COS-Weak: probing the CGM using analogues of weak Mg ii absorbers at z < 0.3

Abstract

We present a sample of 34 weak metal line absorbers at z < 0.3 selected by the simultaneous > 3σ detections of the Si λ1260 and C λ1334 absorption lines, with W r (Si ) < 0.2 Å and W r (C ) < 0.3 Å, in archival HST /COS spectra. Our sample increases the number of known low-z "weak absorbers" by a factor of > 5. The column densities of H and low-ionization metal lines obtained from Voigt profile fitting are used to build simple photoionization models. The inferred densities and line of sight thicknesses of the absorbers are in the ranges of −3.3 < log n H /cm −3 < −2.4 and ∼1 pc -50 kpc (median ≈ 500 pc), respectively. Most importantly, 85% (50%) of these absorbers show a metallicity of [Si/H] > −1.0 (0.0). The fraction of systems showing near-/super-solar metallicity in our sample is significantly higher than in the H -selected sample (Wotta et al. 2016) and the galaxy-selected sample (Prochaska et al. 2017) of absorbers probing the circum-galactic medium at similar redshift. A search for galaxies has revealed a significant galaxy-overdensity around these weak absorbers compared to random positions with a median impact parameter of 166 kpc from the nearest galaxy. Moreover, we find the presence of multiple galaxies in ≈ 80% of the cases, suggesting group environments. The observed dN/dz of 0.8 ± 0.2 indicates that such metal-enriched, compact, dense structures are ubiquitous in the halos of low-z group galaxies. We suggest that these are transient structures that are related to galactic outflows and/or stripping of metal-rich gas from galaxies.

C IV absorbers tracing cool gas in dense galaxy group/cluster environments

“…3.1) suggests log [N(H i) (cm −2 )] 17.25. In addition, the restframe equivalent width of W r (C ii 1335) = 95 ± 21 mÅ (see Figure 2) makes this a weak Mg ii class of absorber which are associated with sub-Lyman limit systems (Churchill & Charlton 1999;Rigby et al 2002;Narayanan et al 2005;Muzahid et al 2018). Considering both these, the true column density is presumably closer to, but lower than log [N(H i) (cm −2 )] ≤ 17.25.…”

Detection of metal-rich, cool-warm gas in the outskirts of galaxy clusters

“…QSO names (5) and redshifts (6) are from Monroe et al (2016). The r 500 values (4), normalized clustocentric impact parameters of the QSO sightlines (7) and absorber redshifts (8) are from Muzahid et al (2018).…”

Properties of the simulated circumgalactic medium

“…The metallicity of the gas is fixed to one-third solar, and is the same for both the accreting gas and the galactic outflows. Outflows are expected to be metal-enriched compared to mostly-pristine cosmological accretion (Muzahid et al 2015;Chisholm et al 2018;Christensen et al 2018), so this fixed metallicity is an idealized simplification, but aligns with some CGM studies that find a metallicity ∼ 0.3Z (Prochaska et al 2017;Muzahid et al 2018). The cooling rate of the gas is calculated assuming optically-thin photoionization equilibrium with a Haardt & Madau (2001) ionizing background.…”