New research, funded by NASA and the University of Tokyo, has shown astronomers may not yet have uncovered the mystery at the heart of one of the Galaxy's oldest star systems, the globular cluster M15.
In September, observations made using NASA's Hubble Space Telescope (HST) seemed to indicate the presence of a mid-size black hole, several thousand times more massive than the Sun, in the hearts of two clusters of stars. The observations were the foundation for a claim that intermediate-mass black holes had at last been discovered. By comparison, stellar-mass black holes are only a few times the mass of our Sun. Galactic-center black holes are millions or even billions of times more massive than our Sun.
New detailed computer simulations, published in the Astrophysical Journal, show a different way to interpret the same data. Instead of a new mid-size black hole of unknown origin in M15, the Hubble data have a more mundane explanation - a dense clump of stellar remnants, the products of normal stellar evolution, in the cluster's core. The cluster may contain a small black hole or maybe none at all. The simulations did not address the other star cluster, called G1, in which evidence for an intermediate-mass black hole was reported at around the same time as the M15 results were released.
The question of what lurks at the center of M15 has kept astronomers busy for at least two decades. At various times, claims have been made that M15 must harbor a central black hole. The claims were based on the high density of stars in the cluster's congested core and on other tantalizing hints. Hints like stars near the center seemed to be orbiting faster than would be expected if only their own gravity were binding them to the cluster. This would be the telltale gravitational signature of an unseen mass hiding in M15's core.
A team of astronomers used the HST for several years to probe M15's secrets. A few months ago, the answer to the decades- long question of the possible black hole in M15 seemed to be at hand. After a painstaking analysis of HST data, using earlier model calculations by Indiana University researchers, the team reported evidence for a central black hole.
Within hours of the team's announcement, astronomers around the world were carefully studying the paper and its exciting conclusions, which were also rather surprising, since the Indiana group had previously published models that produced high velocities near the center of M15 without the need for a black hole.
An international group of researchers, using the world's fastest computer, the GRAPE-6 system in Japan, were engaged in a series of simulations of star clusters that resembled M15. They used the GRAPE-6 to perform independent tests of the M15 black hole claim. The GRAPE's detailed, star-by-star simulations represent the state of the art in cluster modeling. Using this unique tool, the team found they could reproduce the M15 observations without the need for a central black hole. The GRAPE team's members are Jun Makino and Holger Baumgardt, of Tokyo University; Piet Hut, of the Institute for Advanced Study in Princeton, N.J.; Steve McMillan, from Drexel University in Philadelphia; and Simon Zwart, from the University of Amsterdam in the Netherlands.
When the GRAPE researchers notified the HST team of their results, they learned the Hubble team, along with members of the Indiana University group, had reached a similar conclusion. All three groups agreed that a black hole, if present, had to be significantly smaller than originally reported. One of the figures in the original paper, published by the Indiana group, had been labeled incorrectly, throwing off the later analysis of the Hubble observations.
The GRAPE group's results appear in the Jan. 1, 2003, issue of the Astrophysical Journal. The amended Hubble results are in January's Astronomical Journal. An addendum to the earlier paper by the Indiana group will appear in the March 1, 2003, issue of the Astrophysical Journal.
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