Lyman-alpha Absorbers and the Coma Cluster
  The spatial and kinematic distribution of warm gas in and around the Coma
Cluster is presented through observations of Lyman-alpha absorbers using
background QSOs. Updates to the Lyman-alpha absorber distribution found in Yoon
et al. (2012) for the Virgo Cluster are also presented. At 0.2-2.0 R_vir of
Coma we identify 14 Lyman-alpha absorbers (N_HI = 10^{12.8-15.9} cm^-2) towards
5 sightlines and no Lyman-alpha absorbers along 3 sightlines within
3\sigmav_coma. For both Coma and Virgo, most Lyman-alpha absorbers are found
outside the virial radius or beyond 1\sigmav consistent with them largely
representing the infalling intergalactic medium. The few exceptions in the
central regions can be associated with galaxies. The Lyman-alpha absorbers
avoid the hot ICM, consistent with the infalling gas being shock-heated within
the cluster. The massive dark matter halos of clusters do not show the
increasing column density with decreasing impact parameter relationship found
for the smaller mass galaxy halos. In addition, while the covering fraction
within R_vir is lower for clusters than galaxies, beyond R_vir the covering
fraction is somewhat higher for clusters. The velocity dispersion of the
absorbers compared to the galaxies is higher for Coma, consistent with the
absorbers tracing additional turbulent gas motions in the cluster outskirts.
The results are overall consistent with cosmological simulations, with the
covering fraction being high in the observations standing out as the primary
discrepancy.
