Thursday, July 31, 2014

Powering an Ultra-Luminous Galaxy Cluster

In some galaxy clusters, the central intracluster gas can become dense enough to cool radiatively within the cluster’s lifetime. This can drive a continuous flow of cooling gas plunging towards the cluster’s centre. The paucity of such cooling flows suggests that in galaxy clusters where such a process might occur, some form of astrophysical feedback kicks in to prevent the development of a runaway cooling flow. A study by M. McDonald et al. (2012) of the Phoenix Cluster (SPT-CLJ2344-4243) reveals it to be one of the most massive galaxy clusters known. The Phoenix Cluster lies at a distance of almost 6 billion light years, and its mass is estimated to be ~2500 trillion times the Sun’s mass. For comparison, the Milky Way, with its few hundred billion stars, is estimated to be only ~1.5 trillion times the Sun’s mass. The Phoenix Cluster also has ~2 trillion times the Sun’s luminosity.

Artist’s illustration of the Phoenix Cluster, showing the strong flow of cooling gas sinking towards the central galaxy. Image credit: Chandra X-Ray Observatory.

Observations of the Phoenix Cluster by a number of telescopes show the presence of an extremely strong cooling flow bound for the cluster’s centre. The flow rate is estimated to be ~4000 solar masses per year. With such a tremendous inflow of material, the central galaxy of the Phoenix Cluster is undergoing a massive starburst episode, churning out new stars at a remarkable rate of ~750 solar masses per year. For comparison, the present-day star formation rate in the Milky Way is roughly one solar mass per year. The high inflow rate and the high star formation rate suggest that the astrophysical feedback to prevent the formation of a runaway cooling flow has yet to be fully established in the Phoenix Cluster.

At the heat of the central galaxy in the Phoenix Cluster lies a supermassive black hole that is estimated to have a mass of about 20 billion times the mass of the Sun. The supermassive black hole is also accreting material at a prodigious rate of ~60 solar masses per year, or roughly one Earth mass every two seconds. The strong cooling flow in the Phoenix Cluster is believed to be short-lived in contrast to long periods of strong astrophysical feedback; else both the central galaxy and its supermassive black hole would become too massive.

M. McDonald et al. (2012), “A Massive, Cooling-Flow-Induced Starburst in the Core of a Highly Luminous Galaxy Cluster”, arXiv:1208.2962 [astro-ph.CO]