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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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Bursting ‘bubbles’ give our Galaxy gas

The regions of our Galaxy the researchers studied

The regions of our Galaxy the researchers studied. More gas clouds were found in the region on the right than in the region on the left.

  • 650 Milky Way gas clouds studied
  • Each contains 700 times the mass of the Sun
  • Clouds might recycle gas in and out of the Galaxy

Like bubbles bursting on the surface of a glass of champagne, ‘bubbles’ in our Galaxy burst and leave ‘flecks’ of material in the form of clouds of hydrogen gas, researchers using CSIRO’s Parkes telescope have found.

Their study explains the origin of these clouds for the first time.

Swinburne University PhD student Alyson Ford (now at the University of Michigan) and her supervisors; Dr Naomi McClure-Griffiths (CSIRO Astronomy and Space Science) and Felix Lockman (US National Radio Astronomy Observatory), have made the first detailed observations of ‘halo’ gas clouds in our Galaxy.

Just as Earth has an atmosphere, the main starry disc of our Galaxy is surrounded by a thinner halo of stars, gas and ‘dark matter’.

The Parkes radio telescope

The Parkes radio telescope

The halo clouds skim the surface of our Galaxy, sitting 400 to 10,000 light-years outside the Galactic disc. They are big — an average-sized cloud contains hydrogen gas 700 times the mass of the Sun and is about 200 light-years across.

“We’re studying the clouds to understand what role they play in recycling material between the disc and halo,” Dr McClure-Griffiths said.

“The clouds can fall back down into the main body of the Galaxy, returning gas to it.”

Gas is “spritzing” up our Galaxy

The researchers studied about 650 clouds and found striking differences between them in different areas of the Galaxy. One part of the Galaxy had three times as many clouds as another next to it, and the clouds were twice as thick.

The region with lots of thick clouds is where lots of stars form, while the region with fewer clouds also forms fewer stars.

An image made with the Parkes radio telescope of some of the 'halo clouds' above the main body of our Galaxy.

An image made with the Parkes radio telescope of some of the 'halo clouds' above the main body of our Galaxy.

But the halo clouds aren’t found exactly where stars are forming right now. Instead, they seem to be linked to earlier star formation.

Massive stars grow old quickly. After a few million years they shed material into space as a ’wind‘ and then explode.

This violence creates bubbles in the gas in space, like the holes in a Swiss cheese.

“Stellar winds and explosions sweep up gas from the Galactic disc into the lower halo.

“We’ve found this churned-up gas is ‘spritzing’ the surface of the Galactic disc in the form of halo clouds.”

A star-forming region is active for less than a million years, but a super-bubble in the Galaxy takes 20 or 30 million years to form.

“Just as yeast takes a while to make wine bubbly, stars take a while to make the Galaxy bubbly,” Dr McClure-Griffiths said.

The halo clouds are distinct from a larger population of ‘high-velocity clouds’ that also sail outside the galaxy. The halo clouds move in tandem with the rotating Galaxy, while the high-velocity clouds scud along much faster.

This study is the first to accurately locate the halo clouds in relation to the main body of the Galaxy. Its findings were presented overnight at a news conference at a meeting of the American Astronomical Society in Miami, Florida.

Adapted from information issued by CSIRO / A Ford (U. Michigan), N. McClure-Griffiths (CSIRO Astronomy and Space Science) / NASA / JPL-Caltech / David McClenaghan.