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Session 72 - Cosmic Rays and Abundances.
Oral session, Thursday, June 11

[72.05] The Clustering Properties of Low-Redshift QSO Absorption Line Systems Towards the Galactic Poles

D. E. Vanden Berk (U. Texas)

This dissertation presents the clustering properties of a sample of low-redshift Ly\,\alpha and heavy-element QSO absorption line systems obtained with the Faint Object Spectrograph of the Hubble Space Telescope. The QSOs are concentrated in a relatively small area towards the North Galactic Pole, which allows the clustering properties to be examined on transverse scales from about 15 to 200h^-1Mpc (q_0=0.5, \Lambda=0,H_0=100hkm/s/Mpc). The Ly\,\alpha systems are significantly clustered along the lines of sight on scales up to 10h^-1Mpc, and the amplitude of this clustering appears to depend on the limiting equivalent width of the sample. The fraction of Ly\,\alpha systems with W>0.24Åwhich may be clustered as strongly as QSO heavy-element systems is about 40%. At very low redshifts (z<0.18) the majority of Ly\,\alpha systems arise at the same redshifts as strong peaks in the galaxy redshift distribution in the same volume of space. At higher redshifts (0.3 < z < 1.2) there is no clear evidence that the Ly\,\alpha systems are distributed in redshift ``peaks'' like the galaxies. In particular, there is no significant periodicity detected on the scale of 128h^-1Mpc, which had been reported for the galaxies located in the same direction of the sky. There may be an 83h^-1Mpc periodicity associated with the higher equivalent width systems, but it is not highly significant. These results are consistent with a picture of the low-redshift Ly\,\alpha forest in which some fraction of the Ly\,\alpha lines arise in close proximity to galaxies and structures traced by galaxies, but in which a substantial fraction of the systems are also more uniformly distributed on the scales considered here.

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