Mon. Not. R. Astron. Soc., 476, 4965-4986 (2018/June-1)
COS-Weak: probing the CGM using analogues of weak Mg II absorbers at z < 0.3.
MUZAHID S., FONSECA G., ROBERTS A., ROSENWASSER B., RICHTER P., NARAYANAN A., CHURCHILL C. and CHARLTON J.
Abstract (from CDS):
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 C IIλ1334 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 H I 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 nH/cm–3 < -2.4 and ∼1 pc-50 kpc (median ≃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 H I-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 d N/ 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.
© 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society
galaxies: formation - galaxies: haloes - quasar: absorption line
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