Growth and disruption in the Lyra complex
  Nearby clusters of galaxies, z<0.1, are cosmic structures still under
formation. Understanding the thermodynamic properties of merging clusters can
provide crucial information on how they grow in the local universe. A detailed
study of the intra-cluster medium (ICM) properties of un-relaxed systems is
essential to understand the fate of in-falling structures and, more generally,
the virialization process. We analyzed a mosaic of XMM-Newton observations (240
ks) of the Lyra system (z=0.067) that shows a complex dynamical state. We find
the main cluster RXC J1825.3+3026 to be in a late merger phase, whereas its
companion CIZA J1824.1+3029 is a relaxed cool-core cluster. We estimate a mass
ratio of ~1:2 for the pair. No diffuse X-ray emission is found in the region
between them, indicating that these clusters are in a pre-merger phase. We
found evidence of a galaxy group infalling on RXC J1825.3+3026 in an advanced
state of disruption. The Southern Galaxy, one of the brightest galaxies in the
Lyra complex, was very likely at the center of the infalling group. This galaxy
has a gaseous corona indicating that it was able to retain some of its gas
after the ram-pressure stripping of the intra-group medium. In this scenario
the diffuse emission excess observed southwest of RXC J1825.3+3026 could be due
to gas once belonging to the group and/or to cluster ICM dislocated by the
passage of the group. Finally, we identified three high-velocity galaxies
aligned between RXC J1825.3+3026 and the SG, two of these showing evidence of
gas stripped from them during infall. We estimate them to be currently falling
onto the main cluster at an infall velocity of ~ 3000 km/s. Our study of the
Lyra complex provides important clues about the processes presiding over the
virialization of massive clusters in the local Universe.
