The association between gas and galaxies II: The 2-point correlation function

We measure the 2-point correlation function, ξAG, between galaxies and quasar absorption line systems at z < 1, using the dataset of Morris & Jannuzi (2006) on 16 lines-of-sight (LOS) with UV spectroscopy and galaxy multi-object spectroscopy (paper I). The measurements are made in 2D redshift space out to π = 20h Mpc (comoving) along the LOS and out to σ = 2h Mpc projected; as a function of HI column density in the range NHI = 10 − 10 19 cm −2 , also for CIV absorption systems, and as a function of galaxy spectral type. This extends the absorber-galaxy pair analysis of paper I. We find that the amplitude of the peak in ξAG at the smallest separations increases slowly as the lower limit on NHI is increased from 10 to 10 cm , and then jumps sharply (albeit with substantial uncertainties) for NHI > 10 17 cm −2 . For CIV absorbers, the peak strength of ξAG is roughly comparable to that of HI absorbers with NHI > 10 cm , 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 1D ξAG measurements using galaxy sub-samples 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 ξAG is more concentrated to the smallest separations than in the simulations. The latter also display a ‘finger of god’ elongation of ξAG along the line of sight 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 HI-emission-selected galaxies. The physical origin of these ‘fingers of god’ is unclear and we thus highlight several possible areas for further investigation.