Chandra Observations of the Galaxy Cluster A478: The Interaction of Hot Gas and Radio Plasma in the Core, and an Improved Determination of the Compton y-Parameter

Abstract

We present the results from a 42 ks Chandra ACIS observation of the galaxy cluster A478. This cluster is generally considered to be highly relaxed. The Chandra image reveals, for the first time, X-ray cavities in the hot gas within the central 15 kpc radius of A478. Two weak and small (~4 kpc) radio lobes that extend from the central nucleus are detected in a 1.4 GHz VLA observation. The radio lobes are roughly along the direction of the X-ray cavities, but are much smaller than the X-ray cavities. We propose a "doughnut" configuration for the hot gas within the central 15 kpc, created by the interaction of the gas with the radio plasma that originated from the nucleus. The current radio activity of the central radio source is weak (~0.2% of Hydra A) and the total radio power is at least 10 times smaller than the minimum power needed to create the cavities. We compare A478 with other galaxy clusters in which similar X-ray cavities were found. A478 and A4059 host much weaker central radio sources than do others with similarly sized X-ray cavities. On larger scales, deprojected temperature and density profiles are obtained for A478. We used these to derive the Compton y-parameter, for the first time, through direct integration. The result has a much smaller statistical uncertainty than previous ones. This serves as an example of how high-quality X-ray data can help constrain H0. The corresponding H0 also was derived combining the available Sunyaev-Zeldovich effect measurement.