Identifying Circumgalactic Medium Absorption in QSO Spectra: A Bayesian Approach

Abstract

We present a study of candidate galaxy–absorber pairs for 43 low-redshift QSO sightlines (0.06 < z < 0.85) observed with the Hubble Space Telescope/Cosmic Origins Spectrograph that lie within the footprint of the Sloan Digital Sky Survey with a statistical approach to match absorbers with galaxies near the QSO lines of sight using only the SDSS Data Release 12 photometric data for the galaxies, including estimates of their redshifts. Our Bayesian methods combine the SDSS photometric information with measured properties of the circumgalactic medium to find the most probable galaxy match, if any, for each absorber in the line-of-sight QSO spectrum. We find ∼630 candidate galaxy–absorber pairs using two different statistics. The methods are able to reproduce pairs reported in the targeted spectroscopic studies upon which we base the statistics at a rate of 72%. The properties of the galaxies comprising the candidate pairs have median redshift, luminosity, and stellar mass, all estimated from the photometric data, z = 0.13, L = 0.1L *, and log ( M * / M ⊙ ) = 9.7 . The median impact parameter of the candidate pairs is ∼430 kpc, or ∼3.5 times the galaxy virial radius. The results are broadly consistent with the high Lyα covering fraction out to this radius found in previous studies. This method of matching absorbers and galaxies can be used to prioritize targets for spectroscopic studies, and we present specific examples of promising systems for such follow-up.

Investigating Ionization in the Intergalactic Medium

“…The sight lines to PG 1116+215, PG 1216+069, and PG 1424+240 were found to pass within 140 kpc (Tripp et al 2008;Muzahid et al 2018;Scott et al 2021), 94 kpc (Chen & Mulchaey 2009;Scott et al 2021), and 132 kpc (Scott et al 2021), respectively, of galaxies at similar redshifts as these clouds. The virial radii of these galaxies, as shown in Table 2, suggest that the sight lines to PG 1116+215 and PG 1216+06 probe the boundary between the outer CGM and IGM.…”