The Mg II Cross-section of Luminous Red Galaxies

We describe a search for Mg II λλ2796, 2803 absorption lines in Sloan Digital Sky Survey (SDSS) spectra of QSOs whose lines of sight pass within impact parameters ρ ~ 200 kpc of galaxies with photometric redshifts of z = 0.46-0.6 and errors Δz ~ 0.05. The galaxies selected have the same colors and luminosities as the Luminous Red Galaxy (LRG) population previously selected from the SDSS. A search for Mg II lines within a redshift interval of ±0.1 of a galaxy's photometric redshift shows that absorption by these galaxies is rare: the covering fraction is f(ρ) ~= 10%-15% between ρ = 20 kpcandρ = 100 kpc, for Mg II lines with rest equivalent widths of W_r >= 0.6 Å, falling to zero at larger ρ. There is no evidence that W_r correlates with impact parameter or galaxy luminosity. Our results are consistent with existing scenarios in which cool Mg II-absorbing clouds may be absent near LRGs because of the environment of the galaxies: if LRGs reside in high-mass groups and clusters, either their halos are too hot to retain or accrete cool gas, or the galaxies themselves—which have passively evolving old stellar populations—do not produce the rates of star formation and outflows of gas necessary to fill their halos with Mg II-absorbing clouds. In the rarer cases where Mg II is detected, however, the origin of the absorption is less clear. Absorption may arise from the little cool gas able to reach into cluster halos from the intergalactic medium, or from the few star-forming and/or AGN-like LRGs that are known to exist.