An HST/COS legacy survey of intervening SiIII absorption in the extended gaseous halos of low-redshift galaxies

TL;DR

This study uses HST/COS UV spectra to analyze SiIII absorption in the halos of low-redshift galaxies, revealing their distribution, association with galaxies, and the baryonic mass contained in the circumgalactic medium.

Contribution

It provides the first extensive survey of SiIII absorbers at low redshift, linking them to galaxy halos and modeling their absorption cross-section.

Findings

SiIII absorbers are highly correlated with galaxy positions within 200 kpc.

The covering fraction of SiIII in galaxy halos is approximately 70%.

Diffuse gas traced by SiIII contains more baryonic mass than the neutral interstellar medium.

Abstract

Doubly ionized silicon (SiIII) is a powerful tracer of diffuse ionized gas inside and outside of galaxies. It can be observed in the local Universe in ultraviolet (UV) absorption against bright extragalactic background sources. We here present an extensive study of intervening SiIII-selected absorbers and their relation to the circumgalactic medium (CGM) of galaxies at low redshift (z<=0.1), based on the analysis of UV absorption spectra along 303 extragalactic lines of sight obtained with the Cosmic Origins Spectrograph (COS) on board the Hubble Space Telescope (HST). Along a total redshift path of Dz=24 we identify 69 intervening SiIII systems that all show associated absorption from other low and high ions. We derive a bias-corrected number density of dN/dz(SiIII)=2.5 for absorbers with column densities log N(SiIII)>12.2. We develop a geometrical model for the absorption-cross section of the CGM around the local galaxy population and find excellent agreement between the model predictions and the observations. We further compare redshifts and positions of the absorbers with that of ~64,000 galaxies using archival galaxy-survey data. For the majority of the absorbers we identify possible host galaxies within 300 km/s of the absorbers and derive impact parameters rho<200 kpc, demonstrating that the spatial distributions of SiIII absorbers and galaxies are highly correlated. Our study indicates that the majority of SiIII-selected absorbers in our sample trace the CGM of nearby galaxies within their virial radii at a typical covering fraction of ~70 per cent. From a detailed ionization model we estimate that diffuse gas in the CGM around galaxies, as traced by SiIII, contains substantially more baryonic mass than their neutral interstellar medium.