The circumgalactic medium (CGM) of galaxies consists of a multiphase gas with components at very different temperatures, from 10 4 K to 10 7 K. One of the greatest puzzle about this medium is the presence of a large amount of low-temperature (T ∼ 10 4 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 10 5 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.

The characterization of the large amount of gas residing in the galaxy halos, the so called circumgalactic medium (CGM), is crucial to understand galaxy evolution across cosmic time. We focus here on the the cool (T ∼ 104 K) phase of this medium around star-forming galaxies in the local universe, whose properties and dynamics are poorly understood. We developed semi-analytical parametric models to describe the cool CGM as an outflow of gas clouds from the central galaxy, as a result of supernova explosions in the disc (galactic wind). The cloud motion is driven by the galaxy gravitational pull and by the interactions with the hot (T ∼ 106 K) coronal gas. Through a bayesian analysis, we compare the predictions of our models with the data of the COS-Halos and COS-GASS surveys, which provide accurate kinematic information of the cool CGM around more than 40 low-redshift star-forming galaxies, probing distances up to the galaxy virial radii. Our findings clearly show that a supernova-driven outflow model is not suitable to describe the dynamics of the cool circumgalactic gas. Indeed, to reproduce the data, we need extreme scenarios, with initial outflow velocities and mass loading factors that would lead to unphysically high energy coupling from the supernovae to the gas and with supernova efficiencies largely exceeding unity. This strongly suggests that, since the outflows cannot reproduce most of the cool gas absorbers, the latter are likely the result of cosmological inflow in the outer galaxy halos, in analogy to what we have previously found for early-type galaxies.

As the closest L* galaxy to our own Milky Way, the Andromeda galaxy (M31) is an ideal laboratory for studies of galaxy evolution. The AMIGA project has recently provided observations of the cool (T ∼ 104 K) phase of the circumgalactic medium (CGM) of M31, using HST/COS absorption spectra along ∼40 background QSO sightlines, located up to and beyond the galaxy virial radius. Based on these data, and by the means of semi-analytic models and Bayesian inference, we provide here a physical description of the origin and dynamics of the cool CGM of M31. We investigate two competing scenarios, in which (i) the cool gas is mostly produced by supernova(SN)-driven galactic outflows or (ii) it mostly originates from infall of gas from the intergalactic medium. In both cases, we take into account the effect of gravity and hydrodynamical interactions with a hot corona, which has a cosmologically motivated angular momentum. We compare the outputs of our models to the observed covering factor, silicon column density and velocity distribution of the AMIGA absorbers. We find that, to explain the observations, the outflow scenario requires an unphysically large (> 100 per cent) efficiency for SN feedback. Our infall models, on the other hand, can consistently account for the AMIGA observations and the predicted accretion rate, angular momentum and metallicity are consistent with a cosmological infall from the intergalactic medium.

We use spatially-resolved spectroscopy of a distant giant gravitational arc to test orientation effects on Mg ii absorption equivalent width (EW) and covering fraction (〈κ〉) in the circumgalactic medium of a foreground star-forming galaxy (G1) at z ∼ 0.77. Forty-two spatially-binned arc positions uniformly sample impact parameters (D) to G1 between 10 and 30 kpc and azimuthal angles α between 30○ and 90○ (minor axis). We find an EW-D anti-correlation, akin to that observed statistically in quasar absorber studies, and an apparent correlation of both EW and 〈κ〉 with α, revealing a non-isotropic gas distribution. In line with our previous results on Mg ii kinematics suggesting the presence of outflows in G1, at minimum a simple 3-D static double-cone model (to represent the trace of bipolar outflows) is required to recreate the EW spatial distribution. The D and α values probed by the arc cannot confirm the presence of a disc, but the data highly disfavor a disc alone. Our results support the interpretation that the EW-α correlation observed statistically using other extant probes is partly shaped by bipolar metal-rich winds.

How galaxies acquire material from the circumgalactic medium is a key question in galaxy evolution. Recent observations and simulations show that gas recycling could be an important avenue for star formation. This paper presents Keck Cosmic Web Imager integral field unit spectroscopic observations on a type II quasar, Q1517 + 0055 at z = 2.65, a pilot study of our Lyα nebulae sample at z ≈ 2. We revealed diffuse emission of the Lyα 1216, He ii 1640, and C iv 1549 on the projected physical scale of 122, 45, and 79 kpc, respectively. The total Lyα luminosity is L Lyα = 3.04 ± 0.02 × 1044 erg s−1. The line ratio diagnostics shows that He II/Lyα ≈ 0.08, and C IV/Lyα ≈ 0.28, consistent with the photoionization including recombination and photon pumping. We also identify the associated H i and C iv absorption from the spectra. By fitting the spectra, we derive both the column density and the velocity. We find that the velocity profile from both the absorption and the He ii emission exhibit increasing trends. Moreover, both the line ratio diagnostic from the emission and the column density ratio from the absorption confirm that the cool gas metallicity is ≥Z ⊙. From detailed modeling and estimation, gas recycling might be a more plausible interpretation compared with the scenario of a powerful outflow.