Neutral and ionized gas distribution in and around the radio galaxy Coma

H I absorption has been detected with the WSRT against both lobes of the radio galaxy Coma A. This radio galaxy could be expanding in a particularly gas rich environment, perhaps the result from interac-tions/mergers between the dominant giant galaxy (associated with the radio galaxy) and less massive galaxies in the same group. Powerful radio galaxies are frequently associated with kpc-sized emission line nebulosities, extending up to tens of kpc from the nucleus. These nebu-losities can be either lit up by interactions between the radio jet/lobe and the environment or photoionized by anisotropic UV radiation from the active nucleus. Little is known in radio galaxies to what extent this ionized gas is related to the overall ISM. A key element for studies of these extended emission line regions is to know the intrinsic distribution of the gas in and around the galaxy, i.e. to know the distribution of the neutral gas together with that of the ionized gas. Coma A is an ideal object for this study. It is a well-known radio galaxy (z = 0.0857) studied in detail by van Breugel et al. (1985). The radio structure is mainly formed by two lobes (see Fig. 1) and the total size is ∼ 65 kpc (for H • = 75 km s −1 Mpc −1 , 1.45 kpc/arcsec). A spectacular system of interlocking emission line arcs and filaments has been observed in Hα (see Fig. 1) using the Taurus Tunable Filter (on the WHT, Tadhunter et al. 2000). A striking match between the ionized gas and the radio structures (see Fig. 1) makes the origin of the ionized gas even more puzzling and strongly suggests the presence of a complex interaction between the radio structure and the gas around Coma A. 1. HI observations with the Westerbork Synthesis Radio Telescope We have observed Coma A at the frequency of the redshifted H I line (1343 MHz) with the WSRT using 10 MHz bandwidth and a velocity resolution of ∼ 20 km s −1. At the resolution of our observations (about 13 arcsec), the radio structure of Coma A appears just resolved in two structures corresponding to the northern and southern lobes. The preliminary results show that we have detected H I in absorption against both radio lobes. In both lobes the peak of the absorption is at a very low optical depth (∼ 0.4 %). This low optical depth is …