Absorption-Line Detections of $10^{5-6}$ K Gas in Spiral-Rich Groups of Galaxies

TL;DR

This study uses Hubble's COS to detect warm gas in galaxy groups, suggesting a large, hot intra-group medium that is difficult to observe directly, based on absorption lines in quasar spectra.

Contribution

It provides new evidence linking warm absorbers to galaxy groups and proposes the existence of a massive, hot intra-group medium inferred from absorption data.

Findings

Warm absorbers are associated with spiral-rich galaxy groups.

The frequency of warm absorbers suggests large (~1 Mpc) structures.

A hot, diffuse intra-group medium likely exists but remains undetected directly.

Abstract

Using the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST) the COS Science Team has conducted a high signal-to-noise survey of 14 bright QSOs. In a previous paper (Savage et al. 2014) these far-UV spectra were used to discover 14 "warm" ($T > 10^5$ K) absorbers using a combination of broad Ly\alpha\ and O VI absorptions. A reanalysis of a few of this new class of absorbers using slightly relaxed fitting criteria finds as many as 20 warm absorbers could be present in this sample. A shallow, wide spectroscopic galaxy redshift survey has been conducted around these sight lines to investigate the warm absorber environment, which is found to be spiral-rich galaxy groups or cluster outskirts with radial velocity dispersions of \sigma\ = 250-750 km/s. While 2\sigma\ evidence is presented favoring the hypothesis that these absorptions are associated with the galaxy groups and not with the individual, nearest galaxies, this evidence has considerable systematic uncertainties and is based on a small sample size so it is not entirely conclusive. If the associations are with galaxy groups, the observed frequency of warm absorbers (dN/dz = 3.5-5 per unit redshift) requires them to be very large (~1 Mpc in radius at high covering factor). Most likely these warm absorbers are interface gas clouds whose presence implies the existence of a hotter ($T \sim 10^{6.5}$ K), diffuse and probably very massive ($>10^{11}~M_{\odot}$) intra-group medium which has yet to be detected directly.