Black holes are binge eaters

Artist's impression of a supermassive black hole at the core of a galaxy

Artist's impression of a supermassive black hole at the core of a galaxy. Gas clouds intermittently falling into the black hole release lots of energy and cause stars to form, leading to an "active galactic nucleus" or AGN.

  • Galaxies form through the merger of gas clouds
  • Some gas clouds are still coming in, feeding black holes
  • Galactic cores light up when the black holes are “fed”

An on-again, off-again deluge of gas clouds onto giant black holes, could explain why some galaxy cores light up in spectacular fashion.

Galaxies like our own are thought to have formed billions of years ago through the merger of giant clouds of gas, some of which continue to “rain down” onto galaxies from the outside.

Now, new calculations connect the rain of giant clouds of gas with active galactic nuclei (AGN), the extremely bright centres of some galaxies.

If a gas cloud with millions of times more mass than our Sun gets too close to the centre of a galaxy, it can either be consumed by the supermassive black hole that lurks there or, through shockwaves and gravitational collapse, give birth to new stars.

“For a while, people have known that gas clouds are falling onto galaxies, and they’ve also known that active galactic nuclei are powered by gas falling onto supermassive black holes,” says Barry McKernan, a research associate in the Department of Astrophysics at the American Museum of Natural History and an assistant professor at the Borough of Manhattan Community College (BMCC), City University of New York.

“But no one put the two ideas together until now and said, ‘Hey, maybe one is causing the other!'”

It’s thought that every galaxy hosts a supermassive black hole at its centre, yet only a fraction of galactic centres show signs of brighter activity due to black hole “feeding”.

The new research provides an explanation for the apparent conundrum—galactic centres that have sustained recent gas cloud impacts have enough fuel to light up by giving birth to hundreds of stars and feeding the central black hole.

Galactic centres that have not been hit for a while (in cosmic terms that means more than about 10 million years) will be relatively inactive and their cores will appear normal.

“It’s interesting that only some galaxies are active, even though we think every galaxy contains a supermassive black hole,” says K.E. Saavik Ford, a research associate at the Museum and an assistant professor at BMCC.

“The cloud bombardment idea provides an explanation—it’s just random luck.”

Adapted from information issued by the American Museum of Natural History.

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