Too big for its shoes

Just how big can black holes, those much maligned cosmic demons, grow? An Indian astrophysicist at Yale University seems to have found the answer to that.

Priyamvada Natarajan, an associate professor of astronomy and physics at the university and a fellow at the Radcliffe Institute, Harvard, along with co-worker Ezequiel Treister, a postdoctoral fellow at the University of Hawaii, have shown that these all-gobbling cosmic pits appear to curb their own growth — once they accumulate about 10 billion times the mass of the Sun¹. This is the first time someone has worked out an upper limit for the growth of black holes.

The duo used existing optical and X-ray data on these ultra-massive black holes to show that black holes essentially shut themselves off at some point in their evolution. The explanation behind this, they say, could be that black holes eventually reach a point when they radiate so much energy while consuming their surroundings that they end up interfering with the very gas supply that feeds them. This may interrupt nearby star formation.

Natarajan, known in her peer group as Priya, was born and brought up in India and finished high school at the Delhi Public School before moving to the Massachusetts Institute of Technology on a scholarship for undergraduate studies. After a PhD from Trinity College, Cambridge, UK, she was elected to a Title A research fellowship, the first female astrophysicist to get the honour.

"This result has very important implications for our understanding of galaxy and black hole formation and evolution in the Universe," she told Nature India.

Natarajan, whose interests lie in mapping dark matter and dark energy in the universe and black hole physics says black holes in the centers of galaxies appear not only to regulate their own growth but also suppress the formation of stars in the vicinity as they stunt themselves. "This work also has important consequences for the most distant accreting black holes that we are likely to detect," she said.

Black holes are known to exist throughout the universe, with the largest and most massive found at the centers of the largest galaxies. These 'ultra-massive' black holes have been shown to have masses upwards of one billion times that of our own Sun. The new findings have implications for the future study of galaxy formation, since many of the largest galaxies in the universe appear to co-evolve along with the black holes at their centers.

"Evidence has been mounting for the key role that black holes play in the process of galaxy formation," Natarajan said in a Yale University release. "But it now appears that they are likely the prima donnas of this space opera."

Ultra-massive black holes, found at the centers of giant elliptical galaxies in huge galaxy clusters, are the biggest in the known universe. Even the large black hole at the center of our own 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 neighboring gas, dust and stars, seem unable to grow beyond this limit regardless of where — and when — they appear in the universe. "It's not just happening today," Natarajan said. "They shut off at every epoch in the universe."