The Phoenix is a massive type I galaxy cluster (i.e., a cluster that is dominated by a supermassive type-cD galaxy), with a mass around 2×10^15 M☉ (or one million trillions). The most significant mass components are dark matter and the intracluster medium. A type-cD galaxy (or central dominant galaxy) is a subclass of type-D giant elliptical galaxies. These are characterized by large halos of stars and they are located near the centres of massive rich galaxy clusters.
The central galaxy of the cluster has a gigantic stellar halo that extends for over 1.1 million light years (i.e., 22 times the size of our Galaxy) from its centre, making it one of the largest galaxies known. Additionally, the galaxy has a high star formation rate, thus the galaxy is still increasing in size.
The galaxy has an active galactic nuclei and at its heart there is a supermassive black hole with a mass around 20 billion M☉ (this is double the mass of a dwarf galaxy). Additionally, it is growing in mass (and size) at a rate of 60 M☉ per year. This one of the most massive known in the universe. For comparison purposes this is 5,000 more massive than the black hole that sits at the heart of our Galaxy. This corresponds to an event horizon that is 118 billion km across (or 19 times the distance between Sun and Pluto).
But, there are more spectacular features from this galaxy. The galaxy contains a huge amount of hot gas. Analysis of observational data suggests that the hot gas in the central region is cooling at a rate of 3,820 M☉ per year, making it the highest ever measured. Finally, it should be mentioned that there is more normal matter present in this galaxy than the rest of the cluster members combined.
Observations have shown that the galaxy experiences an immense star formation rate. This is estimated at 740 M☉ per year, which 740 times higher than our Galaxy.
Image: Composite image of the Phoenix Cluster. The image was made using data from the Chandra X-ray Observatory (purple), optical from the Hubble Space Telescope (yellow and blue), and radio from the Very Long Array (red). In this the hot gas of the region is depicted in X-rays, while the galaxies are seen in yellow and in blue are areas of cooler gas where stars are formed. Finally, with red jets structures are seen.
Image Credit: X-ray: NASA/CXC/MIT/M.McDonald et al., Radio: NRAO/VLA, Optical: NASA/STScI
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