New observations of the galaxy cluster SPT-CLJ2344-4243 (or Phoenix Cluster) at X-ray, ultraviolet, and optical wavelengths are helping astronomers better understand this extraordinary system. Chandra data reveal large cavities in the X-rays, which are shown with optical data from Hubble. Astronomers think these X-ray cavities were carved out of the surrounding gas by powerful jets of high-energy particles emanating from near a supermassive black hole in the central galaxy of the cluster. Massive filaments of gas and dust, which extend for 160,000 to 330,000 light-years, surround the X-ray cavities. Image credit: X-ray: NASA/CXC/MIT/M.McDonald et al; Optical: NASA/STScI.Galaxy clusters tend to be poor at producing new stars in their centres. They generally have one giant galaxy in their middle that forms stars at a rate significantly slower than most galaxies — including our Milky Way. The central galaxy contains a supermassive black hole roughly a thousand times more massive than the one at the centre of our galaxy. Without heating by outbursts from this black hole, the copious amounts of hot gas found in the central galaxy should cool, allowing stars to form at a high clip. It is thought that the central black hole acts as a thermostat, preventing rapid cooling of surrounding hot gas and impeding star formation.
New data provide more details on how the galaxy cluster SPT-CLJ2344-4243, nicknamed the Phoenix Cluster for the constellation in which it is found, challenges this trend. The cluster has shattered multiple records in the past: In 2012, scientists announced that the Phoenix Cluster featured the highest rate of cooling hot gas and star formation ever seen in the centre of a galaxy cluster, and is the most powerful producer of X-rays of all known clusters. The rate at which hot gas is cooling in the centre of the cluster is also the largest ever observed.
New observations of this galaxy cluster at X-ray, ultraviolet, and optical wavelengths by NASA’s Chandra X-ray Observatory, the Hubble Space Telescope, and the Clay-Magellan telescope located in Chile, are helping astronomers better understand this remarkable object. Clay-Magellan’s optical data reveal narrow filaments from the centre of the cluster where stars are forming. These massive cosmic threads of gas and dust, most of which had never been detected before, extend for 160,000 to 330,000 light-years. This is longer than the entire breadth of the Milky Way galaxy, making them the most extensive filaments ever seen in a galaxy cluster.This animation shows how the Phoenix Cluster appears in different regions of the electromagnetic spectrum. X-rays from the Chandra X-ray Observatory are shown in blue in a separate layer, whereas the optical data from Hubble Space Telescope and Clay-Magellan telescope are shown in red, green and blue in another. Finally, the annotated composite layer shows the cavities — regions with greatly reduced X-ray emission — in the hot gas. The Phoenix Cluster is about 1.5 million light-years across and located about 5.7 billion light-years from Earth in the southern constellation Phoenix. Image credit: X-ray: NASA/CXC/MIT/M.McDonald et al; Optical: NASA/STScI. AN animation by Ade Ashford.These filaments surround large cavities — regions with greatly reduced X-ray emission — in the hot gas. The X-ray cavities can be seen in the composite image that shows the Chandra X-ray data in blue and optical data from the Hubble Space Telescope (red, green, and blue). Astronomers think that the X-ray cavities were carved out of the surrounding gas by powerful jets of high-energy particles emanating from near a supermassive black hole in the central galaxy of the cluster. As matter swirls toward a black hole, an enormous amount of gravitational energy is released. Combined radio and X-ray observations of supermassive black holes in other galaxy clusters have shown that a significant fraction of this energy is released as jets of outbursts that can last millions of years. The observed size of the X-ray cavities indicates that the outburst that produced the cavities in SPT-CLJ2344-4243 was one of the most energetic such events ever recorded.
However, the central black hole in the Phoenix Cluster is suffering from somewhat of an identity crisis, sharing properties with both “quasars”, very bright objects powered by material falling onto a supermassive black hole, and “radio galaxies” containing jets of energetic particles that glow in radio waves, and are also powered by giant black holes. Half of the energy output from this black hole comes via jets mechanically pushing on the surrounding gas (radio-mode), and the other half from optical, UV and X-radiation originating in an accretion disc (quasar-mode). Astronomers suggest that the black hole may be in the process of flipping between these two states.
X-ray cavities located farther away from the centre of the cluster, labelled as “outer cavities”, provide evidence for strong outbursts from the central black hole about a hundred million years ago (neglecting the light travel time to the cluster). This implies that the black hole may have been in a radio mode, with outbursts, about a hundred million years ago, then changed into a quasar mode, and then changed back into a radio mode.
It is thought that rapid cooling may have occurred in between these outbursts, triggering star formation in clumps and filaments throughout the central galaxy at a rate of about 610 solar masses per year. By comparison, only a couple of new stars form every year in our Milky Way galaxy. The extreme properties of the Phoenix Cluster system are providing new insights into various astrophysical problems, including the formation of stars, the growth of galaxies and black holes, and the co-evolution of black holes and their environment.
A supermassive black hole at the heart of galaxy NGC 1097 powers a brilliant nuclear ring of run-away star formation as imaged by the European Southern Observatory’s Very Large Telescope.
Quasars are the discs of hot gas that form around supermassive black holes at the centre of massive galaxies — they are hotter than the surface of the Sun, generating enough light to be seen across the observable universe. New research has revealed quasar wind speeds of 20 percent the speed of light near a supermassive black hole.
This NASA/ESA Hubble Space Telescope image shows NGC 4789A, a dwarf irregular galaxy in the constellation of Coma Berenices. It certainly lives up to its name — the stars that call this galaxy home are smeared out across the sky in an apparently disorderly and irregular jumble, giving NGC 4789A a far more subtle and abstract appearance than its glitzy spiral and elliptical cousins.
