We measure the large-scale clustering of Mg II \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 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 \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 W _ { r } ( 2796 ) 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 Mg II absorber population of W _ { r } ( 2796 ) = 1 - 1.5 Å are found in group-size dark matter halos of \log M _ { h } < 13.4 , whereas absorbers of W _ { r } ( 2796 ) > 1.5 Å are primarily seen in halos of \log M _ { h } < 12.7 .
A larger dataset 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 \sim 0.3 h ^ { -1 } Mpc indicates the presence of cool gas inside the virial radii of the dark matter halos hosting the LRGs .