Like “flower power” tattoos on aging ex-hippy baby
boomers, unexpectedly large numbers of neutron stars and
black holes in elliptical galaxies suggest some of these
galaxies lived through a much wilder youth. The discovery by
NASA’s Chandra X-ray Observatory may require a revision of
theories of how elliptical galaxies evolved.
“For the first time, Chandra has allowed us to distinguish
hundreds of star-like sources that are black holes and
neutron stars in distant elliptical galaxies,” said Craig
Sarazin of the University of Virginia, Charlottesville, who
presented his team’s findings on three elliptical galaxies,
known as NGC 4697, NGC 4649 and NGC 1553, today at the
American Astronomical Society meeting in Albuquerque, N.M.
“The black holes and neutron stars we now see in these
elliptical galaxies are reminders of their very active past.”
Black holes and neutron stars are the “stellar corpses” of
the brightest, most massive and short-lived stars. The
presence of numerous neutron stars and black holes shows
these galaxies once contained many very bright, massive
stars. This is in marked contrast to the present populations
of lower-mass, faint, old stars that now dominate elliptical
galaxies.
The black holes and neutron stars found by Chandra in these
galaxies appear to be members of binary star systems. The
strong pull of gravity from the collapsed star pulls material
off the normal star. This material emits large numbers of X-
rays as it falls into the black hole or neutron star.
The Chandra observations also show most of the binary star
systems that contain black holes are not scattered randomly
among the stars in the elliptical galaxies. Instead, most of
the X-ray binaries are located in “globular star clusters,”
round balls of stars, containing about one million stars in a
region of the galaxy where typically only one would be found.
The high fraction of black hole binaries found in globular
star clusters suggests that the black holes captured a single
star or pulled it away from its original companion. Normally,
the distances between stars in galaxies are too great for
capture to occur, but in the extraordinarily dense
environment of globular clusters they may be much more
common.
“Chandra has shown us that the birthplace of these exotic
black-hole binary systems are in the dense globular
clusters,” said Joel Bregman, a collaborator at the
University of Michigan, Ann Arbor.
“Globular clusters appear to be the ‘singles bars’ of the
stellar world,” said Sarazin, “where a lonely black hole can
go to find a companion.”
Other members of the research team include Elizabeth L.
Blanton, Scott W. Randall and Gregory R. Sivakoff, all of the
University of Virginia, and Jimmy Irwin, University of
Michigan.
The observations were made with Chandra’s Advanced CCD
Imaging Spectrometer, which was built for NASA by Penn State
University, University Park, and the Massachusetts Institute
of Technology, Cambridge. NASA’s Marshall Space Flight
Center, Huntsville, Ala., manages the Chandra program for the
Office of Space Science, Washington. TRW, Inc., Redondo
Beach, Calif., is the prime contractor for the spacecraft.
The Smithsonian’s Chandra X-ray Center controls science and
flight operations from Cambridge, Mass.
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