Black holes in globulars?
The biggest stellar-mass black holes in our galaxy appear to top out at about 15 to 20 times the Sun’s mass. Yet a number of scientists think much larger ones exist in some of the galaxy’s 150 or so globular star clusters. Star-sized black holes likely formed in these clusters early in their histories, more than 10 billion years ago, and fairly quickly sunk to the center. But what happened to these black holes? Some theorists think that they merged with other black holes or neutron stars and grew much bigger, while others suspect that they encountered other stars that then ejected them from the cluster.
Thomas Maccarone of the University of Southampton in England and his colleagues reported in 2007 on the best candidate for a black hole in any globular cluster. They found an X-ray source in a globular circling the giant elliptical galaxy M49, located some 50 million light-years away in the Virgo cluster. The object emits far too many X-rays to be a neutron star and so must be an accreting black hole. Its mass exceeds 20 Suns but could be much higher.
The case for black holes in our galaxy’s globulars is more tenuous. Unless a black hole actively feeds on material from another star, it won’t have an accretion disk that glows in X-rays. The best alternative method is to look at the brightnesses and motions of the stars near a cluster’s center.
Several research teams have examined the Milky Way’s largest globular, Omega Centauri, to do just that. In 2008, a group led by Eva Noyola of the Max Planck Institute for Extraterrestrial Physics in Germany reported a black hole weighing 40,000 solar masses. Just two years later, however, Jay Anderson and Roeland P. van der Marel of the Space Telescope Science Institute in Baltimore, Maryland, found no evidence for a black hole of that size. At this stage, neither side seems to be winning the debate.
A similar argument rages over the relatively nearby globular M15 in Pegasus. Some researchers claim the presence of an intermediate-mass black hole weighing about 4,000 Suns, while others find no such evidence. Earlier this year, Jay Strader of the Harvard-Smithsonian Center for Astrophysics and his colleagues announced that new observations show M15 can’t have a black hole larger than 980 solar masses.