Session 43 - Source Surveys.
Display session, Wednesday, June 11
South Main Hall,

## [43.02] New Insights into QSO Absorption Lines from Multiple Sightlines

A. Crotts (Columbia), S. Burles, D. Tytler (UCSD), Y. Fang (Columbia)

We present Keck HIRES and KPNO 4m/RC Spec spectra of the close QSO triplet 1623+27 (z\approx 2.5, separations s= 127-177 arcsec) and close QSO pair 1517+239, and resulting new insights into the clustering, shape and size of both C IV and Ly \alpha absorption line objects. The cross-correlation of C IV absorbers between sightlines is much weaker than earlier measurements on corresponding scales along single sightlines of the two-point correlation function \xi_a. Internal velocity splittings and/or large deviations from the Hubble flow predominate in establishing \xi_a of C IV absorbers, and that spatial clustering alone cannot explain the strong \xi_a for single-sightline velocity splittings 200 km s^-1<\Delta v <600 km s^-1. For Ly \alpha absorbers, a significant cross-correlation signal persists for velocity differences between sightlines \Delta v < 200 km s^-1, as seen earlier across closer sightline pairs. (Transverse separations for 1623+27 are 0.5-0.7 h^-1 Mpc.) We note that most of the non-random portion of this signal seems to arise from absorbers that span all three sightlines, and show that this indicates a cloud shape on these scales that is close to round in cross-section, with a cross-sectional aspect ratio a < 2 (where a, specifically, is the length-to-diameter ratio for circular-sectioned cylinders). Possibly, the same cross-sectional behavior might arise in disks rather than unflattened clouds, and the kinematics of the objects spanning all three sightlines is consistent with sheets expanding in 2D in the Hubble flow. These results are not highly statistically significant, but do rule out filamentary morphology on proper scales larger than about 0.5 h^-1 Mpc. We also study larger scale structure, and find marginal evidence against the hypothesis that the proximity effect'' exists in the Ly \alpha forest of a background QSO due to a bright foreground QSO. We also analyze the QSO pair 1517+239 (z \approx 1.9, s=102 arcsec), and show that it, along with 1623+27 and other pairs, do not confirm that Ly \alpha cloud size decreases with z.

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The author(s) of this abstract have provided an email address for comments about the abstract: arlin@astro.columbia.edu1