ROSAT observations of the B2 radio galaxies 1615+35 and 1621+38: implications on the radio source confinement

We report X-ray observations of the radio galaxies B2 1615+35 (NGC 6109) and B2 1621+38 (NGC 6137), which belong to poor groups of galaxies and are characterized by a "head-tail" structure. We also analyze the X-ray behaviour of the radio galaxy NGC 6107, present within the field of view and showing a symmetric extended radio structure. The X-ray emission from the group of 1615+35 is complex, including both emission from a hot intergroup gas, and emission from the individual radio galaxies (1615+35 itself, which is pointlike, and NGC 6107, which is extended). In the case of 1621+38, only extended X-ray emission from the galaxy itself is detected. The extended X-ray emission of the galaxies 1621+38 and NGC 6107 is probably related to a gaseous galactic atmosphere. The pointlike emission of the radio galaxy 1615+35, instead, is likely to originate in the nucleus. We discuss the confinement of radio components by the ambient gas, and find that the external thermal pressure exceeds the internal equipartition pressure. The X-ray emitting plasma in the 1615+35 group is characterized by luminosity, temperature and metallitcity typical for groups, i.e. lower than for rich clusters.