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.