The Clustering of MgII Absorption Systems at z=0.5 and Detection of Cold Gas in Massive Halos

We measure the large-scale clustering of MgII \lambda\lambda 2796,2803 absorbers with respect to a population of luminous red galaxies (LRGs) at z \sim 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 \approx 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 \approx 10%. After accounting for these two main uncertainties, we find a 1-\sigma anti-correlation between mean halo bias and absorber strength that translates into a 1-\sigma anti-correlation between mean galaxy mass and W_r(2796). The results indicate that a significant fraction of the MgII absorber population of W_r(2796)=1-1.5 \AA are found in group-size dark matter halos of log M_h < 13.4, whereas absorbers of W_r(2796)>1.5 \AA are seen in halos of log M_h <12.7. A larger dataset would improve the precision of the clustering measurements and the relationship between W_r and halo mass. Finally, the strong clustering of MgII absorbers down to \sim 0.3 h^{-1} Mpc indicates the presence of cool gas inside the virial radii of the halos hosting the LRGs.