Halo Gas Cross Sections And Covering Fractions of MgII Absorption Selected Galaxies

TL;DR

This study investigates the size, covering fraction, and distribution of MgII absorbing gas halos around galaxies, revealing they are patchy, possibly non-symmetric, and influenced by local stochastic processes rather than solely galaxy luminosity.

Contribution

It provides new estimates of halo radii and covering fractions, demonstrating that MgII halos are patchy and not strictly scaled by galaxy luminosity, challenging previous assumptions.

Findings

Halo radii for MgII absorbers are 43-88 kpc depending on luminosity cutoff.

Average covering fraction of MgII halos is approximately 0.5.

Halo gas distribution is patchy and influenced by local stochastic processes.

Abstract

We examine halo gas cross sections and covering fractions, f_c, of intermediate redshift MgII absorption selected galaxies. We computed statistical absorber halo radii, R_x, using current values of dN/dz and Schechter luminosity function parameters, and have compared these values to the distribution of impact parameters and luminosities from a sample of 37 galaxies. For equivalent widths W_r(2796) > 0.3 Ang, we find 43 < R_x < 88 kpc, depending on the lower luminosity cutoff and the slope, beta, of the Holmberg-like luminosity scaling, R \propto L^beta. The observed distribution of impact parameters, D, are such that several absorbing galaxies lie at D > R_x and several non-absorbing galaxies lie at D < R_x. We deduced f_c must be less than unity and obtain a mean of <f_c> ~ 0.5 for our sample. Moreover, the data suggest halo radii of MgII absorbing galaxies do not follow a luminosity scaling with beta in the range of 0.2-0.28, if f_c= 1 as previously reported. However, provided f_c~0.5, we find that halo radii can remain consistent with a Holmberg-like luminosity relation with beta ~ 0.2 and R* = R_x/sqrt(f_c)= 110 kpc. No luminosity scaling (beta=0) is also consistent with the observed distribution of impact parameters if f_c < 0.37. The data support a scenario in which gaseous halos are patchy and likely have non-symmetric geometric distributions about the galaxies. We suggest halo gas distributions may not be govern primarily by galaxy mass/luminosity but also by stochastic processes local to the galaxy.