UNVEILING HIDDEN STRUCTURES IN THE COMA CLUSTER

We have assembled a large data-set of 613 galaxy redshifts in the Coma cluster, the largest presently available for a cluster of galaxies. W e have defined a sample of cluster members complete to b(26.5) = 20.0, using a membership criterion based on the galaxy velocity, when avail able, or on the galaxy magnitude and colour, otherwise. Such a data se t allows us to define nearly complete samples within a region of 1 h(- 1) Mpc radius, with a sufficient number of galaxies per sample to make statistical analyses possible. Using this sample and the ROSAT PSPC X -ray image of the cluster, we have re-analyzed the structure and kinem atics of Coma, by applying the wavelet and adaptive kernel techniques. A striking coincidence of features is found in the distributions of g alaxies and hot intracluster gas. The two central dominant galaxies, N GC 4874 and NGC 4889, are surrounded by two galaxy groups, mostly popu lated with galaxies brighter than b(26.5) = 17 and well separated in v elocity space. On the contrary, the fainter galaxies tend to form a si ngle smooth structure with a central peak coinciding in position with a secondary peak detected in X-rays, and located between the two domin ant galaxies; we suggest to identify this structure with the main body of the Coma cluster. A continuous velocity gradient is found in the c entral distribution of these faint galaxies, a probable signature of t idal interactions rather than rotation. There is evidence for a bound population of bright galaxies around other brightest cluster members. Altogether, the Coma cluster structure seems to be better traced by th e faint galaxy population, the bright galaxies being located in subclu sters. We discuss this evidence in terms of an ongoing accretion of gr oups onto the cluster.