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 Lya forest, we present an analysis of 785 absorption lines in the spectra of ve 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 102A 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 127A, 147A, and 177A 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 di erent view of the nature of the absorbers. The pair samples a scale critical in determining the size upper bound of Lya absorbers, with signicant leverage in redshift compared to previous studies. In the case of the triplet, this represents the spatially densest sample of Lya forest absorbers ever studied and an almost ideally suited probe of the shape of absorbers. We observe a sig-nicant number of Lya lines in common between the triplet sight lines, for lines stronger than rest equivalent width (and no detected metal lines) and velocity di erences up to 200 km s~1, W 0 [ 0.4 A corresponding to a two-point correlation function on scales of 0.50.8 h~1 Mpc with m \ 1.88 ~0.50 `0.78 SzT \ 2.14 and inconsistent at the 99.999% level with the absence of any clustering. These data also show that a signicant fraction of the Lya forest absorbers spans all three sight lines to the W 0 [ 0.4 A KP triplet, indicating that the strong-lined absorbers are consistent with nearly round shapes, chosen from a range of possible cylinders of di erent elongations. This may be inconsistent with results from hydrodynamic/gravitational simulations of H I in the early Universe, indicating that the theoretical counterparts of Lya forest clouds are long and lamentary. Furthermore, there is a probable correlation of with *v suggestive of the clouds being attened and expanding with the Hubble ow in their long W 0 dimension, as would be indicative of sheets or laments. This is supported by the uniformity of line strengths between the three sight lines for