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

TL;DR

This study provides a comprehensive survey of MgII absorbing gas around 380 galaxies at redshifts below 0.5, revealing how absorption properties depend on galaxy characteristics and environment, and constraining the nature of circumgalactic gas.

Contribution

It offers the first complete census of MgII absorption around a large, diverse galaxy sample at low redshift, analyzing dependencies on galaxy properties and environment.

Findings

MgII equivalent width depends on halo radius, luminosity, and stellar mass.

High MgII covering fraction within 40 kpc, declining rapidly beyond.

Velocity dispersion consistent with virial motion, constraining clump mass and accretion rates.

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 $M_{\rm B}=-16.7$ 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($L_{\rm B}$) and stellar mass ($M_{\rm star}$) 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 $\langle \kappa \rangle$ is high with $\langle \kappa \rangle\gtrsim 60$% at $<40$ kpc for isolated galaxies and declines rapidly to $\langle \kappa \rangle\approx 0$ at $d\gtrsim100$ kpc. Within the gaseous radius, $\langle \kappa \rangle$ depends sensitively on both $M_{\rm star}$ 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 $m_{\rm cl} \gtrsim 10^5 \,\rm M_\odot$ and cool gas accretion rate of $\sim 0.7-2 \,M_\odot\,\rm yr^{-1}$. 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.