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.
