The Relationship between Galaxies and Low-Redshift Weak Lyα Absorbers in the Directions of H1821+643 and PG 1116+215

To study the nature of low-z Lyα absorbers in the spectra of QSOs, we have obtained high signal-to-noise ratio (S/N) UV spectra of H1821+643 (zem = 0.297) and PG 1116+215 (zem = 0.177) with the Goddard High-Resolution Spectrograph on the Hubble Space Telescope. The spectra have minimum S/Ns of ~70-100 and 3 σ limiting equivalent widths of 50-75 mÅ at a resolution of ~150 km s-1. Excluding lines within 3000 km s-1 of zem, we detect 26 Lyα lines with Wr > 50 mÅ toward H1821+643 and 13 toward PG 1116+215 (comparable to the 13 Lyα lines observed toward 3C 273 by Morris et al.), which implies a density of 102 ± 16 lines per unit redshift for Wr > 50 mÅ and zabs < 0.28. The two-point velocity correlation function shows marginal evidence of clustering of Lyα lines on ~500 km s-1 scales, but only if the weakest lines are excluded. We have also used the Wisconsin-Indiana-Yale-NOAO (WIYN) Observatory to measure galaxy redshifts in the ~1° fields centered on each QSO in order to study the relationship between the Lyα absorbers and galaxies. We find 17 galaxy-absorber pairs within projected distances of 1 h−175 Mpc with velocity separations of 350 km s-1 or less. Monte Carlo simulations show that if the Lyα lines are randomly distributed, the probability of observing this many close pairs on the two sight lines is 3.6 × 10-5. We find that all galaxies with projected distances ρ ≤ 600 h−175 kpc from the QSO sight lines have associated Lyα absorbers within 1000 km s-1, and the majority of these galaxies have absorbers within 350 km s-1. We also find that the Lyα equivalent width is anticorrelated with the projected distance of the nearest galaxy out to at least ρ ≈ 600 h−175 kpc. For ρ > 600h−175 kpc, we find galaxies that do not have associated Lyα lines, but nevertheless the anticorrelation persists if we select galaxies with ρ ≲ 2 h−175 Mpc that are within 500 or 1000 km s-1 of a Lyα absorber. This anticorrelation has a high significance but should be interpreted cautiously because there are potential selection biases that could lead to an artificial correlation. Statistical tests also show that the Lyα absorbers are not randomly distributed with respect to the galaxies. Splitting the sample into roughly equal sets with Wr > 100 mÅ and Wr < 100 mÅ shows that the weakest absorbers are not randomly distributed either. Comparison of the nearest neighbor distances of the weaker and stronger absorbers suggests that the weakest absorbers are less closely associated with galaxies, but the difference is not yet statistically significant. We find several galaxy groups that do not have clearly associated Lyα absorbers. However, given the projected distance of the nearest galaxy, we do not necessarily expect to find detectable Lyα lines in these groups based on the equivalent width-projected distance anticorrelation. Furthermore, we find several counterexamples of comparable galaxy groups that do have associated Lyα lines. As in previous studies, we find some Lyα absorbers in regions apparently devoid of galaxies, although this may be caused by the limited spatial extent and/or limited depth of the redshift survey. The equivalent width distributions of the absorbers apparently in voids and nonvoid absorbers are statistically indistinguishable, but the sample is small. We discuss the nature of the Lyα absorbers in light of the new data. The observations are consistent with the hypothesis that many of the low-redshift Lyα absorption lines with rest equivalent widths in the range from 50 to ~500 mÅ trace the overall gas distributions in the large-scale structures of galaxies rather than the gaseous halos of individual galaxies. Other phenomena may also cause Lyα absorption lines.