Observations of A4059 with Chandra, Hubble Space Telescope, and the very large array: Unraveling a complex cluster/radio galaxy interaction

Abstract

We present a detailed reanalysis of Chandra data for the galaxy cluster A4059 and its central radio galaxy, PKS 2354-35. We also present new 1.4 and 4.7 GHz CnB-array radio data from the Very Large Array, 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 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 ∼1′ structures than the previous high-quality maps), we fail to find a detailed correspondence between the ∼1′ 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 that 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 ∼30 kpc southwest of the cluster center that has been noted in these previous analyses. 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 southwest ridge and find none of them to be entirely satisfactory. In our preferred model, the southwest ridge is due to radiative cooling induced by an interaction between a radio galaxy-driven disturbance and a preexisting bulk ICM flow. The presence of such a bulk flow (with a velocity of ∼500 km 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/subcluster merger - The presence of a prominent dust lane in the cD galaxy of A4059, ESO 349 - G010, is circumstantial evidence for just such a merger.