1999ApJS..122..355V

We examine the clustering properties of low-redshift Lyα and heavy-element QSO absorption line systems seen in the spectra of 13 QSOs at the Galactic poles. This is the densest sample of ∼1° separated QSOs observed spectroscopically with the Hubble Space Telescope to date. At the median redshift of the Lyα sample (z~0.7), the QSO lines of sight are separated on transverse scales from about 15 to 200 h^–1 Mpc (q₀=0.5, H=100 h km.s^–1.Mpc^–1), allowing the three-dimensional clustering of the absorbers to be examined on those scales. The Galactic poles are also regions where relatively deep and wide-field galaxy redshift surveys have taken place, so the distributions of galaxies and Lyα systems can be compared within the same volume of space. There are 545 total absorption lines detected in the complete sample from 13 QSOs. We identify 307 Lyα systems, of which 18 contain heavy-element lines. We confirm the relatively slow redshift number density evolution for Lyα systems at z≲1. There are also five likely C IV doublets in our sample, for which the Lyα line is not accessible. The main results are summarized as follows:

• (1.) Most, but not all, of the very low-z (z<0.2) Lyα systems appear to be associated with large-scale structures traced by luminous galaxies, based upon the spectrum of the low-redshift QSO PG 1309+355.
• (2.) On scales of 2.5 to 5 h^–1 Mpc, the line-of-sight correlation function shows a positive and significant clustering amplitude, ξ_los=0.95±0.28, for systems with rest equivalent width, W, exceeding 0.24 Å. Most of this signal can be accounted for by a small number of Lyα ``clumps'', which typically contain about five systems spanning ∼20 h<sup>–1</sup> Mpc. There is qualitative evidence that the strength of the correlation function rises with the limiting Lyα rest equivalent width.
• (3.) Over half of the heavy-element systems are either in groups or have a nearby ``clump'' of Lyα lines. However, there is only weak evidence for a significant heavy-element/Lyα cross-correlation function.
• (4.) There is no clear evidence that the Lyα systems are distributed in redshift ``peaks'' and ``voids'' as is characteristic for the galaxies. In particular, there is no significant periodicity detected on the scale of 128 h^–1 Mpc, which had been detected for the galaxies located in the same direction of the sky. There is marginal evidence for a 41.5 h^–1 Mpc ``periodicity'' in the distribution, which is seen for all equivalent width-limited samples.
• (5.) The three-dimensional Lyα radial correlation function shows no significant departures from a random sample at any scale or using any limiting equivalent width. This indicates that if there are sheetlike structures spanning separations on the order of 30 h^–1 Mpc or more, they are not clearly traced by Lyα systems.
• (6.) The fraction of Lyα lines with W>0.24 Å, which may be clustered in the same way as the heavy-element systems detected mostly at higher redshifts, is about 39%±6%. This value is likely to depend on the limiting equivalent width and redshift of the sample.

These results are consistent with a picture of the Lyα forest at low redshift in which some fraction of the Lyα 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 and are largely unrelated to the distribution of luminous galaxies. The results also appear to be in broad agreement with numerical simulations of the intergalactic medium, which suggest that the denser regions traced by luminous galaxies are also traced by the stronger Lyα systems, but that Lyα systems also arise in the more general underlying distribution of matter in the universe.