Astronomers Discover Densest Galaxy in Nearby Universe: Ultra-Compact Dwarf Galaxy

Astronomers have officially discovered the densest galaxy in the nearby universe--so far. The ultra-compact dwarf galaxy is packed with an extraordinary amount of stars and reveals a little bit more about galaxy evolution.

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

The astronomers first spotted the galaxy, known as M60-UCD1, from the W.M. Keck Observatory on the summit of Mauna Kea, Hawaii. They also collected further data with the Multiple Meter Telescope in Arizona and employed Chandra data. In the end, they found that the galaxy probably weighs 200 million times more than our Sun and that about half of this mass is found within a radius of only about 80 light years. This makes the density of stars about 15,000 times greater than found in Earth's neighborhood in the Milky Way.

The discovery of ultra-compact galaxies like this one is still relatively new--only within the past 10 years or so. It's only now that technology has advanced enough that astronomers can see individual features instead of assuming the brilliant sources of light were either single stars or very-distant galaxies.

"Twenty years ago we couldn't have done this," said Strader in a news release. "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."

This particular galaxy has a bright X-ray source at its center. One explanation for this source is a giant black hole weighing in at some 10 million times the mass of the sun. Yet further research is still needed to confirm this theory.

Currently, scientists are 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. Since large black holes aren't found in star clusters, it's likely the massive black hole in this galaxy was produced by collisions between the galaxy and one or more nearby galaxies. In fact, the galaxy may be the remnant of a much larger galaxy.

"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, co-author of the new study, in a news release. "This leaves behind just the very dense nucleus of the former galaxy, and an overly massive black hole."

The researchers are still investigating this ultra-dense galaxy. The findings could allow them to better understand its intriguing past and show how it fits into the galactic evolutionary chain.

The findings are published in The Astrophysical Journal Letters.