The COS CGM Compendium. III. Metallicity and Physical Properties of the Cool Circumgalactic Medium at z ≲ 1

Abstract

We characterize the metallicities and physical properties of cool, photoionized gas in a sample of 152 z  ≲ 1 strong Ly α forest systems (SLFSs, absorbers with 15 < log  N H i < 16.2). The sample is drawn from our Cosmic Origins Spectrograph (COS) circumgalactic medium compendium (CCC), an ultraviolet survey of H i -selected circumgalactic gas around z  ≲ 1 galaxies that targets 261 absorbers with 15 < log  N H i  < 19. We show that the metallicity probability distribution function of the SLFSs at z  ≲ 1 is unimodal, skewed to low metallicities with a mean and median of [X/H] = −1.47 and −1.18 dex. Very metal-poor gas with [X/H] < −1.4 represents about half of the population of absorbers with 15  < log  N H i  < 19 absorbers within the Evolution and Assembly of GaLaxies and their Environments (EAGLE) high-resolution simulations.