High-Ionization Quasar Absorption Lines: A Test of the Existence of Hot Gas in Spiral-Rich Groups

Abstract

ROSAT studies of poor groups of galaxies have revealed hot, diffuse gas only in groups dominated by E/S0 galaxies. Here we investigate the possibility that a similar, but cooler, component exists in spiral-rich groups and can be detected through absorption in QSO spectra. We find that for the expected temperature, density, and metal abundance of the intragroup medium, various lines should be detectable. At T ~ 2 × 106 K,O VI, Ne VIII, Mg X, and Si XII lines will be seen. Strong Lyman series lines are also expected. A collisionally ionized gas at these temperatures will not produce much C IV and N V absorption, since it requires a multiphase medium to produce spectra that show all of the above ions. In addition, some lines of sight will likely pass only through the intragroup gas, producing pure high-ionization absorption systems (i.e., O VI lines without associated C IV and N V lines). Recent Hubble Space Telescope (HST) and Keck observations have revealed the presence of absorption lines with some of the properties we predict for the intragroup medium in spiral-rich groups. The discovery of a hot intragroup medium in spiral-rich groups would have many important consequences for extragalactic astronomy, including the possible pressure confinement of low-ionization absorption-line clouds.