24 September 2013

My god, it's full of stars. Astronomers discover densest galaxy

by Will Parker

Alpha Centauri, our closest neighboring star lies about 4 light years distant; now imagine as many as 10,000 stars crammed into that relatively small space. Astronomers say that's about the density of the newly discovered ultra-compact dwarf galaxy known as M60-UCD1.

"This galaxy is more massive than any ultra-compact dwarfs of comparable size," said Jay Strader, the Michigan State University scientist who led the discovery. "And [M60-UCD1] is arguably the densest galaxy known in the local universe."

M60-UCD1 was found in what's known as the Virgo cluster of galaxies, a collection of galaxies located about 54 million light years from our own Milky Way. Until recently, astronomers assumed dwarf galaxies were either single stars or very-distant galaxies.

What makes it so remarkable is that about half of its mass is found within a radius of only about 80 light years. This would make the density of stars about 15,000 times greater than found in Earth's neighborhood 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 galaxy was discovered using NASA's Hubble Space Telescope with follow-up observations performed by NASA's Chandra X-ray Observatory and ground-based optical telescopes. The 6.5-meter Multiple Mirror Telescope in Arizona was used to study the amount of elements heavier than hydrogen and helium in stars in M60-UCD1. The values were found to be similar to our Sun. "The abundance of heavy elements in this galaxy makes it a fertile environment for planets and, potentially, life to form," said co-discoverer Anil Seth, of the University of Utah.

Astronomers are now trying to determine if M60-UCD1 and other ultra-compact dwarf galaxies are either born as jam-packed star clusters or if they are galaxies that get smaller because they have stars ripped away from them. The mass of the galaxy and the Sun-like abundances of elements favor the idea that it is the remnant of a much larger galaxy.

If this stripping did occur, then the galaxy was originally 50 to 200 times more massive than it is now. It is possible that this stripping took place long ago and that M60-UCD1 has been stalled at its current size for several billion years. The researchers estimate that M60-UCD1 is more than about 10 billion years old.

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Source: Michigan State University, NASA