The peculiar cooling flow cluster RX J0820.9+0752

Abstract

We present observations of the cluster of galaxies associated with the X-ray source RX J0820.9+0752 and its dramatic central cluster galaxy in the optical and X-ray wavebands. Unlike other cooling flow central cluster galaxies studied in detail, this system does not contain a powerful radio source at its core, and so provides us with an important example of where we expect to see only the processes caused directly by the cooling flow itself. A 9.4-ks Chandra observation shows that the hot intracluster gas is cooling within a radius of 20 kpc at a rate of a few tens of solar masses per year. The temperature profile is typical of a cooling flow cluster and drops to below 1.8 keV in the core. Optical images taken with the AAT and Hubble Space Telescope show that the central galaxy is embedded in a luminous (LHα∼ 5 × 1042 erg s−1), extended line-emitting nebula that coincides spatially with a bright excess of X-ray emission, and separate, off-nucleus clumps of blue continuum that form part of a patchy structure arcing away from the main galaxy. The X-ray/Hα feature is reminiscent of the 40-kpc long filament observed in A1795 which is suggested to be a cooling wake, produced by the motion of the central cluster galaxy through the intracluster medium. We present optical spectra of the central cluster galaxy and its surroundings, and find that the continuum blobs show stronger line emission, differing kinematic properties and more extreme ionization ratios than the surrounding nebula. Accounting for the strong intrinsic reddening and its significant variation over the extent of the line-emitting region, we have fitted the continuum spectra of the blobs and the nucleus using empirical stellar spectra from a library. We found that continuum emission from early main-sequence stars can account for the blue excess light in the blobs. Kinematical properties associate the gas in the system with a nearby secondary galaxy, suggesting some kind of tidal interaction between the two. We suggest that the secondary galaxy has moved through the cooling wake produced by the central cluster galaxy, dragging some of the gas out of the wake and triggering the starbursts found in the blobs.