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 illustrated in this artist concept image. Credit: NRAO/AUI/NSF; D. Berry/SkyWorks; ALMA (ESO/NAOJ/NRAO)
Findings suggest 2 dining styles for black holes. For the first time, astronomers have detected billowy clouds of cold, clumpy gas streaming toward a black hole, at the center of a massive galaxy cluster. The clouds are traveling at up to 355km/s and may be only 150 light years away from its edge, almost certain to fall into the black hole. The observations represent the first direct evidence to support the hypothesis that black holes feed on clouds of cold gas. The results suggest fueling a black hole – accretion – is a whole lot more chaotic, clumpy than thought.
Given new observations, black holes probably have 2 ways of feeding: For most of the time, they may slowly graze on a steady diet of diffuse hot gas. Once in a while, they may quickly gobble up clumps of 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,” Assistant Prof McDonald says. “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.”
The researchers made their detection using ALMA telescope, designed to see the oldest, most distant galaxies in the universe. The team focused ALMA’s telescopes 1 billion light years away, on the central galaxy in the Abell 2597 Cluster, a galaxy 10s of 1000s of light years across. This particular galaxy is among the brightest in the universe, as it is likely producing many new stars.
The team originally wanted to get a sense for how many stars this cluster was churning out, so they mapped all the cold gas within the cluster. This cold gas has cooled and condensed out of the diffuse halo of hot gas surrounding a cluster, forming clumps. It is the collapse of cold gas that creates new stars, especially in the cluster’s central galaxy. “In the center of a cluster, there’s a single massive galaxy, the big daddy galaxy of the cluster,” McDonald says. “It’s sitting at the bottom of a gravitational funnel, and all the gas from a thousand galaxies is available to it. These are the galaxies that are the most massive, with the most massive black holes in the universe, and the most potential for star formation.”
The researchers used ALMA to map the spectral signatures, or radio emissions, from the galaxy cluster, looking specifically for CO the presence of which usually indicates very cold gas, -200F. They mapped carbon monoxide across the entire galaxy cluster and found that as they looked further into the cluster, they encountered progressively cooler gas, from millions of degrees Fahrenheit to subzero temperatures.
At the very center, just at the edge of the cluster’s supermassive black hole, the researchers discovered something quite unexpected: the shadows of 3 very cold, very clumpy gas clouds. The shadows were cast against bright jets of material spewing from the black hole, suggesting that these clouds were very close to being consumed by the black hole.
The team estimated the velocities of the 3 clouds to be 240, 275, and 355 km/s, with all 3 headed toward the black hole. They will likely be absorbed into its accretion disc. While ALMA was only able to see 3 clouds near the black hole, there may be even more in the vicinity, setting the black hole up for quite a feast.
http://news.mit.edu/2016/scientists-observe-supermassive-black-hole-feeding-cold-gas-0608
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