The association between gas and galaxies III: The Cross-correlation of
  Galaxies and Ly-alpha Absorbers at z ~ 1
  We have measured the 2D 2-point correlation function, Xi_AG, between low
column density Ly-alpha absorbers and galaxies at a redshift z ~ 1. We measured
Ly-alpha absorbers between redshifts z=0.68 to 1.51 over a total redshift path
length of Delta z=1.08 from HST STIS E230M absorption spectra towards the
quasars HE 1122-1648 (z=2.4) and PKS 1127-145 (z=1.187). The column density of
the Ly-alpha absorbers ranged from 13.2< log(N_HI (cm^-2))<17.4, with a median
column density of log(N_HI)=14.0 . A total of 193 galaxy redshifts within the
surrounding 6.8' x 5.7' field of view of both quasars were identified in a R
magnitude limited survey (21.5<R_Vega<24.5) using the FORS2 spectrograph at the
VLT, of which 95 were higher than the minimum redshift z=0.68 to be used in the
correlation function. A 3 sigma upper-limit of Xi_AG=2.8 was found when 145
Ly-alpha absorber-galaxy pairs were binned in redshift space, in a bin of size
Delta sigma=1.0, Delta pi=2.0 h^-1 Mpc along the projected separation and line
of sight distances respectively. The upper-limit in the cross-correlation was
found to be 5.4 sigma lower than the central peak in the galaxy
auto-correlation within the same redshift range, Xi_GG, which was in our data
equal to 10.7+/-1.4. Thus we have shown for the first time that the clustering
between low column density absorbers and galaxies at a redshift of 1 is weaker
than that between galaxies at the same redshift.
