Black holes really do get a bad rap. For their latest “crime,” astronomers at the University of Nottingham and Imperial College London have described how supermassive black holes can inhibit star formation in their host galaxies.
As far as we can tell, all massive galaxies have a supermassive black hole at the centers. Moreover, the masses of the black hole and the host galaxy seem to be correlated, so even though the black hole is many times smaller than the entire galaxy, they seem to “know” about each other. Thus their evolution may be intimately related.
Black holes grow by accreting matter onto them with their strong gravitational pull. When gas is falling in at a high enough rate, it forms an accretion disk around the black hole that shines brightly, generating radiation across all wavelengths. At this point, when the black hole is feeding, we call it an ‘active galactic nucleus.’
New observations, led by Asa Bluck, characterize the x-ray output of active galactic nuclei and find them to be the main generators of x-ray radiation in galaxies, despite being a tiny fraction of their host galaxies size. In fact, enough energy is spewed forth to strip the galaxies of all their star-producing gas 25-times over!
And so, once again, black holes are denigrated as killers. Not only can they disturb the star-forming ability of their host galaxies, but this very mechanism can shut down the accretion of matter onto the black hole itself, eventually turning off the “active phase.”
However, I have to stand up for these black holes. It has also been shown that supermassive black hole accretion often accompanies extreme periods of star formation in galaxies, called starbursts. The same inflow of gas that feeds the black hole simultaneously powers the birth of many, many stars. The largest of these stars die in incredible supernova explosions, which can also cutoff future star formation.
Active galaxies and star formation are really complex, and sometimes messy, processes. I wouldn’t dump all the blame for shutting down star formation on supermassive black holes, as powerful as they may be. They are just playing their part in the complicated and fascinating evolution of galaxies.
Image: Artist’s impression of an active galactic nucleus (NASA)