The association between gas and galaxies - III. The cross-correlation of galaxies and Lyα absorbers at z ∼ 1

We have measured the two-dimensional two-point correlation function, xi AG, between low-column-density Ly alpha absorbers and galaxies at a redshift z similar to 1. We measured Ly alpha absorbers between redshifts z = 0.68 and 1.51 over a total redshift path length of Delta z = 1.08 from Hubble Space Telescope Space Telescope Imaging Spectrograph 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(10) [N(HI)(cm(-2))] <= 17.4, with a median column density of log(10) [N(HI)(cm(-2))] = 14.0. A total of 193 galaxy redshifts within the surrounding 6.8 x 5.7 arcmin(2) field of view of both quasars were identified in a R-magnitude-limited survey (21.5 <= R(Vega) <= 24.5) using the Focal Reducer and Low Dispersion Spectrograph (FORS2) at the Very Large Telescope, of which 95 were higher than the minimum redshift of 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(70)(-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 autocorrelation 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.