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  i < 10 15  cm −2 ) Lyman α (Lyα) 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 −1 100  Mpc and s 0,gg = 3.1 ± 0.5  h −1 100  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 −1 100  Mpc and r 0,gg = 3.5 ± 0.7  h −1 100  Mpc , respectively) on scales from 1 to 10  h −1 100  Mpc , ruling out the minihalo model for the confinement Lyα 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 ⊙ ) = 14.2  h −1 100 , similar to those of galaxy groups. The flattening of the cross‐correlation at separations less than ∼600  h −1 100  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 .