The Clustering of MgII Absorption Systems at z=0.5 and Detection of Cold Gas in Massive Halos
Abstract
We measure the large-scale clustering of MgII λλ 2796,2803 absorbers with respect to a population of luminous red galaxies (LRGs) at z 0.5. From the cross-correlation measurements between MgII absorbers and LRGs, we calculate the mean bias of the dark matter halos in which the absorbers reside. We investigate systematic uncertainties in the clustering measurements due to the sample selection of LRGs and due to uncertainties in photometric redshifts. First, we compare the cross-correlation amplitudes determined using a it flux-limited LRG sample and a volume-limited one. The comparison shows that the relative halo bias of MgII absorbers using a flux-limited LRG sample can be overestimated by as much as ≈ 20%. Next, we assess the systematic uncertainty due to photometric redshift errors using a mock galaxy catalog with added redshift uncertainties comparable to the data. We show that the relative clustering amplitude measured without accounting for photometric redshift uncertainties is overestimated by ≈ 10%. After accounting for these two main uncertainties, we find a 1-σ anti-correlation between mean halo bias and absorber strength that translates into a 1-σ anti-correlation between mean galaxy mass and Wr(2796). The results indicate that a significant fraction of the MgII absorber population of Wr(2796)=1-1.5 are found in group-size dark matter halos of log Mh < 13.4, whereas absorbers of Wr(2796)>1.5 are seen in halos of log Mh <12.7. A larger dataset would improve the precision of the clustering measurements and the relationship between Wr and halo mass. Finally, the strong clustering of MgII absorbers down to 0.3 h-1 Mpc indicates the presence of cool gas inside the virial radii of the halos hosting the LRGs.