Big black holes may not stop star birth

New study suggests models may have given these supermassive beasts too much credit

Monster black holes definitely suck, but they might not be the cosmic bullies that astronomers think they are.

STARS AND BLACK HOLES Astronomers compared the total light emanating from galaxies (left) to only the light associated with their central supermassive black holes (right) to show that star formation in the galaxies stopped at least 100 million years before the central regions powered by the black holes attained their peak brightness. Kevin Schawinski, Sloan Digital Sky Survey

Researchers often cite the whirlpool of activity generated by matter spiraling into supermassive black holes — which weigh millions to billions of times the sun’s mass and sit at the cores of galaxies — to explain why star formation halts and galaxies never exceed a maximum size. Models suggest that fierce winds or powerful radio jets produced as black holes pull in new material can warm up or even expel cold gas, keeping galaxies from growing bigger and depleting them of the raw material for making stars.

But a new study of 177 relatively nearby galaxies known to harbor luminous, actively feeding supermassive black holes reveals that star formation stopped at least 100 million years before the black holes had reached their maximum size and strength. That suggests that either the black holes aren‘t the main culprit in suppressing star formation or that an earlier, gentler phase of these compact objects — before they packed on so much material — somehow produced enough fireworks to disrupt star formation.

To scrutinize the ages of stars, Kevin Schawinski and Meg Urry of Yale University and their colleagues examined visible-light images of galaxies and carefully subtracted the central source of light from each. The central light is produced by the supermassive black holes’ activity. Removing that light source was a critical step, says Urry, because the bluish light radiated by material falling onto a supermassive black hole can mimic the color of light from young stars in a galaxy.

After separating out the black holes’ contribution to the radiation, it became clear that the galaxies’ youngest stars, as indicated by their green or reddish tinge, hadn’t formed any more recently than 100 million years before the black holes had reached their peak brightness and activity, the team reports in the Feb. 10 Astrophysical Journal Letters. The stars ought to have had bluer colors if their formation had ceased any more recently, Urry says.       

“The black hole may still be involved, but the [old] scenario doesn’t seem to hold up in the observations,” Schawinski says.

Whatever is stopping the star formation is either an earlier, less active phase, of the black hole or something else altogether, Urry adds.

It’s been pretty well established that radio jets emanating from supermassive black holes can blow bubbles in surrounding gas, heating and blowing some — but not all — of the gas out of a galaxy, comments Tim Heckman of Johns Hopkins University in Baltimore. But the “cartoon picture” that some percentage of a black hole’s energy is transformed into winds and jets that blow out all the gas isn’t supported by observations, he says. It may be that supermassive black holes play a subtler, housekeeping role in regulating star formation, in which the compact bodies don’t allow star birth to increase beyond a certain rate.

Schawinski and his colleagues limited their study to nearby galaxies because those are the only ones for which astronomers have high-resolution visible-light images, a prerequisite for accurately assessing and removing the central source of light produced by a supermassive black hole. To examine galaxies further back in time, the team will have to conduct the same type of study at longer, infrared wavelengths, since cosmic expansion shifts the light of more distant objects to longer wavelengths. Ultrasharp infrared images may not be available until astronauts install the Hubble Space Telescope’s Wide Field Camera 3, now slated for a May servicing mission.

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