Modeling the distribution of Mg II absorbers around galaxies using Background Galaxies & Quasars

TL;DR

This study combines galaxy and quasar spectra to model MgII absorption distribution around galaxies, revealing asymmetric absorption near the minor axis and different gas distributions at varying impact parameters.

Contribution

It introduces a joint analysis method and models the azimuthal dependence of MgII absorption, highlighting the geometry and origins of circumgalactic gas.

Findings

MgII absorbers are predominantly near the minor axis within 50 degrees.

Absorption strength varies with impact parameter, indicating different gas distributions.

MgII gas within 40 kpc is likely wind-driven, while farther gas may be inflowing or disk-aligned.

Abstract

We present joint constraints on the distribution of MgII absorption around galaxies, by combining the MgII absorption seen in stacked background galaxy spectra and the distribution of host galaxies of strong MgII systems from the spectra of background quasars. We present a suite of models that predict, the dependence of MgII absorption on a galaxy's apparent inclination, impact parameter(b) and azimuthal angle. The variations in the absorption strength with azimuthal angles provide much stronger constraints on the intrinsic geometry of the MgII absorption than the dependence on the galaxy's inclination. Strong MgII absorbers (W_r(2796)>0.3) are asymmetrically distributed in azimuth around their host galaxies:72% of the absorbers studied and 100% of the close-in absorbers within b<35 kpc, are located within 50deg of the host galaxy's projected minor axis. Composite models consisting either of a simple bipolar component plus a spherical or disk component, or a single highly softened bipolar distribution, can well represent the azimuthal dependencies observed in both the datasets. Simultaneously fitting both datasets to the composite model, bipolar cone is confined to 50deg of the minor axis and contains 2/3 of the total MgII absorption. The single softened cone model has an exponential fall off with azimuth with an exponential scale-length in opening angle of 45deg. We conclude that the distribution of MgII gas at low impact parameters is not the same as that found at high impact parameters. MgII absorption within 40 kpc primarily arises from cool MgII gas entrained in winds. Beyond 40 kpc, there is evidence for a more symmetric distribution, significantly different from that closer into the galaxies. Here a significant component appears aligned more with the disk and is possibly inflowing, perhaps as part of a galactic fountain or the inflow of material from further out in the system.