Cool circumgalactic gas of passive galaxies from cosmological inflow

¹ Kapteyn Astronomical Institute, University of Groningen, Landleven 12, 9747 AD Groningen, The Netherlands
e-mail: afruni@astro.rug.nl
² Department of Physics, ETH Zürich, Wolfgang-Pauli-Strasse 27, 8093 Zürich, Switzerland

Received: 31 December 2018
Accepted: 13 March 2019

Abstract

The circumgalactic medium (CGM) of galaxies consists of a multi-phase gas with components at very different temperatures, from 10⁴ K to 10⁷ K. One of the greatest puzzles about this medium is the presence of a large amount of low-temperature (T ∼ 10⁴ K) gas around quiescent early-type galaxies (ETGs). Using semi-analytical parametric models, we describe the cool CGM around massive, low-redshift ETGs as the cosmological accretion of gas into their dark matter halos, resulting in an inflow of clouds from the external parts of the halos to the central galaxies. We compare our predictions with the observations of the COS-LRG collaboration. We find that inflow models can successfully reproduce the observed kinematics, the number of absorbers and the column densities of the cool gas. Our MCMC fit returns masses of the cool clouds of about 10⁵ M_⊙ and shows that they must evaporate during their journey due to hydrodynamic interactions with the hot gas. We conclude that the cool gas present in the halos of ETGs likely cannot reach the central regions and feed the galaxy star formation, thus explaining why these passive objects are no longer forming stars.