Characterizing the chemically enriched circumgalactic medium of ~38 000 luminous red galaxies in SDSS DR12

We report a definitive detection of chemically enriched cool gas around massive quiescent galaxies at z ≈ 0.4-0.7. The result is based on a survey of 37 621 luminous red galaxy (LRG)- quasi-stellar object pairs in SDSS DR12 with projected distance d < 500 kpc. The LRGs are characterized by a predominantly old stellar population (age ≳ 1 Gyr) with 13 per cent displaying [O II] emission features and LINER-like spectra. Both passive and [O II]-emitting LRGs share the same stellar mass distribution with a mean of 〈log (M*/M_⊙)〉 ≈ 11.4 and a dispersion of 0.2 dex. Both LRG populations exhibit associated strong MgII absorbers out to d < 500 kpc. The mean gas covering fraction at d ≲ 120 kpc is 〈κ〉_MgII > 15 per cent and declines quickly to 〈κ〉_MgII ≈ 5 per cent at d ≲ 500 kpc. No clear dependence on stellar mass is detected for the observed Mg II absorption properties. The observed velocity dispersion of MgII-absorbing gas relative to either passive or [O II]-emitting LRGs is merely 60 per cent of what is expected from virial motion in these massive haloes. While no apparent azimuthal dependence is seen for 〈κ〉_MgII around passive LRGs at all radii, a modest enhancement in 〈κ〉_MgII is detected along the major axis of [O II]-emitting LRGs at d < 50 kpc. The suppressed velocity dispersion of MgII-absorbing gas around both passive and [O II]-emitting LRGs, together with an elevated 〈κ〉_MgII along the major axis of [O II]-emitting LRGs at d < 50 kpc, provides important insights into the origin of the observed chemically enriched cool gas in LRG haloes. We consider different scenarios and conclude that the observed Mg II absorbers around LRGs are best explained by a combination of cool clouds formed in thermally unstable LRG haloes and satellite accretion through filaments.