OBSERVATIONAL CONSTRAINTS ON THE NATURE OF LOW-REDSHIFT LY-ALPHA ABSORBERS

We present results from a spectroscopic and imaging survey of galaxies in the fields of quasars from the Hubble Space Telescope (HST) Quasar Absorption Line Key Project. The aim of this survey is to identify ga laxies within 3.5' from the quasar sightline, to a limiting, integrate d gamma-band magnitude m(r) = 22.5. The data are then compared to the HST homogeneous sample of Ly alpha-only absorbers in order to put cons traints on the nature of these absorbers, and in particular on their r elation to galaxies. We have obtained spectra for 81 objects in three quasar fields and identified 66 galaxies (success rate of 81%) at reds hifts in the range z = 0.0500-0.7974, and at linear impact parameters to the quasar sightlines spanning D=57-2380 h(50)(-1) kpc. Among these galaxies, 19 are at less than 750 km s(-1) from a Ly alpha absorber, and only one clearly does not give rise to any absorption. Three other galaxies are at the redshifts of metal-rich absorption systems, of wh ich one belongs to a cluster with altogether 19 identified galaxies at the quasar redshift. The analysis of our sample combined with those o f previous studies shows that:1) the redshift agreements of the Ly alp ha absorber-galaxy associations cannot be due to chance coincidence; 2 ) there is no clear anti-correlation between the Ly alpha rest-frame e quivalent width and the impact parameter for the whole sample (w(r),mi n=0.10 Angstrom). When only the strongest lines are considered (w(r) g reater than or equal to 0.24 Angstrom), w(r) (Ly alpha) and D are marg inally anti-correlated. Lanzetta et al. (1995) found a stronger anti-c orrelation which could be due, at least in part, to the existence of m etal-rich absorbers in their sample. Our results suggest that most Ly alpha absorbers are not gaseous clouds that belong in a strict sense t o galaxies, as is the case for Mg rr absorbers. The size of Ly alpha g alactic halos can be inferred from the variation with D of the fractio n of associations to the total number of galaxies at impact parameters < D. This fraction drops from 1 to similar to 0.65 at D similar to 20 0h(50)(-1) kpc and flattens at larger values of D (greater than or sim ilar to 300h(50)(-1) kpc). This leads to Ly alpha galactic halos sizes about three times larger than the inner Mg It halo region; 3) there i s no correlation between the galaxy luminosity and the impact paramete r. This again suggests that significant Ly alpha clouds do not belong to individual galaxies, but instead are distributed in the local large -scale structure. For the smaller impact parameters, this could reflec t a link between D and the total galaxy mass rather than its luminous mass; 4) the HWHM=120 km s(-1) of the relative velocity distribution o f the Ly alpha absorber-galaxy associations is consistent with either galaxy rotation velocities or the local velocity dispersion in large-s cale structures.