The association between gas and galaxies - II. The two-point correlation function

Abstract

We measure the two-point correlation function, TAG, between galaxies and quasar absorption-line systems at z 10 17 cm -2 . For C IV absorbers, the peak strength of ξ AG is roughly comparable to that of Hi absorbers with N HI > 10 16.5 cm -2 , consistent with the finding that the Civ absorbers are associated with strong Hi absorbers. We do not reproduce the differences reported by Chen et al. between ID TAG measurements using galaxy subsamples of different spectral types. However, the full impact on the measurements of systematic differences in our samples is hard to quantify. We compare the observations with smoothed particle hydrodynamical (SPH) simulations and discover that in the observations TAG is more concentrated to the smallest separations than in the simulations. The latter also display a 'finger of god' elongation of TAG along the LOS in redshift space, which is absent from our data, but similar to that found by Ryan-Weber for the cross-correlation of quasar absorbers and H I-emission-selected galaxies. The physical origin of these 'fingers of god' is unclear, and we thus highlight several possible areas for further investigation.