Cross correlation of Lyman-α absorbers with gas-rich galaxies

The H I Parkes All Sky Survey (HIPASS) galaxy catalogue is cross-correlated with known low redshift, low column density (NHI < 10 cm) Lyman-α absorbers from the literature. The redshift-space correlation is found to be similar in strength to HIPASS galaxy self-clustering (correlation length s0,ag = 6 ± 4 and s0,gg = 3.1 ± 0.5 h −1 100 Mpc respectively). In realspace the cross-correlation is stronger than the galaxy auto-correlation (correlation length r0,ag = 7.2 ± 1.4 and r0,gg = 3.5 ± 0.7 h −1 100 Mpc respectively) on scales from 1 − 10 h 100 Mpc, ruling out the mini-halo model for the confinement Lyα absorbers at the 99 percent confidence level. Provided that the cause of the strong cross-correlation is purely gravitational, the ratio of correlation lengths suggest that absorbers are embedded in dark matter haloes with masses log(M /M⊙) = 14.2 h −1 100 , similar to those of galaxy groups. The flattening of the cross-correlation at separations less than ∼ 600 h 100 kpc could correspond to the thickness of filaments in which absorbers are embedded. This work provides indirect statistical evidence for the notion that galaxy groups and large-scale filaments, particularly those that comprise gas-rich galaxies, are the dominant environments of low column density Lyα absorbers at z = 0.