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 IIλ1260 and CIIλ1334 absorption lines, with Wr (Si II) < 0.2 Å and Wr(C II) < 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 HI 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 -1.0(0.0). The fraction of systems showing near-/supersolar metallicity in our sample is significantly higher than in the HI-selected sample of Wotta et al., and the galaxy-selected sample of Prochaska et al., 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 per cent of the cases, suggesting group environments. The observed dN/dz of 0.8 ± 0.2 indicates that such metalenriched, compact, dense structures are ubiquitous in the haloes 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.