The Chemical and Ionization Conditions in Weak Mg II Absorbers

TL;DR

This study analyzes the chemical and ionization conditions of 100 weak Mg II absorbers in quasar spectra, revealing their association with galaxy halos and intergalactic structures, and inferring their metallicity and astrophysical origins.

Contribution

It provides the first comprehensive analysis of weak Mg II absorbers' chemical and ionization states, linking them to galaxy halos and intergalactic environments with inferred metallicities.

Findings

Weak Mg II clouds often have solar or higher metallicity.

Two main populations: halo gas and Type Ia enriched intergalactic gas.

Redshift-dependent prevalence of different absorber types.

Abstract

We present an analysis of the chemical and ionization conditions in a sample of 100 weak Mg II absorbers identified in the VLT/UVES archive of quasar spectra. Using a host of low ionization lines associated with each absorber in this sample, and on the basis of ionization models, we infer that the metallicity in a significant fraction of weak Mg II clouds is constrained to values of solar or higher, if they are sub-Lyman limit systems. Based on the observed constraints, we present a physical picture in which weak Mg II absorbers are predominantly tracing two different astrophysical processes/structures. A significant population of weak Mg II clouds, those in which N(Fe II) is much less than N(Mg II), identified at both low (z ~ 1) and high (z ~ 2) redshift, are potentially tracing gas in the extended halos of galaxies, analogous to the Galactic high velocity clouds. These absorbers might correspond to alpha-enhanced interstellar gas expelled from star-forming galaxies, in correlated supernova events. On the other hand, N(FeII) approximately equal to N(Mg II) clouds, which are prevalent only at lower redshifts (z < 1.5), must be tracing Type Ia enriched gas in small, high metallicity pockets in dwarf galaxies, tidal debris, or other intergalactic structures.