The velocity field of the Lyra complex
  The formation of cosmic structure culminates with the assembly of galaxy
clusters, a process quite different from cluster to cluster. We present the
study of the structure and dynamics of the Lyra complex formed of the two
clusters RXC J1825.3+3026 and CIZA J1824.1+3029, very recently studied using
both X-ray and radio data. This is the first analysis based on kinematics of
member galaxies. New spectroscopic data for 285 galaxies were acquired at the
Italian Telescopio Nazionale Galileo and used in combination with PanSTARRS
photometry. The result of our member selection is a sample of 198 galaxies. For
RXCJ1825 and CIZAJ1824 we report the redshifts, z=0.0645 and z=0.0708, the
first estimates of velocity dispersion, sigmav=995 and 700 km/s, and dynamical
mass, M200=1.1E15 and 4E14 Msun. The past assembly of RXCJ1825 is traced by the
two dominant galaxies, both aligned with the major axis of the galaxy
distribution along the East-West direction, and by a minor North-East
substructure. We also detect a quite peculiar high velocity field in the
South-West region of the Lyra complex. This feature is likely related to a high
velocity, very luminous galaxy, suggested to be the central galaxy of a group
in interaction with RXCJ1825 by very recent studies based on X-ray and radio
data. The redshift of the whole Lyra complex is z=0.067. Assuming that the
redshift difference between RXCJ1825 and CIZAJ1824 is due to the relative
kinematics, the projected distance between the cluster centers is 1.3 Mpc and
the los velocity difference is 1750 km/s. A dynamical analysis of the system
shows that the two clusters are likely to be gravitationally bound, in a
pre-merger phase, with CIZAJ1824 in front of RXCJ1825 and going toward it. Our
results corroborate a picture where the Lyra region is the place of a very
complex scenario of cluster assembly.
