Astronomers working with the Hubble Space Telescope and the Chandra Xray Observatory have discovered a nearby galaxy which crams an incredible number of stars into a very small space. The galaxy, named M60-UCD1, is classed as an Ultra-Compact Dwarf Galaxy and spans only 160 light years across. The Milky Way, by contrast, is almost a thousand times larger than this. M60-UCD1 was discovered by the Hubble Space Telescope, and lies along a line of site very close to the more famous NGC 4649 elliptical galaxy in the constellation Virgo (also catalogued as M60 in the Messier list). It lies about 54 million light years away from Earth, which in intergalactic terms puts it in the Milky Way’s neighbourhood. Follow-up observations made by Chandra orbiting observatory and the 6.5-meter Multiple Mirror Telescope in Arizona, revealed further details on the dwarf’s composition. The image above is a Hubble image with the Chandra X-ray data superimposed in pink.

The galaxy is incredibly dense, containing about 200 million Sun’s worth of stars (by mass) and cramming most of those into an area with a radius of only 80 light years. This is some 15,000 times denser than our own galaxy, and means that individual stars are on average 25 times closer to each other than the stars in our own stellar neighbourhood. These stars have been analysed with ground-based telescopes and found to have a metallicity similar to that of our own Sun. “The abundance of heavy elements in this galaxy makes it a fertile environment for planets and, potentially, for life to form,” said Anil Seth of the University of Utah, who has studied this galaxy.

Ultra-compact dwarf galaxies are still something of a mystery to astronomers, who are trying to work out how such tiny galaxies form. The two leading ideas are that they begin life as unusually large and rich stellar clusters, or if they were once regular sized galaxies that have been disrupted by interactions with other galaxies. The second scenario would imply that the bulk of the galaxies original hundred billion stars would have been lost in the interaction, and that the remainder would have settled very close to the old core.

One aspect of M60-UCD1 that is immediately obvious from the Chandra data is that it’s core is very luminous in X-ray light. Such active galactic nuclei are known to be caused by supermasive black holes drawing in surrounding gas and dust. As this material accretes into a rapidly spinning disk, friction and tidal forces heat it to temperatures of millions of degrees, causing it to shine brightly in X-rays. But so far as we know, supermassive black hole’s only form in the cores of galaxies, and this together with the abundance of elements heavier than Helium suggests that the “Stripped Galaxy” model of formation is correct. The SMBH at the core of M60-UCD1 is estimated to have a mass of about 10 million Sun’s.

The original galaxy formed around 10 billion years ago, but the postulated interaction that stripped away almost 99% of its mass could have happened at any time since then. “We think nearly all of the stars have been pulled away from the exterior of what once was a much bigger galaxy,” said Duncan Forbes of Swinburne University in Australia, another collaborator in the study. “This leaves behind just the very dense nucleus of the former galaxy, and an overly massive black hole.”

These results appear online and have been published in the September 20th issue of The Astrophysical Journal Letters. The first author is Jay Strader, of Michigan State University in East Lansing, MI. The co-authors are Anil Seth from University of Utah, Salt Lake City, UT; Duncan Forbes from Swinburne University, Hawthorn, Australia; Giuseppina Fabbiano from Harvard-Smithsonian Center for Astrophysics (CfA), Cambridge, MA; Aaron Romanowsky from San Jos’e State University, San Jose, CA; Jean Brodie from University of California Observatories/Lick Observatory, Santa Cruz, CA; Charlie Conroy from University of California, Santa Cruz, CA; Nelson Caldwell from CfA; Vincenzo Pota and Christopher Usher from Swinburne University, Hawthorn, Australia, and Jacob Arnold from University of California Observatories/Lick Observatory, Santa Cruz, CA.

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