The M15 globular star cluster is close enough that individual star speeds can be measured. By contrast, the G1 observations rely on measurements of the collective properties of many stars. In either case, a black hole can be identified by using a common Hubble black-hole-hunting technique, which searches for a rise in velocities toward the cluster center. Stars close to the black-hole "whirlpool" orbit at a faster rate, in keeping with fundamental laws of orbital motion around a massive central body, as described by Johannes Kepler four centuries ago.
Black holes cannot be seen directly. Some emit X-rays, or show other telltale evidence of their presence, when they capture nearby material. However, the dark objects in the centers of G1 and M15 are quiet. Nonetheless, they are presumed to be black holes because of their small size and large mass. An alternative explanation would be to assume that the centers of these clusters harbor a swarm of neutron stars or other exotic objects that sank to the cluster's center. However, theoretical studies do not predict swarms that are massive enough to account for the Hubble observations.
Astronomers have searched for black holes in globular clusters for nearly 30 years. The roadblock has been the fact that ground-based telescopes cannot easily resolve the stars closest to the suspected black hole. As far back as the 1970s, hunting for globular-cluster black holes was recognized as a task suited for Hubble Space Telescope's exquisite resolution, which is needed for looking close to a black hole. The researchers say that the quest is now over.
To further understand these issues, it is now extremely important to search for black holes in other star clusters as well. Some globular clusters are so close to us that, if they had black holes, we would be able to probe closer to these monsters than we have ever been able to before.
The members of the G1 research team are Michael Rich, Karl Gebhardt and Luis Ho. The members of the M15 research team are Roeland Van Der Marel, Joris Gerssen, Karl Gebhardt, Puragra Guhathakurta, Ruth Peterson and Carlton Pryor.
