Rapid Star Formation Observed in the Phoenix Cluster

The hot gas in Phoenix is giving off copious amounts of X-rays and cooling quickly over time, especially near the center of the cluster, causing gas to flow inwards and form huge numbers of stars at the base of the flows. These features are shown in this artist’s impression of the central galaxy, with hot gas shown in red, cooler gas shown in blue, the gas flows shown by the ribbon-like features and the newly formed stars in blue, in the outer part of the galaxy. Credit: NASA/CXC/M.Weiss

Using the Chandra X-Ray Observatory, the Magellan telescopes in Chile, the South Pole Telescope in Antarctica, and other facilities, a team of astronomers discovered the Phoenix Cluster, which is located 5.7 billion light-years away and is undergoing a massive burst of star formation with about 740 solar-masses worth of new stars being created every year.

Most galaxies lie in clusters, groupings of several to many thousands of galaxies. Our Milky Way galaxy itself is a member of the “Local Group,” a band of about fifty galaxies whose other large member is the Andromeda Galaxy about 2.3 million light-years away. The closest large cluster of galaxies to us is the Virgo Cluster, with about 2000 members, whose center is about 50 million light-years away. The space between all these galaxies is not empty, but is filled with hot gas whose temperature is of order ten million kelvin, or even higher.

The development of clusters is a key feature of galaxy evolution, as hundreds or often thousands of galaxies are bound together by their mutual gravitational attractions. Clustering influences how a particular member galaxy will evolve, and reflects the larger cosmic structures and physical processes. Astronomers use observations of faint, distant galaxies to check for consistency in their models of galaxy evolution, and to refine the parameters. The task of deciphering clustering in the early universe is difficult, however, because distant galaxies are faint and only the brightest ones can be seen. Still, research to date has been very productive. Observations of galaxy clusters have recently shed important new light on everything from dark matter in the universe to early cosmological phases following the Big Bang.

A composite X-ray (purple), optical (red/green/blue), and UV (blue) image of the newly discovered Phoenix Cluster. This galaxy cluster has been dubbed the Phoenix Cluster because it is located in the constellation of the Phoenix, and because new stars are forming there at the highest rate ever observed in a cluster. Credit: X-ray: NASA/CXC/MIT/M.McDonald; UV: NASA/JPL-Caltech/M.McDonald; Optical: AURA/NOAO/CTIO/MIT/M.McDonald; Illustration: NASA/CXC/M.Weiss

Despite many successes, current models of galaxy clustering suffer from several glaring omissions. One relates to the fate of its hot, X-ray emitting gas. In the cores of clusters the gas is heated to extreme temperatures by supernovae (the explosive deaths of stars) and other processes. The mystery is why this hot gas does not cool more efficiently and sink towards the center of the cluster. The common surmise has been that outflowing jets from supermassive black holes, or other kinds of feedback, inhibit the formation of such “cooling flows,” but establishing the details of this mechanism has been elusive.

Harvard-Smithsonian Center for Astrophysics (CfA) astronomers Ryan Foley, Matt Ashby, Bill Forman, Steve Murray, Brian Stalder, Tony Stark, Chris Stubbs, and Alex Vikhlinin are part of a team of astronomers who have discovered one of the largest and most luminous galaxy clusters in the universe, the so-called Phoenix Cluster, about 5.7 billion light-years away. Using the Chandra X-Ray Observatory, the Magellan telescopes in Chile, the South Pole Telescope in Antarctica, and a suite of other space and ground-based facilities, the team finds that the cluster (at least currently) is undergoing a massive burst of star formation with about 740 solar-masses worth of new stars being created every year (for reference, the Milky Way forms about one star per year).

This was unexpected: most clusters support only modest star formation. At the same time, the team finds that the dominant black hole in the cluster does not show especially powerful jet activity. Writing in last week’s issue of the journal Nature, the team concludes that this massive starburst is powered by the infalling material. They speculate that in this cluster the dominant black hole, although steadily growing in mass, has jets that are unable to halt the “cooling flow.”The implication is that either this cluster is of a very rare kind, or that with subsequent evolution it will develop more effective and customary ways to inhibit the infalling material.

Reference: “A massive, cooling-flow-induced starburst in the core of a luminous cluster of galaxies” by M. McDonald, M. Bayliss, B. A. Benson, R. J. Foley, J. Ruel, P. Sullivan, S. Veilleux, K. A. Aird, M. L. N. Ashby, M. Bautz, G. Bazin, L. E. Bleem, M. Brodwin, J. E. Carlstrom, C. L. Chang, H. M. Cho, A. Clocchiatti, T. M. Crawford, A. T. Crites, T. de Haan, S. Desai, M. A. Dobbs, J. P. Dudley, E. Egami, W. R. Forman, G. P. Garmire, E. M. George, M. D. Gladders, A. H. Gonzalez, N. W. Halverson, N. L. Harrington, F. W. High, G. P. Holder, W. L. Holzapfel, S. Hoover, J. D. Hrubes, C. Jones, M. Joy, R. Keisler, L. Knox, A. T. Lee, E. M. Leitch, J. Liu, M. Lueker, D. Luong-Van, A. Mantz, D. P. Marrone, J. J. McMahon, J. Mehl, S. S. Meyer, E. D. Miller, L. Mocanu, J. J. Mohr, T. E. Montroy, S. S. Murray, T. Natoli, S. Padin, T. Plagge, C. Pryke, T. D. Rawle, C. L. Reichardt, A. Rest, M. Rex, J. E. Ruhl, B. R. Saliwanchik, A. Saro, J. T. Sayre, K. K. Schaffer, L. Shaw, E. Shirokoff, R. Simcoe, J. Song, H. G. Spieler, B. Stalder, Z. Staniszewski, A. A. Stark, K. Story, C. W. Stubbs, R. Šuhada, A. van Engelen, K. Vanderlinde, J. D. Vieira, A. Vikhlinin, R. Williamson, O. Zahn and A. Zenteno, 15 August 2012, Nature.
DOI: 10.1038/nature11379