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NASA's Hubble and Chandra Find Evidence for Densest Nearby Galaxy

For Release: September 24, 2013


Sagittarius A*
Credit: X-ray: NASA/CXC/MSU/J.Strader et al, Optical: NASA/STScI
Press Image and Caption

The densest galaxy in the nearby part of the Universe may have been found. Packed with an extraordinary number of stars, this unusual galaxy is providing astronomers with clues to its intriguing past and how it fits into the galactic evolutionary chain.

The galaxy, known as M60-UCD1, is a type of “ultra-compact dwarf galaxy”. It was discovered with NASA’s Hubble Space Telescope and follow-up observations were done with NASA’s Chandra X-ray Observatory and ground-based optical telescopes.

Observations from the W. M. Keck Observatory on the summit of Mauna Kea, Hawaii, characterized it as the most luminous known galaxy of its type and one of the most massive, weighing 200 million times more than our Sun.

What makes M60-UCD1 so remarkable is that about half of this 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, meaning that the stars are about 25 times closer.

"Traveling from one star to another would be a lot easier in M60-UCD1 than it is in our galaxy," said Jay Strader of Michigan State University in Lansing, first author of a new paper describing these results. "But it would still take hundreds of years using present technology."

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-author Anil Seth of the University of Utah.

Another intriguing aspect of M60-UCD1 is that the Chandra data reveal the presence of a bright X-ray source in its center. One explanation for this source is a giant black hole weighing in at some 10 million times the mass of the Sun.

Astronomers 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. Large black holes are not found in star clusters, so if the X-ray source is in fact due to a massive black hole, it was likely produced by collisions between the galaxy and one or more nearby galaxies. The mass of the galaxy and the Sun-like abundances of elements also favor the idea that the galaxy is 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 co-author Duncan Forbes of Swinburne University in Australia. "This leaves behind just the very dense nucleus of the former galaxy, and an overly massive black hole."

If this stripping did occur, then the galaxy was originally 50 to 200 times more massive than it is now, which would make the mass of its black hole relative to the original mass of the galaxy more like the Milky Way and many other galaxies. 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.

The density of stars in the galaxy is so high astronomers do not expect to find a signature of dark matter in the motion of stars. However, these galaxies are considered likely to contain some dark matter. If they do, they are important for making comparisons with computer simulations of the formation of galaxies, which typically predict a larger number of clumps of dark matter than observed.

M60-UCD1 is located near a massive elliptical galaxy NGC 4649, also called M60, about 60 million light years from Earth. These results appear online and have been published in the September 20th issue of The Astrophysical Journal Letters.

NASA's Marshall Space Flight Center in Huntsville, Ala., manages the Chandra Program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory controls Chandra's science and flight operations from Cambridge, Mass.

For Chandra images, multimedia and related materials, visit:

For an additional interactive image, podcast, and video on the finding, visit:

Media contacts:
J.D. Harrington
Headquarters, Washington

Megan Watzke
Chandra X-ray Center, Cambridge, Mass.

Visitor Comments (7)

Interstellar space travel would be
much more feasible in M60 UCD1 than
in our galaxy. What is the average
distance between stars in M60 UCD1?

Posted by Howard Zirkin on Saturday, 10.1.16 @ 15:09pm

How does the mass of this compare with the average mass of globular clusters? Are we talking order of mag. larger? Was this thought to be a globular orbiting M80, and found to be much heavier?

Posted by Bruce Salem on Monday, 10.21.13 @ 16:30pm

For Ronnie, 9.26.13
The stars in M60 UCD1 are 25 times closer than those in the vicinity of our sun, not 25%. Gravity goes as the square of the distance, so average gravity overall would be 625 times greater than that in our vicinity. Still, it's not nearly enough to disrupt spacecraft, since there is still a huge distance between stars in that galaxy.

Posted by Jim Harwood on Saturday, 10.5.13 @ 23:58pm

Thank you very much for the text, photos and explanations !
from Sofia, Bulgaria

Posted by Anna Geo-Georgieva on Sunday, 09.29.13 @ 10:30am

If there are that many stars stacked on top of a giant black hole wouldn't the black hole become active again?

Posted by Charlie on Thursday, 09.26.13 @ 18:59pm

That is amazing, but if the galaxy M60 UCD1 is only 80 light years and the stars are almost 25% closer than our star, wouldn't the gravitational pull be 25% greater. Could any of our crafts handle the pressure?

Posted by Ronnie on Thursday, 09.26.13 @ 15:56pm

Oh, how I wish I would have had the wisdom to study astronomy and other related fields as I grew up. Being older now, I can only appreciate the tremendous advancement your field has made over the decades. I thrill at the sight of so many spectacular photos and distant galaxies that you investigate. Please, please continue your amazing discoveries so we all may enjoy your work!

Posted by Kenneth B Curtis on Thursday, 09.26.13 @ 11:15am