We measure the large-scale clustering of Mg II λλ 2796,2803 absorbers with respect to a population of luminous red galaxies (LRGs) at z ~ 0.5. From the cross-correlation measurements between Mg II absorbers and LRGs, we calculate the mean bias of the dark matter halos in which the absorbers reside. We investigate possible 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 flux-limited LRG sample and a volume-limited one. The comparison shows that the relative halo bias of Mg II 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 1σ anticorrelation between mean halo bias and absorber strength Wr (2796) that translates into a 1σ anticorrelation between mean galaxy mass and Wr (2796). The results indicate that a significant fraction of the Mg II 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 primarily seen in halos of log Mh 12.7. A larger data set would improve the precision of both the clustering measurements and the relationship between equivalent width and halo mass. Finally, the strong clustering of Mg II absorbers down to scales of ~0.3 h –1 Mpc indicates the presence of cool gas inside the virial radii of the dark matter halos hosting the LRGs.
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