Astronomers have witnessed a cosmic weather event that has never been seen before — a cluster of giant gas clouds raining in on the supermassive black hole at the center of a very distant giant elliptical galaxy.
Abell 2597 BCG: 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 millimeter-wavelength light emitted by electrons whizzing around powerful magnetic fields generated by the galaxy’s supermassive black hole. The shadow indicates that cold clouds of molecular gas are raining in on the black hole. Image credit: B. Saxton / NRAO / AUI / NSF / G. Tremblay et al. / NASA / ESA / Hubble / ALMA / ESO / NAOJ.
The observation marks the first direct evidence to support the theory that black holes feed on clouds of cold gas.
Using the Atacama Large Millimeter/submillimeter Array (ALMA), the astronomers found that the feeding process was ‘chaotic’ and ‘clumpy,’ as opposed to a smooth, simple and clean process that was previously hypothesized.
“It was magical being able to see evidence of these clouds accreting onto the supermassive black hole,” said co-author Dr. Timothy Davis of Cardiff University.
“At that very moment, nature gave us a clear view of this complicated process, allowing us to understand supermassive black holes in a way that has never been possible before.”
“The data has provided us with a snapshot of what is happening around the black hole at one precise time, so it’s possible that the black hole has an ever bigger appetite and is devouring even more of these cold clouds of gas surrounding it.”
Previous models have suggested that the gradual growth of supermassive black holes – a process known as accretion – occurs when surrounding hot gas accumulates smoothly onto the black hole, much like a slow graze.
But the very first observations made by the astronomers suggest that in addition to this, supermassive black holes may occasionally quickly gobble up faster-moving cold gas as it comes nearby.
“This diffuse, hot gas is available to the black hole at a low level all the time, and you can have a steady trickle of it going in,” said co-author Prof. Michael Macdonald, from MIT’s Kavli Institute for Astrophysics and Space Research.
“Every now and then, you can have a rainstorm with all these droplets of cold gas, and for a short amount of time, the black hole’s eating very quickly. So the idea that there are these two dinner modes for black holes is a pretty nice result.”
The astronomers used ALMA to peer into Abell 2597, a group of galaxies about one billion light-years away.
At its core is Abell 2597 BCG (Brightest Central Galaxy). It spans some tens of thousands of light-years across and is one of the brightest in the Universe.
Suffusing the space between Abell 2597’s galaxies is a diffuse atmosphere of hot ionized gas, which was previously observed with NASA’s Chandra X-ray Observatory.
Deep in the heart of Abell 2597 BCG, astronomers see a small cluster of giant gas clouds raining in on the central supermassive black hole. They were revealed by the billion light-year-long shadows they cast toward Earth. These data present the first observational evidence for predicted chaotic cold accretion onto a supermassive black hole. Image credit: NRAO / AUI / NSF / Dana Berry / SkyWorks / ALMA / ESO / NAOJ.
The scientists were interested in discerning how many stars were being born in Abell 2597 BCG and therefore went about measuring cold gas – stars are formed when cold gas collapses.
“This very, very hot gas can quickly cool, condense, and precipitate in much the same way that warm, humid air in Earth’s atmosphere can spawn rain clouds and precipitation,” said lead author Dr. Grant Tremblay, from Yale University.
“The newly condensed clouds then rain in on the galaxy, fueling star formation and feeding its supermassive black hole.”
Near the center of Abell 2597 BCG the team discovered just this scenario: three massive clumps of cold gas are careening toward the supermassive black hole in the galaxy’s core at about 800,000 miles per hour.
Each cloud contains as much material as a million Suns and is tens of light-years across.
Normally, objects on that scale would be difficult to distinguish at these cosmic distances, even with ALMA’s amazing resolution. They were revealed, however, by the billion-light-year-long ‘shadows’ they cast toward Earth
“We got very lucky. We could probably look at 100 galaxies like this and not see what we saw just by chance,” Prof. MacDonald said.
“Seeing three shadows at once is like discovering not just one exoplanet, but three in the first try. Nature was very kind in this case.”
The results will appear in the journal Nature on June 9, 2016.
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Grant R. Tremblay et al. Cold, clumpy accretion onto an active supermassive black hole. Nature, published online June 9, 2016
