The Baryon Content of Dark Matter Halos: Empirical Constraints from MgII Absorbers

TL;DR

This study empirically constrains the extent and distribution of MgII absorbing gas around galaxies, revealing a scaling relation with galaxy luminosity and a boundary beyond which no absorption occurs, informing baryon content in halos.

Contribution

It provides the first empirical scaling relation for MgII gas extent around galaxies and demonstrates a boundary radius linked to galaxy luminosity and dark matter halos.

Findings

MgII absorber strength decreases with impact parameter.

A clear boundary exists at R_gas beyond which no MgII absorption is detected.

Gas radius scales with galaxy luminosity as R_gas = 91 x (L_B/L_{B_*})^{0.35} h^{-1} kpc.

Abstract

We have initiated a survey of MgII 2796, 2803 absorbers near known galaxies at projected distances rho<~100 h^-1 kpc to a background QSO, in order to constrain the extent and covering fraction of Mg^+ ions around galaxies of different luminosity and mass. The current sample consists of 13 galaxy and absorber pairs and 10 galaxies that do not produce MgII absorption lines to within sensitive upper limits. The redshifts of the galaxy and absorber pairs range from z = 0.2067 to 0.892 with a median of z = 0.3818. We find that galaxies at larger impact parameters produce on average weaker MgII absorbers. This anti-correlation is substantially improved when accounting for the intrinsic luminosities of individual galaxies. In addition, there exists a distinct boundary at rho=R_gas, beyond which no MgII absorbers are found. A maximum likelihood analysis shows that the observations are best described by an isothermal density profile and a scaling relation R_gas = 91 x (L_B/L_{B_*})^(0.35 +/- 0.05) h^-1 kpc (or 69 h^-1 kpc at W(2796)=0.3 Ang) with a mean covering factor of <kappa>=80-86%. Together with the scaling relation between halo mass and galaxy luminosity inferred from halo occupation studies, this scaling of R_gas indicates that gas radius is a fixed fraction of the dark matter halo radius. We compare our results with previous studies and discuss the implications of our analysis for constraining the baryon content of galactic halos and for discriminating between competing scenarios for understanding the nature of the extended gas.