Reobservation of Close QSO Groups: The Size Evolution and Shape of Lyman Alpha Clouds

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 and a triplet. 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. The pair samples a scale critical to determining the size upper bound of Ly alpha clouds, with significant leverage in redshift. For the triplet, this represents the spatially densest sample of Ly alpha forest clouds ever studied, and a nearly 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 > 0.4A and velocity differences to 200 km/s, corresponding to a two-point correlation function 0.72 on scales 0.5 to 1 Mpc/h with =2.29, and inconsistent with no clustering the 98% level. These data also show that a significant fraction of the W_o > 0.4A Ly alpha clouds span all three sightlines to the QSO triplet, indicating that the strong-lined objects are consistent with nearly round shapes, chosen from a possible range of cylinders of various elongations. Furthermore, there is a probable correlation of W_o with velocity difference suggestive of the clouds being flattened and expanding with the Hubble flow in their long dimension, as would be indicative of sheets or filaments. When the QSO pair through the Ly alpha forest is included with other pairs at different redshifts and sightline separations, one finds no strong evidence for evolution of the cloud size with redshift. (abridged abstract)