M60-UCD1: An Ultra-Compact Dwarf Galaxy
The densest galaxy in the nearby Universe may have been found, as described
in our latest press
release. The galaxy, known as M60-UCD1, is located near a massive elliptical
galaxy NGC 4649, also called M60, about 54 million light years
from Earth.
This composite image shows M60 and the region around it, where data from
NASA's Chandra X-ray Observatory are pink
and data from NASA's Hubble Space Telescope (HST) are red, green and blue. The
Chandra image shows hot gas and double stars containing black holes
and neutron
stars and the HST image reveals stars in M60 and neighboring galaxies
including M60-UCD1. The inset is a close-up view of M60-UCD1 in an HST
image.
Packed with an extraordinary number of stars, M60-UCD1 is an "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.
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
Keck 10-meter telescope in Hawaii. Remarkably, 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.
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.
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.
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.
These results appear online and have been published in the September 20th
issue of The Astrophysical Journal Letters. The first author is Jay Strader, of
Michigan State University in East Lansing, MI. The co-authors are Anil Seth from
University of Utah, Salt Lake City, UT; Duncan Forbes from Swinburne University,
Hawthorn, Australia; Giuseppina Fabbiano from Harvard-Smithsonian Center for
Astrophysics (CfA), Cambridge, MA; Aaron Romanowsky from San Jos'e State
University, San Jose, CA; Jean Brodie from University of California
Observatories/Lick Observatory, Santa Cruz, CA; Charlie Conroy from University
of California, Santa Cruz, CA; Nelson Caldwell from CfA; Vincenzo Pota and
Christopher Usher from Swinburne University, Hawthorn, Australia, and Jacob
Arnold from University of California Observatories/Lick Observatory, Santa Cruz,
CA.
Image credit: X-ray: NASA/CXC/MSU/J.Strader et al, Optical: NASA/STScI
NASA
Guillermo Gonzalo Sánchez Achutegui
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