AChandraX‐Ray Observation of A1991: The Late Stages of Infall?

We present results from a 38 ks Chandra X-ray observation of the z=0.059galaxy cluster A1991. The cluster has a bright X-ray core and a centraltemperature gradient that declines inward from 2.7 keV at 130 kpc toapproximately 1.6 keV at the cluster center. The radiative cooling time of thegas in the inner 10 kpc is about 0.5 Gyr, and rises to 1 Gyr at a radius of 20kpc. The cooling rate of the gas within the latter radius is less than about 25solar mass/yr. The Chandra ACIS-S3 image shows that the intracluster medium hasan asymmetric surface brightness distribution with respect to the centralgalaxy. Bright knots of soft X-ray emission embedded in a cometary structureare located approximately 10 arcsec north of the optical center of the cDgalaxy. Unlike the structures seen in other cooling flow clusters, the knotshave no obvious association with the radio source. The structure's temperatureof 0.83 keV makes it nearly 1 keV cooler than its surroundings, and its mass is3.4E+09 solar masses. Based on its bow-shaped appearance and modestoverpressure with respect to its surroundings, we interpret the structure as acool mass concentration that is breaking apart as it travels northward throughthe center of the cluster.Comment: Accepted for publication in Ap.