How to Build a Supermassive Black Hole
by Christopher Wanjek
Recent Chandra observations suggest that monster black holes form from the merger of their smaller brethren.
NASA astronomer Kim Weaver has got that sinking feeling. You know, it's that unsettling notion you get when you sift through your X-ray data and, to your surprise, find mid-sized black holes sinking toward the center of a galaxy, where they merge with others to form a single supermassive black hole.
Could such a thing be true? These would be the largest mergers since America On Line bought Time-Warner, and perhaps even more violent. The process would turn a starburst galaxy inside out, making it more like a quasar host galaxy.
Using the Chandra X-Ray Observatory, Weaver saw a hint of this fantastic process in a relatively nearby starburst galaxy named NGC 253 in the constellation Sculptor. She noticed that starburst galaxies -- those gems set aglow in a colorful life cycle of hyperactive star birth, death, and renewal -- seem to have a higher concentration of mid-mass black holes compared to other galaxies.
Mid-mass black holes are a new category of object first identified in 1999. With the mass of 100 to 10,000 Suns, they are more massive than black holes that form from stellar collapse, yet smaller than the million- to billion-solar-mass monsters that lurk in the cores of active galaxies. The more conservative astronomer is apt to use the term "ultraluminous X-ray source," or ULX, instead of mid-mass black hole, for there is no proof these objects are true black holes. What astronomers are seeing is a frenzy of hot gas shining brilliantly in X-rays and confined to a relatively tiny region.
But assuming these sources are indeed mid-mass black holes, these objects are usually located outside the cores of galaxies. With the angular resolution afforded by Chandra, Weaver could zoom in on NGC 253 and see four black holes located within about 3000 light-years from the core. That's close, and it may imply that the objects are gravitating toward the center of the galaxy.
What would happen if black holes got together and mingled? A wild party. A hundred 10,000- solar-mass black holes could create a single million-solar-mass black hole. Such black holes are thought to power quasars, a type of active galactic nuclei (AGN). Maybe this is how some supermassive black holes form -- through mergers of smaller black holes. No one is sure.
Weaver cannot confirm that the black holes are sinking; they are merely close to the core. In support of her claim, she has found tantalizing in NGC 253 -- in the form of X-ray spectra different from that of starburst activity -- that iron-rich gas is falling onto a budding supermassive black hole. "Could this be a starburst galaxy that is transforming itself into a quasar-like galaxy as we watch?" asks Weaver. "We have known for several years that starburst activity can be associated with AGN activity. In NGC 253, Chandra may have found a causal connection."
Weaver admits this sinking black hole idea is speculative, but it is not without its theoretical base. As early as the 1970s, Scott Tremaine of Princeton University suggested that the gravitational friction created when globular clusters pass background stars could cause the clusters to lose their orbital velocity around the galaxy and spiral into the core. Such a process, called dynamical friction, could form a 50 million-solar-mass core in 10 billion years.
Demos Kazanas of the Theory Group at NASA's Goddard Space Flight Center says that supermassive black holes could start small and build from any old junk -- stars, gas, mid-mass black holes. They don't discriminate. But the problem with the merging mid-mass black-hole hypothesis, he says, is getting enough black holes close to the galactic core and have them merge within a couple billion years. We're simply not seeing enough of these objects for such a scenario.
Andy Ptak and Ed Colbert of Johns Hopkins University, who were the first to identify mid-mass black holes, pored through old data from the retired ROSAT X-ray satellite to get a feel for black-hole populations. They estimate that at least a fifth of nearby galaxies have at least one midmass black hole. (They use the term IXO for Intermediate-Luminosity X-ray Objects, not ULX; I told you this was new territory.) Colbert says that IXOs are common. The idea that they can sink and merge, he says, is mere speculation until many more are found. Yet considering that only a couple of years ago the total number of these things was zero, Weaver might be on to something.
Mid-mass black holes are too far away to detect velocities and see whether they are sinking. So Weaver's next step is to search for ULXs in other galaxies to see if they gather near the centers an indication, but not proof, that they are heading toward a crazy black hole mixer going down in the core.
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