M60-(NGC 4649) - M60-UCD1 (inset)
Credit:
X-ray: NASA/CXC/MSU/J.Strader et al, Optical: NASA/STScI
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.
Fast Facts for M60-UCD1:
Scale: Image is 3.2 arcmin across (about 50,000 light years
Category: Normal Galaxies & Starburst Galaxies
Coordinates (J2000): RA 12h 43m 40.30s | Dec +11° 32' 58.00"
Constellation: Virgo
Observation Date: 6 pointings between April 2000 and August 2011
Observation Time: 85 hours 32 min (3 days 13 hours 32 min)
Obs. ID: 784, 8182, 8507, 12975, 12976, 14328
Instrument: ACIS
References: Strader, J. et al, 2013, ApJ 775, 6; arXiv:1307.7707
Color Code: X-ray (Pink); Optical (Red, Green, Blue)
Distance Estimate: About 54 million light years
