Lone Black Holes Discovered Adrift in the Galaxy

Donald Savage

Headquarters, Washington, DC

(Phone: 202/358-1547)

Ray Villard

Space Telescope Science Institute, Baltimore, MD

(Phone: 410/338-4514)

RELEASE: 00-4

LONE BLACK HOLES DISCOVERED ADRIFT IN THE GALAXY

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.

“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 today in Atlanta at the 195th meeting of the American
Astronomical Society.

The findings also suggest that stellar mass black holes do not
require some sort of interaction in a double star system to form,
but may also be produced in the collapse of isolated massive stars,
as has long been proposed by stellar theorists.

The black hole’s gravity acts like a powerful lens, bending
the light of a background star so that it appears as two separate
images when the black hole slowly drifts in front of it. The
bending angle is about 100 times smaller than the angular
resolution of Hubble, so the two distorted images of the background
star cannot be separated even in high-resolution Hubble images.

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.

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.

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.

NOTE TO EDITORS: Related images are available on the Internet at: