Papers › COS-Weak: Probing the CGM using analogs of weak Mg II absorbers at z < 0.3
COS-Weak: Probing the CGM using analogs of weak Mg II absorbers at z < 0.3
Sowgat Muzahid, Gloria Fonseca, Amber Roberts, Benjamin Rosenwasser, Philipp Richter, Anand Narayanan, Chris Churchill, Jane Charlton
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We present a sample of 34 weak metal line absorbers at z< 0.3 complied via the simultaneous detections (3σ) of the SiIIλ1260 and CIIλ1334 absorption lines, with Wᵣ(SiII)<0.2 \AA\ and Wᵣ(CII)<0.3 \AA, 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 using CLOUDY. The inferred densities and total hydrogen column densities are in the ranges of -3.3 < logn_H/cm⁻³ < -2.4 and 16.0 < logN_H/cm⁻²<20.3, respectively. The line of sight thicknesses of the absorbers have a wide range of ∼1 pc$-$50 kpc with a median value of $\sim$500 pc. The high-ionization OVI absorption, detected in 12/18 cases, always stems from a different gas-phase. Most importantly, 85% (50%) of these absorbers show a metallicity of [Si/H] > -1.0 (0.0). The fraction of systems showing high metallicity (i.e., [Si/H]>-1.0) in our sample is significantly higher than the HI-selected sample (Wotta et al. 2016) and the galaxy-selected sample (Prochaska et al. 2017) of absorbers probing the circum-galactic medium (CGM) at similar redshift. A search for galaxies has revealed a significant galaxy-overdensity around these weak absorbers compared to random places with a median impact parameter of 166 kpc to 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 galaxies that are in groups. We suggest that these are transient structures that are related to outflows and/or stripping of metal-rich gas from galaxies.
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