Thermal and non-thermal plasmas in the galaxy cluster 3C 129

We describe new Chandra spectroscopy data of the cluster which harbours the prototypical 'head-tail' radio galaxy 3C 129 and the weaker radio galaxy 3C 129.1. We combined the Chandra data with Very Large Array (VLA) radio data taken at 0.33, 5 and 8 GHz (archival data) and 1.4 GHz (new data). We also obtained new H I observations at the Dominion Radio Astrophysical Observatory (DRAO) to measure the neutral hydrogen column density in the direction of the cluster with arcmin angular resolution. The Chandra observation reveals extended X-ray emission from the radio galaxy 3C 129.1 with a total luminosity of 1.5 × 10⁴¹ erg s^-1. The X-ray excess is resolved into an extended central source of ≃2 arcsec (1 kpc) diameter and several point sources with individual luminosities up to 2.1 × 10⁴⁰ erg s^-1. There is no evidence for a correlation between the 3C 129.1 X-ray and radio morphology. In the case of the radio galaxy 3C 129, the Chandra observation shows, in addition to core and jet X-ray emission reported in an earlier paper, some evidence for extended, diffuse X-ray emission from a region east of the radio core. The 12 × 36 arcsec² (6 kpc × 17 kpc) region lies 'in front' of the radio core, in the same direction into which the radio galaxy is moving. We use the radio and X-ray data to study in detail the pressure balance between the non-thermal radio plasma and the thermal intracluster medium (ICM) along the tail of 3C 129 which extends over 15 arcmin (427 kpc). Depending on the assumed lower energy cut-off of the electron energy spectrum, the minimum pressure of the radio plasma lies a factor of between 10 and 40 below the ICM pressure for a large part of the tail. We discuss several possibilities to explain the apparent pressure mismatch.