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 (N-H < 10(15) cm(-2)) Lyman alpha (Ly alpha) absorbers from the literature. The redshift-space correlation is found to be similar in strength to HIPASS galaxy self-clustering (correlation length s(0,ag)= 6 +/- 4 h(100)(-1) Mpc and s(0,gg)= 3.1 +/- 0.5 h(100)(-1) Mpc, respectively). In real space the cross-correlation is stronger than the galaxy auto-correlation (correlation length r(0,ag)= 7.2 +/- 1.4 h(100)(-1) Mpc and r(0,gg)= 3.5 +/- 0.7 h(100)(-1) Mpc, respectively) on scales from 1 to 10 h(100)(-1) Mpc, ruling out the minihalo model for the confinement Ly alpha absorbers at the 99 per cent 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-circle dot) = 14.2 h(100)(-1), similar to those of galaxy groups. The flattening of the cross-correlation at separations less than similar to 600 h(100)(-1) 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 Ly alpha absorbers at z = 0.