Baryon content and dynamic state of galaxy clusters: XMM-Newton
  observations of A1095 and A1926
  We have initiated a program to study the baryon content and dynamic state of
galaxy clusters. Here we present results primarily from XMM-Newton observations
of two optically-selected galaxy clusters, A1095 ($z \simeq 0.210$) and A1926
($z \simeq 0.136$). We find that both of them are actually cluster pairs at
similar redshifts. We characterize the temperatures of these individual
clusters through X-ray spectral fits and then estimate their gravitational
masses. We show a rich set of substructures, including large position offsets
between the diffuse X-ray centroids and the brightest galaxies of the clusters,
which suggests that they are dynamically young. For both A1095 and A1926, we
find that the mass required for the cluster pairs to be bound is smaller than
the total gravitational mass. Thus both cluster pairs appear to be ongoing
major mergers. Incorporating SDSS and NVSS/FIRST data, we further examine the
large-scale structure environment and radio emission of the clusters to probe
their origins, which also leads to the discovery of two additional
X-ray-emitting clusters ($z \simeq 0.097$ and $z \simeq 0.147$) in the field of
A1926. We estimate the hot gas and stellar masses of each cluster, which
compared with the expected cosmological baryonic mass fraction, leave ample
room for warm gas.
