Pressure Balance between Thermal and Non - Thermal Plasmas in the 3 C 129 Cluster

With new Chandra observations of the cluster containing the two radio galaxies 3C129 and 3C129.1, we have made a fit to the X-ray surface brightness to obtain thermal pressures. VLA data at 1.4 GHz have been obtained to complement previous maps at 0.33 GHz (Lane et al. 2002) and at 5 and 8 GHz (Taylor et al. 2001). From these radio data, we are able to derive the minimum non-thermal pressure of various emitting volumes along the tail of 3C129 and in the lobes of 3C129.1. Under the assumption that the non-thermal plasma excludes significant thermal plasma, we may expect pressure balance for most features since ram pressure should be important only close to the cores of the galaxies. Since we find that the minimum non-thermal pressures are generally only a factor of a few below estimates of the ambient thermal pressure , we conclude that it is unlikely that relativistic protons contribute significantly to the total pressure. Reasonable contributions from low energy electrons and filling factors in the range 0.1 to 1 suffice to achieve pressure balance. Although we do not find strong signatures for the exclusion of hot gas from the radio structures, we find soft features near the cores of both galaxies suggestive of cool gas stripping and hard features associated with radio jets and possibly a leading bow shock. 3C129 and 3C129.1 are two radio galaxies in a nearby cluster of galaxies (fig. 1) which lies close to the galactic plane. We proposed Chandra X-ray observations in order to find signatures of hydrodynamic interactions between the hot intracluster gas and the radio lobes and tails and to estimate the non-thermal pressures within the radio structures by determining the external gas pressure and assuming pressure balance. It seemed likely that given the long tail of 3C129 (≈ 0.5 Mpc projected length), that most of the source was close to the plane of the sky and that projection effects would thus be minimal. Since tailed radio 1 2 Author & Co-author galaxies are found only in clusters of galaxies, it also seemed likely that the projected distance between the cluster center and 3C129 was not significantly less than the actual distance: 3C129 was already known to lie close to the edge of the detected gas distribution (Leahy & Yin, 2000). For these reasons, we believed 3C129 was a good target for detecting the sort of depression in X-ray …