$^1$Department of Physics and CSST, Ewha University, Seoul 120-750, Korea $^2$Center for High Energy Physics, Kyungpook National University, Daegu 702-701, Korea $^3$JILA, Campus Box 440, University of Colorado, Boulder CO 80309 $^4$Dept. of Astronomy, University of Maryland, College Park, MD 20742 $^5$Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85740 Garching, Germany $^6$CASA, Campus Box 389, University of Colorado, Boulder CO 80309 $^7$Dept. of Physics, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250

Abstract:

We present a detailed reanalysis of Chandra X-ray Observatory data for the galaxy cluster Abell 4059 and its central radio galaxy, PKS2354-35. We also present new 1.4GHz and 4.7GHz CnB-array radio data from the Very Large Array¹, as well as a short archival WFPC2 image from the Hubble Space Telescope. The presence of a strong interaction between this radio galaxy and the intracluster medium (ICM) was suggested by Huang & Sarazin (1998) on the basis of a short observation by the High Resolution Imager on ROSAT, and confirmed in our preliminary analysis of the Chandra/ACIS-S data. In particular, X-ray imaging clearly shows two cavities within the ICM that are approximately aligned with the radio-galaxy axis. However, using our new radio maps (which are at lower frequencies and better matched to searching for $\sim 1{\rm\thinspace arcmin}$ structures than the previous high-quality maps) we fail to find a detailed correspondence between the $\sim 1{\rm\thinspace arcmin}$ scale radio-lobes and the ICM cavities. This suggests that the cavities are ``ghosts'' of a previous burst of powerful activity by PKS 2354-35. This is supported by detailed, spatially-resolved, X-ray spectroscopy which fails to find any shock-heated ICM, suggesting that the cavities are evolving subsonically. We also examine the nature of the central asymmetric ridge (or bar) of X-ray emission extending for $\sim30{\rm\thinspace kpc}$ south-west (SW) of the cluster center that has been noted in these previous analyzes. We find the ridge to be denser and cooler than, but probably in pressure balance with, its surroundings. The thermal evolution of this structure seems to be dominated by radiative cooling, possibly enhanced by the radio-galaxy ICM interaction. We discuss several possible models for the formation of this SW ridge and find none of them to be entirely satisfactory. In our preferred model, the SW ridge is due to radiative cooling induced by an interaction between a radio-galaxy driven disturbance and a pre-existing bulk ICM flow. The presence of such a bulk flow (with a velocity of $\sim 500{\rm\thinspace km}\,{\rm\thinspace s}^{-1}$ projected on the plane of the sky) is suggested by the off-center nature of the pair of X-ray cavities. Such a bulk flow can be created during a cluster/sub-cluster merger -- the presence of a prominent dust-lane in the cD galaxy of Abell 4059, ESO 349-G010, is circumstantial evidence for just such a merger.

galaxies:jets, galaxies:clusters:individual (Abell 4059), X-rays: galaxies: clusters