Supermassive black hole spotted devouring surprising meal

Scientists have directly observed a supermassive black hole feeding for the first time – and learned it isn't eating the kind of meal that they had assumed.

Black hole consumes cold, clumpy clouds of gas — not the hot plasma scientists had expected

Cold molecular gas clouds condense out of the hot, ionized gas that suffuses the space between the galaxies in this cluster in an artist's conception. New ALMA data show that these clouds are raining in on the galaxy, plunging toward the supermassive black hole at its centre. (NRAO/AUI/NSF; Dana Berry / SkyWorks; ALMA (ESO/NAOJ/NRAO))
Scientists have directly observed a supermassive black hole feeding for the first time — and they learned it isn't eating the kind of meal that they had assumed.

The supermassive black hole at the centre of the Abell 2497 Brightest Cluster Galaxy doesn't appear to favour hot meals as previously expected. Instead, it's dining on very cold clouds of molecular hydrogen, with dashes of carbon monoxide and other "impurities" thrown in, reports a new study published today in the journal Nature by an international team of scientists, including several Canadians.

"It's surprising, because this gas is very, very cold and very, very clumpy," Grant Tremblay, lead author of the study, told CBC News.

"State of the art theory and simulation always assume that the black hole feeds from a smooth, spherical inflow of very, very hot plasma … and our observations are totally inconsistent with this."

That means astrophysicists may need to revise their theories about how black holes interact with the massive galaxies that surround them, and their theories about how such massive galaxies evolve, added Tremblay, who is an Einstein Fellow at Yale University.

Enormous galaxy

Abell 2497 Brightest Cluster Galaxy is the biggest galaxy in a huge cluster of up to 100 galaxies located about 1.2 billion light years away.

"It is an absolutely enormous galaxy," Tremblay said. "It's about 300,000 light years across." That's about three times wider than our own Milky Way galaxy.

Abell 2497 is located in a place in the night sky that would be visible from Canada, but is so far away that it can only be seen with powerful telescopes, even though it is extremely bright.

University of Manitoba astrophysicists Christopher O'Dea and Stefi Baum, Tremblay's former PhD supervisors and co-authors of the new paper, first reported hints of hydrogen gas clouds falling into the black hole at the centre of Abell 2497 in 1994.

Tremblay and a group of collaborators recently made new observations of the galaxy and its black hole with the Atacama Large Millimeter/submillimeter Array (ALMA) telescope in Chile.

'Big, bright light bulb'

The telescope is designed to see radio waves. The brightest source of radio waves in Abell 2497 is the black hole itself, which generates an intense magnetic field. The electrons spiralling around that magnetic field generate huge quantities of radio waves visible to ALMA.

This composite image shows what Abell 2597 Brightest Cluster Galaxy actually looks like through a telescope. The background image (blue) is from the NASA/ESA Hubble Space Telescope. The foreground (red) is ALMA data showing the distribution of carbon monoxide gas in and around the galaxy. The pull-out box shows the ALMA data of the "shadow" (black) produced by absorption of the millimetre-wavelength light emitted by electrons whizzing around powerful magnetic fields generated by the galaxy's supermassive black hole. (B. Saxton (NRAO/AUI/NSF)/G. Tremblay et al./NASA/ESA Hubble/ALMA (ESO/NAOJ/NRAO))

"What we see is a big, bright light bulb," Tremblay said.

While Abell 2497 and its galaxy cluster sit in a giant ball of hot gas millions of light years across, near the centre of the galaxy are patches of gas that have cooled down into cold, clumpy clouds.

Tremblay said ALMA is also very sensitive to radio waves from cold, molecular gases. It can't actually see hydrogen, which makes up the bulk of the cold clouds scattered through the centre of a galaxy, but it can see carbon monoxide, an "impurity" whose presence indicates that the gas clouds are very cold — less than -129 C.

The carbon monoxide in the clouds cast characteristic "shadows" of certain colours in the presence of the bright light from the black hole, allowing the carbon monoxide to be detected, Tremblay said.

A physics phenomenon called the Doppler effect means those colours shift relative to the normal colour of carbon monoxide shadows depending on whether the clouds are moving toward or away from you. It's very similar to the way the pitch of an ambulance siren gets higher or lower depending on whether the ambulance is moving toward or away from you.

Deep in the heart of the Abell 2597 Brightest Cluster Galaxy, astronomers see a small cluster of giant gas clouds raining in on the central black hole, as portrayed in this artist's conception. They were revealed by the billion light-year-long shadows they cast toward Earth. These ALMA data present the first observational evidence for predicted "chaotic, cold" accretion on a supermassive black hole. (NRAO/AUI/NSF; Dana Berry / SkyWorks; ALMA (ESO/NAOJ/NRAO))

The shift in colours indicate that the clumps of carbon monoxide are moving toward the black hole and away from Earth at about 300 kilometres per second.

O'Dea said those colour shifts are "the definite proof that something is falling into the black hole."

Tremblay agreed that the observations show the gas "will eventually become fuel for the black hole within a very short period of time, if not almost immediately. The black hole will ultimately consume this gas, without a doubt."

Tremblay said the researchers have calculated that there are enough cold gas clouds in the galaxy that the black hole can be fuelled with nothing but cold gas.

No evidence of hot meals

Meanwhile, no one has ever observed a black hole consuming hot gas, despite all the theories that they do, O'Dea said. He suggested that the hot gas theory was preferred because it was mathematically simpler to calculate and simulate what was going on with hot gas than cold gas.

"The infall of cold gas is very messy — it's lumpy, it comes in asymmetrically, you have to dissipate energy," he said.

On the other hand, he added, astrophysicists have long known that cold gas clouds existed near black holes.

"We see it in the nuclei of galaxies, we see the cold gas forming stars. It's sort of been there all along, and now we just now are realizing that it's falling into the black hole and providing the energy for the activity that we see."