Warm-hot gas in groups and galaxies toward H2356-309
  We present a detailed analysis of the galaxy and group distributions around three reported X-ray absorption line systems in the spectrum of the quasar H2356-309. Previous studies associated these absorbers with known large-scale galaxy structures (i.e., walls and filaments) along the line of sight. Such absorption lines typically trace 10^{5-7} K gas, and may be evidence of the elusive warm-hot intergalactic medium (WHIM) thought to harbor the bulk of the low-redshift "missing baryons;" alternatively, they may be linked to individual galaxies or groups in the filaments. Here we combine existing galaxy survey data with new, multi-object Magellan spectroscopy to investigate the detailed galaxy distribution near each absorber. All of these three absorption systems are within the projected virial radii of nearby galaxies and/or groups, and could therefore arise in these virialized structures rather than (or in addition to) the WHIM. However, we find no additional galaxies near a fourth "void" absorber recently found in the spectrum, suggesting that this system may indeed trace gas unassociated with any individual halo. Though the number of known systems is still small, spatial coincidences suggest that some X-ray absorbers lie in galaxy and/or group environments, though others could still trace the large-scale filamentary WHIM gas predicted by simulations. 
Rik J. Williams, John S. Mulchaey, Juna A. Kollmeier (Carnegie   Observatories)
astro-ph.CO astro-ph.GA