Cl1205+44: an atypic cluster/group of galaxies observed with XMM-Newton and Chandra.

We selected a number of distant cluster candidates from ROSAT PSPC data in the SHARC survey; part of these were observed with optical and infrared imaging through various filters. Optical colours indicate that galaxies in Cl1205+44 are very red and suggest a redshift greater than 1. However, optical redshifts of galaxies obtained very recently by J. Mulchaey in this area show a concentration of galaxies around z=0.33. Our recent observations of this object with XMM-Newton and Chandra confirm the presence of a faint extended X-ray source; if the redshift is fixed to 0.33, the X-ray gas has a temperature of 2.7 keV, and a bolometric luminosity of 2.4 10**43 ergs. Cl1205+44 seems therefore to be a poor structure at redshift 0.33. Several points make it, however, remarkable. Galaxies are very red for a z=0.33 structure. There is only a very low number of galaxies in the X-ray emission area, consistently with what would happen with a poor group or a fossil group of galaxies. However, the (Lx/Tx) value for Cl1205+44 does not follow the relation for nearby groups or clusters (Jones L.R. et al. 2003), while it falls on the Lx/Tx relation for intermediate redshift (z=0.3) rich clusters (Novicki et al. 2002), without being rich enough to be such a rich structure. This possibly suggests an evolution of the Lx-Tx relation for both groups and clusters of galaxies. We present here an analysis of the main X-ray properties of Cl1205+44 and explain why this structure is possibly filling the gap between groups and clusters of galaxies.