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

ABSTRACT

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= 20 h⁻¹  Mpc (comoving) along the LOS and out to  σ= 2 h⁻¹  Mpc projected; as a function of H i column density in the range  N_H I= 10¹³–10¹⁹ cm⁻² , also for C iv 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_H I  is increased from 10¹³ to  10¹⁶ cm⁻² , and then jumps sharply (albeit with substantial uncertainties) for  N_H I > 10¹⁷ cm⁻² . For C iv absorbers, the peak strength of  ξ_AG  is roughly comparable to that of H i absorbers with  N_H I > 10^16.5 cm⁻² , consistent with the finding that the C iv absorbers are associated with strong H i 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_AG  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.