Reobservation of Close QSO Groups: The Size Evolution and Shape of Lyα Forest Absorbers

In order to study the size and shape of the absorbers that lie in front of the QSOs, in particular the Lyα forest, we present an analysis of 785 absorption lines in the spectra of five QSOs in close groupings: a pair (LB 9605: 1517+2357 at z = 1.834 and LB 9612: 1517+2356 at z = 1.903, with a separation of 102'' 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'' 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 Lyα absorbers, with significant leverage in redshift compared to previous studies. In the case of the triplet, this represents the spatially densest sample of Lyα forest absorbers ever studied and an almost ideally suited probe of the shape of absorbers. We observe a significant number of Lyα lines in common between the triplet sight lines, for lines stronger than rest equivalent width W0 > 0.4 Å (and no detected metal lines) and velocity differences up to 200 km s-1, corresponding to a two-point correlation function ξ=1.88+ 0.78−0.50 on scales of 0.5-0.8 h-1 Mpc with ⟨z⟩ = 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 W0 > 0.4 Å Lyα forest absorbers spans all three sight lines 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 Lyα forest clouds are long and filamentary. Furthermore, there is a probable correlation of W0 with Δv 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 line strengths between the three sight lines for W0 > 0.4 Å. We conclude, tentatively, that the W0 > 0.4 Å Lyα forest objects are sheetlike. In contrast, the weaker lines, 0.2 Å > W0 > 0.4 Å, show no evidence of spanning the sight lines 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 sight-line pair is included with other pairs at different redshifts and sight-line separations, one finds no strong evidence for evolution of Lyα absorber size with redshift. We also show that there is no evidence of large-scale structure in the Lyα 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 sight-line two-point cross-correlation function for C IV lines ξ=2.05+ 1.82−1.21 over scales of 0.5-1 h-1 Mpc. This is significantly weaker than ξ measured by autocorrelation along single sight lines for 200 km s-1 < Δv < 600 km s-1, suggesting that most of the latter signal may be due to the internal motions within absorbers that are smaller than 0.5 h-1 Mpc.