J.D. Harrington
Headquarters, Washington
202-358-5241
j.d.harrington@nasa.gov
Megan Watzke
Chandra X-ray Center, Cambridge, Mass.
617-496-7998
mwatzke@cfa.harvard.edu
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RELEASE 13-291
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NASA's Hubble and Chandra Find Evidence for Densest Nearby Galaxy
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Astronomers using NASA's Hubble Space Telescope and Chandra X-ray Observatory
and telescopes on the ground may have found the most crowded galaxy in our
part of the universe.
The ultra-compact dwarf galaxy, known as M60-UCD1, is packed with an
extraordinary number of stars and may be the densest galaxy near Earth. It is
providing astronomers with clues to its intriguing past and its role in the
galactic evolutionary chain.
M60-UCD1, estimated to be about 10 billion years old, is near the massive
elliptical galaxy NGC 4649, also called M60, about 54 million light years
from Earth. It is the most luminous known galaxy of its type and one of the
most massive, weighing 200 million times more than our sun, based on
observations with the W.M. Keck Observatory 10-meter telescope in Hawaii.
What makes M60-UCD1 so remarkable is that about half of this mass is found
within a radius of only about 80 light years. The density of stars is about
15,000 times greater -- meaning the stars are about 25 times closer to each
other -- than in Earth’s neighborhood in the Milky Way galaxy.
"Traveling from one star to another would be a lot easier in M60-UCD1 than it
is in our galaxy, but it would still take hundreds of years using present
technology," said Jay Strader of Michigan State University in Lansing.
Strader is the lead author of a paper about the research, which was published
Sept. 20 in The Astrophysical Journal Letters.
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, for life to form," said co-author
Anil Seth of the University of Utah.
Another intriguing aspect of M60-UCD1 is the presence of a bright X-ray
source in its center, revealed in Chandra data. One explanation for this
source is a giant black hole weighing in at about 10 million times the mass
of our sun.
Astronomers want to find out whether M60-UCD1 was born as a jam-packed star
cluster or became more compact as stars were ripped away from it. 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
M60-UCD1 and one or more nearby galaxies. M60-UCD1's great mass and the
abundances of elements heavier than hydrogen and helium are also arguments
for the theory it 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 originally was 50 to 200 times
more massive than it is now, and the mass of its black hole relative to the
original mass of the galaxy would be more like that of the Milky Way and many
other galaxies. The stripping could have taken place long ago and M60-UCD1
may have been stalled at its current size for several billion years.
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:
http://www.nasa.gov/chandraFor an additional interactive image, podcast, and video on the finding,
visit:
http://chandra.si.edu
