Acknowledgment: J. Tonry
(University of Hawaii), P. Cote (Dominion Astrophysical Observatory),
and G. Fabbiano (Harvard-Smithsonian Center for Astrophysics).
Astronomers have found an unlikely object in an improbable place: a
monster black hole lurking inside one of the tiniest galaxies known.
Though the black hole is five times the mass of the black hole at the
center of our Milky Way, it is inside a galaxy that crams 140 million
stars within a diameter of about 300 light-years, only 1/500th of our
galaxy's diameter.
The dwarf galaxy containing the black hole, called M60-UCD1, is the
densest galaxy ever seen. If you lived inside of it, the night sky
would dazzle with at least 1 million stars visible to the naked eye (as
opposed to 4,000 stars in our nighttime sky, as seen from Earth's
surface).
The finding implies that there are many other very compact galaxies
in the universe that contain supermassive black holes. The observation
also suggests that dwarf galaxies may actually be the stripped remnants
of larger galaxies that were torn apart during collisions with yet
other galaxies — rather than small islands of stars born in isolation.
"We don't know of any other way you could make a black hole so big in
an object this small," said University of Utah astronomer Anil Seth,
lead author of an international study of the dwarf galaxy published in
Thursday's issue of the journal Nature.
His team of astronomers used the Hubble Space Telescope and the
Gemini North 8-meter optical and infrared telescope on Hawaii's Mauna
Kea to observe M60-UCD1 and measure the black hole's mass. The sharp
Hubble images provide information about the galaxy's diameter and
stellar density. Spectroscopy with Gemini measures the stellar motions
as affected by the black hole's pull. These data are used to calculate
the mass of the unseen black hole.
Black holes are gravitationally collapsed, ultracompact objects that
have a gravitational pull so strong that even light cannot escape.
Supermassive black holes — those with the mass of at least 1 million
stars like our Sun — are thought to be at the centers of many galaxies.
The black hole at the center of our Milky Way galaxy has the mass of 4
million suns, but as heavy as that is, it is less than 0.01 percent of
the Milky Way's total mass. By comparison, the supermassive black hole
at the center of M60-UCD1 is a stunning 15 percent of the small
galaxy's total mass. "That is pretty amazing, given that the Milky Way
is 500 times larger and more than 1,000 times heavier than the dwarf
galaxy M60-UCD1," Seth said.
One explanation is that M60-UCD1 was once a large galaxy containing
10 billion stars, but then it passed very close to the center of an
even larger galaxy, M60, and in that process all the stars and dark
matter in the outer part of the galaxy got torn away and became part of
M60.
The team believes that M60-UCD1 may eventually be pulled back to
merge with the center of M60, which has its own monster black hole,
weighing a whopping 4.5 billion solar masses (more than 1,000 times
bigger than the black hole in our galaxy). When that happens, the black
hole in M60-UCD1 will merge with the far more massive black hole in
M60. The galaxies are 50 million light-years away.
The Hubble Space Telescope is a project of international cooperation
between NASA and the European Space Agency. NASA's Goddard Space Flight
Center in Greenbelt, Md., manages the telescope. The Space Telescope
Science Institute (STScI) in Baltimore conducts Hubble science
operations. STScI is operated for NASA by the Association of
Universities for Research in Astronomy, Inc., in Washington.
CONTACT
Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4514
Lee Siegel
University of Utah, Salt Lake City, Utah
801-581-8993
Anil Seth
University of Utah, Salt Lake City, Utah
801-585-7793
Source: Hubble Site
