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 $z<0.3$ complied via the simultaneous detections ($3\sigma$) of the SiII$\lambda 1260$ and CII$\lambda 1334$ absorption lines, with $W_r$(SiII)$<0.2$ Å and $W_r$(CII)$<0.3$ 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 using CLOUDY. The inferred densities and total hydrogen column densities are in the ranges of $-3.3<\log n_{\mathrm{H}}/\mathrm{c}{\mathrm{m}}^{-3}<-2.4$ and $16.0<\log N_{\mathrm{H}}/\mathrm{c}{\mathrm{m}}^{-2}<20.3$, respectively. The line of sight thicknesses of the absorbers have a wide range of $\sim$1 pc$-$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 $\sim 80$% of the cases, suggesting group environments. The observed $d\mathcal{N}/dz$ of $0.8\pm 0.2$ 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.

arXiv commentPublished (2018MNRAS.476.4965M) after minor revision. Appendix A is newly added!

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