Black Holeson the Loose
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Jan 14, 2000: Two international teams of astronomers using
NASA's Hubble Space Telescope and ground-based telescopes in
Australia and Chile have discovered the first examples of isolated
stellar-mass black holes adrift among the stars in our galaxy.
All previously known stellar black holes have been found in orbit
around normal stars, with their presence determined by their
effect on the companion star. The two isolated black holes were
detected indirectly by the way their extreme gravity bends the
light from a more distant star behind them.
Right:A & B Two images
of a crowded starfield as seen through a ground-based telescope
show the subtle brightening of a star due to the effect of gravitational
microlensing, where an invisible but massive foreground object
passes in front of the star and amplifies its light. The dark
lensing object is estimated to be a six-solar-mass black hole
that is drifting alone among the stars. Right: C
A NASA Hubble Space Telescope image of the same field clearly
resolves the lensed star and yields its true brightness. [more
information]
"These results suggest that black holes are common, and
that many massive but normal stars may end their lives as black
holes instead of as neutron stars," said David Bennett of
the University of Notre Dame, South Bend, IN. Bennett presented
his team's results yesterday in Atlanta at the 195th meeting
of the American Astronomical Society.
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However, the black hole's gravity also magnifies these stellar images, causing them to brighten as the black hole passes in front. Bennett's team was searching for these passages, also called gravitational microlensing events.
Careful
analysis of the two events reveals that the lensing objects are
each approximately six times the mass of the Sun. If the objects
were ordinary stars with this mass they would be bright enough
to outshine the more distant background source star. The masses
are also too large to be white dwarfs or neutron stars. This
leaves a black hole as the best explanation. Above: A diagram showing gravitational microlensing by a black hole. Light shines from a distant star and passes near enough to a black hole on the way to Earth. The black hole bends the light, which produces microlensed images. These images appear as a single brightened star to an observer on Earth.
This microlensing detection technique, combined with Hubble's
extraordinary resolution to pinpoint the lensed star, opens the
possibility for searching for lone black holes and assessing
whether they contribute to the galaxy's long-sought "dark
matter."
These microlensing events were discovered in 1996 and 1998 by
the Massive Compact Halo Object (MACHO) collaboration with the
National Science Foundation, using the 1.3 meter telescope at
the Mt. Stromlo Observatory in Canberra, Australia, while the
magnification was still increasing. The prompt discovery and
announcement of these events enabled precise follow-up observations
by the Global Microlensing Alert Network from the .9 meter telescope
at Cerro Tololo Inter-American Observatory and by the Microlensing
Planet Search project using the 1.9 meter telescope at Mt. Stromlo.
The MACHO team surveys tens of millions of stars in the direction
of the center of our galaxy, where the star field is very crowded,
increasing the chances for seeing rare gravitational microlensing
events. The two events were also of exceptionally long duration,
lasting 800 and 500 days respectively, which suggests that the
lensing objects have a high mass.
Right: This diagram shows various positions of Earth
with respect to a distant star and a somewhat closer black hole.
Only when Earth, the black hole, and the star are aligned will
an observer on Earth see a brightened image of the distant star.
Follow-up observations were done with Hubble to clearly identify
the lensed star for the first event and make a precise measurement
of its brightness after the lensing event. The Hubble frame indicates
that the lensed star was blended with two neighboring stars of
similar brightness which could not be separated in the poorer-resolution,
ground-based images. Hubble's identification of the lensed star
allowed for an accurate estimate of the mass of the black hole.
The 1998 event was brighter, and modeling of the ground-based
measurements enabled astronomers to determine the brightness
of the lensed star, but this determination awaits confirmation
with future Hubble images.
The Space Telescope Science Institute is operated by the Association
of Universities for Research in Astronomy, Inc. for NASA, under
contract with NASA's Goddard Space Flight Center, Greenbelt,
MD. The Hubble Space Telescope is a project of international
cooperation between NASA and the European Space Agency.
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