29th September 2013

Astronomers discover densest galaxy ever

Imagine the distance between the Sun and the star nearest to it – Alpha Centauri. That's about 4.2 light years. Now, imagine as many as 10,000 of our Suns crammed into that relatively small space.

The diagram above shows the density of a galaxy that was recently discovered by an international team of astronomers led by a Michigan State University faculty member.

"This galaxy is more massive than any ultra-compact dwarfs of comparable size," said Jay Strader, MSU assistant professor of physics and astronomy, "and is arguably the densest galaxy known in the local universe."

As detailed in a recent edition of Astrophysical Journal Letters, the ultra-compact dwarf galaxy was found in the Virgo cluster of galaxies, located 54 million light years from our own Milky Way. What makes this galaxy – known as M60-UCD1 – so remarkable is that approximately half of its mass is found within a radius of only 80 light years. This would make the density of stars around 15,000 times greater than found in Earth's neighbourhood in the Milky Way.

"Traveling from one star to another would be a lot easier in M60-UCD1 than it is in our galaxy," Strader said. "Since the stars are so much closer in this galaxy, it would take just a fraction of the time."

The discovery of ultra-compact galaxies is relatively new – only within the past 10 years or so. Until then, astronomers could see these "things" way off in the distance, but assumed they were either single stars or very distant galaxies.

Another intriguing aspect of this galaxy is the presence of a bright X-ray source in its centre. One explanation for this is a giant black hole, weighing in at some 10 million times the mass of our sun.

Astronomers are trying to determine whether M60-UCD1 and other ultra-compact dwarf galaxies are either born as really jam-packed star clusters, or if they are galaxies that gradually get smaller because of stars being ripped away from them. The possible massive black hole, combined with the high galaxy mass and Sun-like levels of elements found in the stars, favour the latter idea. A giant black hole at the centre of M60-UCD1 helps tip the scales against the scenario where this galaxy was once a star cluster, since such large black holes are not found in these types of objects.

The galaxy was discovered using NASA's Hubble Space Telescope. Follow-up observations were done with NASA's Chandra X-ray Observatory and ground-based optical telescopes, including the Keck 10-metre telescope in Hawaii.

"Twenty years ago, we couldn't have done this," Strader said. "We didn't have Hubble or Chandra. This is one of those projects where you bring together the full force of NASA's great observatories, plus ground-based resources."

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