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

We measure the two-point correlation function, ξ_AG, between galaxies and quasar absorption-line systems at z < 1, using the data set of Morris & Jannuzi on 16 lines-of-sight (LOS) with ultraviolet (UV) spectroscopy and galaxy multi-object spectroscopy (Paper I). The measurements are made in 2D redshift space out to π = 20h^-1 Mpc (comoving) along the LOS and out to σ = 2h^-1 Mpc projected; as a function of HI column density in the range N_HI = 10¹³-10¹⁹cm^-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 ξ_AG at the smallest separations increases slowly as the lower limit on N_HI is increased from 10¹³ to 10¹⁶cm^-2, and then jumps sharply (albeit with substantial uncertainties) for N_HI > 10¹⁷cm^-2. For CIV absorbers, the peak strength of ξ_AG is roughly comparable to that of HI absorbers with N_HI > 10^16.5cm^-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 1D ξ_AG 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 ξ_AG is more concentrated to the smallest separations than in the simulations. The latter also display a `finger of god' elongation of ξ<sub>AG</sub> 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 HI-emission-selected galaxies. The physical origin of these `fingers of god' is unclear, and we thus highlight several possible areas for further investigation.