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.
