that most of the latter signal may be due to the internal motions within absorbers which are smaller than 0.5 h ^ { -1 } Mpc .