Hole in the Middle: Are midsize black holes the missing link?

Two teams of astronomers reported this week that they had confirmed the existence of a new class of black hole.

MIDDLING BLACK HOLE. Artist’s view of a black hole at the core of a globular cluster. G. Bacon/NASA

Most astronomers now accept the reality of black holes, objects whose immense gravity sucks in everything around them. Observations have indicated that these gravitational monsters come in two sizes: baby black holes just a few times more massive than the sun and supermassive black holes weighing as much as a billion suns. In contrast, the newly found class consists of middleweights, ranging between 100 to 10,000 times the mass of the sun.

Studies unveiled Sept. 17 at a NASA briefing in Washington, D.C., focus on two globular clusters, which are crowded groupings of 10,000 to a million or so stars.

Until recently, astronomers had considered the gravity of a globular cluster to be too weak to hold on to a black hole even if one managed to form there. But recent calculations have shown that if the initial black hole were 30 to 50 times the sun’s mass, it could grow into an intermediate-weight black hole. Further, ultrabright X-ray sources concentrated in star-forming regions hint at the presence of midsize black holes (SN: 5/1/99, p. 286).

But astronomers have wanted more compelling evidence, such as measurements of the mass of these putative black holes. To gather that evidence, a team including Karl Gebhardt of the University of Texas at Austin and Roeland Van Der Marel and Joris Gerssen of the Space Telescope Science Institute in Baltimore studied the motions of stars within the globular cluster M15, the densest known in the Milky Way. Using the Hubble Space Telescope, the team measured a component of the velocity of individual stars orbiting within a fraction of a light-year of M15’s crowded core.

Hubble’s imaging spectrograph revealed that the stars close to the core move just as fast as those farther out, a strong indication that an ultradense object lurks at the globular cluster’s core. Gerssen calculates that a black hole located there would have a mass of 4,000 suns.

Gebhardt, Michael Rich of the University of California, Los Angeles, and their collaborators examined another globular cluster, G1, which is part of the nearby galaxy Andromeda. The cluster lies 2.2 million light-years from Earth, 70 times farther than M15.

G1 resembles a miniversion of M32, a bright satellite galaxy of Andromeda known to harbor a central black hole. Because G1 lies so far from Earth, Hubble couldn’t track the motion of the cluster’s individual stars. Instead, it measured the average velocity of groups of stars that lie at varying distances from the cluster’s center. The researchers conclude that a black hole 20,000 times the sun’s mass lies at the core of G1.

“This is a lot closer to being conclusive evidence” of midsize black holes than previous observations, comments Cole Miller of the University of Maryland in College Park. Also, because the ages of both of these globular clusters are known–12 billion years for M15 and 10 billion for G1–these studies mark the first time astronomers have determined the age of black holes, Rich adds.

The presence of midsize black holes may explain how supermassive black holes form, Gebhardt notes. One clue is that despite their extraordinary difference in mass, both midsize and supermassive black holes obey the same rule: Their mass is 0.5 percent that of the hub of stars and gas that surrounds them. That pattern suggests that black holes within globular clusters merge to build supermassive black holes. And this suggests that the merging of globular clusters built the cores of galaxies, Gebhardt adds.

“The findings are telling us something important about galaxy formation,” says Van Der Marel. Right now, “we’re just not sure what.”


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