Ophiuchus: an optical view of a very massive cluster of galaxies hidden
  behind the Milky Way
  The Ophiuchus cluster, at a redshift z=0.0296, is known from X-rays to be one
of the most massive nearby clusters, but due to its very low Galactic latitude
its optical properties have not been investigated in detail. We discuss the
optical properties of the galaxies in the Ophiuchus cluster, in particular with
the aim of understanding better its dynamical properties. We have obtained deep
optical imaging in several bands with various telescopes, and applied a
sophisticated method to model and subtract the contributions of stars in order
to measure galaxy magnitudes as accurately as possible. The colour-magnitude
relations obtained show that there are hardly any blue galaxies in Ophiuchus
(at least brighter than r'<=19.5), and this is confirmed by the fact that we
only detect two galaxies in Halpha. We also obtained a number of spectra with
ESO-FORS2, that we combined with previously available redshifts. Altogether, we
have 152 galaxies with spectroscopic redshifts in the 0.02<=z<=0.04 range, and
89 galaxies with both a redshift within the cluster redshift range and a
measured r' band magnitude (limited to the Megacam 1x1 deg^2 field). A complete
dynamical analysis based on the galaxy redshifts available shows that the
overall cluster is relaxed and has a mass of 1.1x10^15 solar masses. The
Sernal-Gerbal method detects a main structure and a much smaller substructure
that are not separated in projection. From its dynamical properties derived
from optical data, the Ophiuchus cluster seems to be overall a relaxed
structure, or at most a minor merger, though in X-rays the central region
(radius ~ 150 kpc) may show evidence for merging effects.
