New Haven: There may be a limit to how big the universe’s biggest black holes can get, says research led by a Yale University astrophysicist of Indian origin.

They were once considered rare and exotic objects, but black holes are now known to exist throughout the universe. The largest and most massive ones among them are found at the centre of the largest galaxies. These “ultra-massive” black holes have been shown to have masses upwards of one billion times that of the sun.

Priyamvada Natarajan, an Associate Professor of astronomy and physics at Yale University and a fellow at the Radcliffe Institute for Advanced Study, and Ezequiel Treister, a postdoctoral fellow at the University of Hawaii, have now shown that even the biggest of these gravitational monsters cannot keep growing forever. Instead, once they accumulate about 10 billion times the mass of the sun they appear to curb their own growth.

These ultra-massive black holes, found at the centre of giant elliptical galaxies in huge galaxy clusters, are the biggest in the known universe. Even the large black hole at the centre of the Milky Way galaxy is thousands of times less massive than these behemoths. But these gigantic black holes, which accumulate mass by sucking in matter from neighbouring gas, dust and stars, seem unable to grow beyond this limit regardless of where, and when, they appear in the universe.

The study, appearing in the Monthly Notices of the Royal Astronomical Society(MNRAS), represents the first time an upper mass limit has been derived for black holes. Dr. Natarajan and Dr. Treister used available optical and X-ray data of the ultra-massive black holes to show that, in order for those various observations to be consistent the black holes must essentially shut off at some point in their evolution.

One possible explanation Dr. Natarajan has put forth: the black holes eventually reach the point when they radiate so much energy as they consume their surroundings that they end up interfering with the very gas supply that feeds them, which may interrupt nearby star formation. The findings have implications for the study of galaxy formation.

“Evidence has been mounting for the key role that black holes play in the process of galaxy formation,” said Dr. Natarajan. “But it now appears that they are likely [to be] the prima donnas of this space opera.”