Few astronomers doubt the existence of black holes, even though their intense light-trapping gravity prevents them from being spotted directly. But just how these ultradense collections of matter form isn't as well understood. Now a group of Italian researchers has come up with evidence supporting a leading theory that suggests they form in the center of a galaxy when clusters of ancient stars, called globular clusters, coalesce.
In spiral galaxies, such as our own Milky Way, millions of young stars orbit the galactic center in a more or less flat disc. Globular clusters and other ancient stars called halo stars, on the other hand, rotate about the galactic core in a more diffuse sphere, known as a halo. Because past theories of galactic evolution have pointed to the clusters as a possible fuel source for black holes, University of Rome astronomer Roberto Capuzzo-Dolcetta and his colleagues took a close look at the globular clusters and halo stars at the center of three spiral galaxies--the Milky Way, the Andromeda Galaxy (M31), and M87.
Current theories about the formation of spiral galaxies suggest that when they begin to take shape, halo stars and globular clusters should follow a similar distribution pattern. Yet, when Capuzzo-Dolcetta and Laura Vignola used the Hubble Space Telescope and its ground-based kin to plot the location and distance of globular clusters and halo stars to the galactic centers, they made a surprising find: Many of the globular clusters near the galactic core were missing. "The globular-cluster distribution is less concentrated at the center than [that of] the halo stars," says Capuzzo-Dolcetta, who has submitted these findings to the journal Astronomy and Astrophysics. After crunching the numbers in a computer model, Capuzzo-Dolcetta and Andrea Tesseri estimated that our galaxy has lost about 30% of its original globular clusters, while Andromeda has lost 25% and M87, 50%. The researchers have submitted these calculations to another journal, Monthly Notices of the Royal Astronomical Society.
The likely explanation, say the researchers, is that clusters near the center of the halo have coalesced into black holes. Capuzzo-Dolcetta suggests that because globular clusters are so much more massive than halo stars, they experience a much stronger gravitational pull. This dissipates their rotational energy more quickly than the halo stars, causing them to fall toward the galaxy core, where they merge, forming an ultrahigh density of matter that "can be the beginning of the process of black-hole formation." Charles Bailyn, an astronomer at Yale University in New Haven, Connecticut, agrees that the theory is plausible, but he cautions: "The question about these theories is the time scales--how fast should this happen?"