Supermassive Black Holes Grow Faster Than Galaxies, Says Nasa

Nasa astronomers have found that supermassive black holes grow faster than galaxies themselves. They discovered this with the Nasa's Chandra X-ray Observatory.

Earlier, astronomers believed that super massive black holes and the bulge of stars at the centre of the host galaxy grow at the same rate - the bigger the bulge, the bigger the black hole. Now, astronomers have discovered that black holes grow much faster than the galaxies.

Scientists discovered this when they were studying two galaxies NGC 4342 and NGC 4291. NGC 4342 and NGC 4291 are close to earth in cosmic terms, at distances of 75 million and 85 million light years.

During the study, scientists were analysing the mass of the black hole and the mass of the galaxy in NGC 4342 and NGC 4291. They were stunned to find that the black holes were 10 to 35 times more massive than the galaxy.

Astronomers believe that two supermassive black holes and their evolution are tied to their dark matter halos and did not grow in tandem with the galactic bulges. In this view, the black holes and dark matter halos are not overweight, but the total mass in the galaxies is too low.

"This gives us more evidence of a link between two of the most mysterious and darkest phenomena in astrophysics - black holes and dark matter - in these galaxies," said Akos Bogdan, scientist at the Harvard-Smithsonian Centre for Astrophysics (CfA), in a statement.

Astronomers earlier believed that the main reason behind this loss in mass of the galaxies was tidal stripping. Tidal stripping occurs when some of a galaxy's stars are stripped away by gravity during a close encounter with another galaxy.

If such tidal stripping had taken place, the halos mostly would have been missing because dark matter extends farther away from the galaxies; it is more loosely tied to them than the stars and more likely to be pulled away.

To find out about tidal stripping, astronomers used Chandra to look for evidence of hot, X-ray-emitting gas around the two galaxies. They found that the hot gas was distributed widely around NGC 4342 and NGC 4291, implying that each galaxy has an unusually massive dark matter halo and that tidal stripping is unlikely.

"This is the clearest evidence we have, in the nearby universe, for black holes growing faster than their host galaxy," said Bill Forman, astronomer at the Harvard-Smithsonian Center for Astrophysics. "It's not that the galaxies have been compromised by close encounters, but instead they had some sort of arrested development."

Then how can the mass of a black hole grow faster than the stellar mass of its host galaxy?

According to Nasa astronomers, a large concentration of gas spinning slowly in the galactic centre is what the black hole consumes. The blackhole grows quickly compared to galaxy, and as it grows, the amount of gas it can accrete, or swallow, increases along with the energy output from the accretion. So there by it leads to increase in the mass of the black hole. Once the black hole reaches a critical mass, outbursts powered by the continued consumption of gas prevent cooling and limit the production of new stars.

So this is probably the reason as to why black holes have more mass than their galaxies

"It's possible that the supermassive black hole reached a hefty size before there were many stars at all in the galaxy," said Bogdan. "That is a significant change in our way of thinking about how galaxies and black holes evolve together."