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Densest Galaxy Observed

By Stephen Luntz

An ultra-compact dwarf galaxy 54 million light years from Earth is the densest yet seen – and by quite a margin.

M60 is a giant galaxy visible in small telescopes. Like most large galaxies it is surrounded by globular clusters and dwarf galaxies. An international team of astronomers examining Hubble images of the area noticed one nearby object was bright enough to warrant further investigation.

“We used the Keck telescope to analyse a spectrum and confirmed the distance is the same as M60, allowing us to convert its angular size and brightness to physical size and luminosity,” says Prof Duncan Forbes of the Swinburne Centre for Astrophysics and Supercomputing.

M60-UCD1 turns out to be the brightest, and one of the heaviest, ultra-compact dwarf (UCD) galaxies ever observed, with a mass around 200 million times that of the Sun. Half of this is packed into a central region of just 80 light years, a density 15,000 times greater than our region of the Milky Way. “The nearest star to the Sun is some 4 light years away,” says Forbes, “but in this case there would be as many as 10,000 stars within a distance of 4 light years”.

The finding, published in Astrophysical Journal Letters, sheds light on the debate over whether UCDs are the result of globular clusters combining or larger galaxies having their outer edges stripped away in close encounters with bigger objects. The central cores of substantial galaxies are often as dense as that of M60-UVD1, but their average density is brought down by extensive outer limits.

“It seems likely that M60-UCD1 was once 50–200 times larger, but at some point in its 10 billion year life it got too close to M60 and lost its outer edges. Forbes says: “At the moment this is displaced on a graph from other galaxies, but we expect with a more systematic survey we will find others that fill the gap”.

Forbes is keen to investigate the black hole that is suspected to lie at M60-UCD1’s centre, using adaptive optics on the Keck telescope to examine the motion of stars at the galaxy’s heart. “If they are moving fast we can infer extra mass,” he says.

“Extreme objects often lead to new avenues of research. We look forward to seeing where this discovery will lead.”