This composite image shows the hot gas around the elliptical galaxy M84 in an X-ray image (blue) and a radio image (red). The bubbles (some visible in the hot gas) were generated by jets coming out of M84's central black hole. The top bubble (blue) is bursting and the hot relativistic gas, shown in red, is spilling out.
Credit: X-ray (NASA/CXC/MPE/A.Finoguenov et al.); Radio (NSF/NRAO/VLA/ESO/R.A.Laing et al); Optical (SDSS)
Like cosmic bubble makers, some black holes spew out behemoth blobs of hot gas into their home galaxies.
The bubbles ultimately pop, and their gassy contents keep both the black hole and its galaxy from ballooning to mega sizes, a new study finds.
The results apply to elliptical galaxies and their supermassive black holes, which can weigh as much as a billion suns or more. Our galaxy, the Milky Way, is a spiral galaxy. And while it houses a supermassive black hole, the researchers say the same process might not apply to it.
Black hole bubbles
The researchers focused on the supermassive black hole at the center of the elliptical galaxy M84, which is about 55 million light-years from Earth. (A light-year is the distance light will travel in a year, or about 6 trillion miles, or 10 trillion km.) They combined data collected by NASA?s Chandra X-Ray Observatory and results from a black-hole computer simulation.
They noticed huge bubbles, or cavities, of hot plasma (ionized gas) rising up from the tips of the black hole's pair of laser-like jets. (As material falls into the gravitational clutches of a black hole, the energy can be spit out as jets of radiation and high-speed particles.) They estimate the bubbles are about 13,000 light-years across and they are launched from jets about every 10 million years.
The X-ray images showed that, like Russian dolls, each bubble has a smaller bubble tucked inside of it and so on. When the outer bubble bursts, spilling its gaseous guts, there's another inside waiting to pop as well. That continuous bubble-popping provides a constant input of heat into the surrounding interstellar gas.
"We think certain instabilities are formed on the interface between the bubble and the surrounding medium and these instabilities shred and puncture this bubble, and the stuff that is inside them, this hot plasma, is spilling out and mixing with the surrounding gas," said researcher Mateusz Ruszkowski, an astronomer at the University of Michigan.
The jolts of heat stem the food supply to the central black hole and slow down star formation nearby.
Over time, black holes grow in heft as their gravity pulls in surrounding gases. Because cool gas is denser, it sinks to the center of galaxies ? and toward the black hole ? faster. If the gas around the black hole is kept warm, it sinks toward the black hole at a slower rate.
"In this way, you can feed the black hole and add more and more mass to it," Ruszkowski told SPACE.com. "If there's no mechanism to prevent the cooling that is essentially triggering this feeding process then the black hole would grow in an uncontrollable fashion."
But, he added, "nobody in the field thinks this is happening," he said. The new results, which are detailed in the Oct. 20 issue of Astrophysical Journal, reveal a mechanism for continuous heating of the interstellar material, he said.
A similar mechanism keeps star formation in check and in turn the mass of the home galaxy.
Stars are thought to form as dense clouds of gas and dust collapse under their gravity. Over time, the material heats up and ultimately the tight bundle becomes a full-fledged star powered by thermonuclear fusion of hydrogen and other light elements in its core.
The cooler the material, the more likely the clumps of gas and dust will succumb to the force of gravity and collapse into luminous stars.
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