Cosmic dust in MgII absorbers

TL;DR

This paper measures the cosmic dust and neutral hydrogen content in MgII absorbers, suggesting their outflow origin and estimating their impact on galaxy evolution over cosmic timescales.

Contribution

It provides the first estimates of dust and gas densities in MgII absorbers and links their properties to galaxy outflows and evolution.

Findings

Dust density in MgII absorbers is comparable to galactic disks.

Neutral hydrogen in MgII clouds is about 5% of stellar hydrogen.

MgII absorber presence lasts for a few billion years.

Abstract

MgII absorbers induce reddening on background quasars. We measure this effect and infer the cosmic density of dust residing in these systems to be \Omega\ ~ 2e-6, in units of the critical density of the Universe, which is comparable to the amount of dust found in galactic disks or about half the amount inferred to exist outside galaxies. We also estimate the neutral hydrogen abundance in MgII clouds to be \Omega\ ~ 1.5e-4, which is approximately 5% of hydrogen in stars in galaxies. This implies a dust-to-gas mass ratio for MgII clouds of about 1/100, which is similar to the value for normal galaxies. This would support the hypothesis of the outflow origin of MgII clouds, which are intrinsically devoid of stars and hence have no sources of dust. Considerations of the dust abundance imply that the presence of MgII absorbers around galaxies lasts effectively for a few Gyr. High redshift absorbers allow us to measure the rest-frame extinction curve to 900 Angstroms at which the absorption by the Lyman edge dominates over scattering by dust in the extinction opacity.