COS-Weak: probing the CGM using analogs 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 > 3 sigma detections of the Si II lambda 1260 and CII lambda 1334 absorption lines, with W-r (Si II) < 0.2 angstrom and W-r (C II) < 0.3 angstrom, 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 < log n(H)/cm(-3) < -2.4 and similar to 1 pc-50 kpc (median approximate to 500 pc), respectively. Most importantly, 85 per cent (50 per cent) of these absorbers show a metallicity of [Si/H] > -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 approximate to 80 per cent 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 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.