Cosmic Blowout: Black holes spew as much as they consume

Notorious for gorging on matter, the supermassive black holes at the cores of galaxies may blow out as much material as they swallow. A study reported last week suggests that during the roughly 10 billion years that these black holes power the brilliant light beacons known as quasars, they also generate high-speed winds that eject an amount of gas equivalent to billions of suns. These winds, which contain oxygen, carbon, and iron, may seed the black hole’s host galaxy–and possibly intergalactic space–with the elements necessary for life.

WIND MACHINE. Illustration of a massive black hole flanked by swirling gas (green). The disk’s intense heat propels some of the gas in the top and bottom layers to lift off. X rays generated near the disk’s inner edge then force this gas into cone-shaped winds (red) and accelerate it. Chandra/M. Weiss

WIND MACHINE. Illustration of a massive black hole flanked by swirling gas (green). The disk’s intense heat propels some of the gas in the top and bottom layers to lift off. X rays generated near the disk’s inner edge then force this gas into cone-shaped winds (red) and accelerate it.
Chandra/M. Weiss

The winds travel at 20 to 40 percent of the speed of light and demonstrate that black holes, though they comprise only one-thousandth of a galaxy’s mass, “can exert a profound influence on galaxy evolution,” comments theorist Mitchell C. Begelman of the University of Colorado in Boulder.

In the study, George Chartas of Pennsylvania State University in State College and his colleagues examined the X-ray spectra of two quasars. Each quasar’s light emanates from a rotating black hole. For gas to spiral into a spinning black hole and fuel a quasar, it must first form a rotating disk that permits material to move inward while radiating energy. The hotter, inner part of the disk radiates mostly X rays, while the cooler, outer portion radiates less-energetic, ultraviolet light.

Researchers had already observed that ions in the outer part of disks absorb some of the ultraviolet light. This process accelerates the ions outward in a wind that ejects annually as much material from a black hole as the sun contains.

Astronomers had inferred the existence of a more energetic wind generated by inner-disk ions that absorb X rays but had no direct evidence for it.

In the new observations, reported at a meeting of the American Astronomical Society in Mont Tremblant, Quebec, Chartas’ group took advantage of a type of gravitational mirage.

A cluster of galaxies lying between each quasar and Earth creates the mirage. Each cluster acts as a magnifying lens, greatly brightening a quasar’s light. Studying the amplified light of quasar APM 08279+525 with NASA’s Chandra X-ray Observatory and quasar PG1115+080 with the European Space Agency’s XMM-Newton satellite, Chartas’ team detected a wind of inner-disk ions. The observations mark the first time that astronomers have found a wind from a black hole’s inner disk transporting oxygen, carbon, and iron ions into space. These elements are among the building blocks of future star generations and of life.

Begelman suspects that such winds could be widespread among black holes, not just those with quasars. He suggests that when hard-to-detect black holes, like the one at the Milky Way’s center, gorge on gas, they convert most of their radiation into superfast winds instead of producing fireworks.

Black hole winds may be linked to another type of galactic wind. Last year, researchers reported that many galaxies in the early universe generated winds that blew material from one galaxy to another (SN: 4/20/02, p. 244: Available to subscribers at Cosmic Remodeling: Superwinds star in early universe). Slower than the black hole winds but covering a wider area, these winds were probably driven by a string of supernovas, the explosions of massive stars. Tom Abel of Penn State, not a member of Chartas’ group, speculates that supernova-driven winds were so efficient because winds from a supermassive black hole had already cleared a path through the galaxy.

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