A complete census of circumgalactic Mg II at redshift <i>z</i> ≲ 0.5

This paper presents a survey of Mgii absorbing gas in the vicinity of 380 random galaxies, using 156 background quasi-stellar objects (QSOs) as absorption-line probes. The sample comprises 211 isolated (73 quiescent and 138 star-forming galaxies) and 43 non-isolated galaxies with sensitive constraints for both Mgii absorption and H alpha emission. The projected distances span a range from d = 9 to 497 kpc, redshifts of the galaxies range from z = 0.10 to 0.48, and rest-frame absolute B-band magnitudes range from M-B = -16.7 to -22.8. Our analysis shows that the rest-frame equivalent width of Mgii, W-r(2796), depends on halo radius (R-h), B-band luminosity(L-B), and stellar mass (M-star) of the host galaxies, and declines steeply with increasing d for isolated, star-forming galaxies. At the same time, W-r(2796) exhibits no clear trend for either isolated, quiescent galaxies or non-isolated galaxies. In addition, the covering fraction of Mgii absorbing gas kappa is high with kappa greater than or similar to 60 percent at <40 kpc for isolated galaxies and declines rapidly to kappa approximate to 0 at d greater than or similar to 100 kpc. Within the gaseous radius, the incidence of Mgii gas depends sensitively on both M-star and the specific star formation rate inferred from H alpha. Different from what is known for massive quiescent haloes, the observed velocity dispersion of Mgii absorbing gas around star-forming galaxies is consistent with expectations from virial motion, which constrains individual clump mass to and cool gas accretion rate of . Finally, we find no strong azimuthal dependence of Mgii absorption for either star-forming or quiescent galaxies. Our results demonstrate that multiple parameters affect the properties of gaseous haloes around galaxies and highlight the need of a homogeneous, absorption-blind sample for establishing a holistic description of chemically enriched gas in the circumgalactic space.