A Complete Census of Circumgalactic MgII at Redshift z<~ 0.5

Abstract

We present 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 Ha 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 $M_{\mathrm{B}}=-16.7$ 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($L_{\mathrm{B}}$) and stellar mass ($M_{\mathrm{star}}$) of the host galaxies, and declines steeply with increasing $d$ for isolated, star-forming galaxies. $W_r$(2796) exhibits no clear trend for either isolated, quiescent galaxies or non-isolated galaxies. The covering fraction of MgII absorbing gas $⟨\kappa ⟩$ is high with $⟨\kappa ⟩\gtrsim 60$% at $<40$ kpc for isolated galaxies and declines rapidly to $⟨\kappa ⟩\approx 0$ at $d\gtrsim 100$ kpc. Within the gaseous radius, $⟨\kappa ⟩$ depends sensitively on both $M_{\mathrm{star}}$ and the specific star formation rate inferred from Ha. Different from massive quiescent halos, the observed velocity dispersion of MgII gas around star-forming galaxies is consistent with expectations from virial motion, which constrains individual clump mass to $m_{\mathrm{cl}}\gtrsim 10^5{\mathrm{M}}_{\odot }$ and cool gas accretion rate of $\sim 0.7-2M_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$. We find no strong azimuthal dependence of MgII absorption for either star-forming or quiescent galaxies. Our results highlight the need of a homogeneous, absorption-blind sample for establishing a holistic description of chemically-enriched gas in the circumgalactic space.

20 pages, 11 figures, accepted for publication in the MNRAS