Can black hole belching teach us about galactic evolution?

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Mar 7, 2017

Supermassive black holes belch — to an extreme. They discharge winds at warp speed when the feeding frenzy on gas and debris from their accretion discs gets to be overkill. This discharge of gases is known as outflow, which is common crass behavior among the immense black holes gorging on star stuff in galactic centers. Astronomers have now observed the most intense outflow yet, and it could help explain the evolution of galaxies.

The massive belch in question is coming from galaxy IRAS 13224-3809. Its onslaught of gases was found to be hundreds of times faster than anything a black hole has ever burped before, with temperatures fluctuating so frequently it could only mean they were being influenced by the accretion disc's X-ray emissions. Outflow regulates the expansion and evolution of a galaxy by eliminating surrounding gas and suppressing the development of nascent stars. The extreme speed of the IRAS outflow could also make it possible to observe processes that would usually take months in only several hours. This makes it obvious why scientists are so interested in what a black hole disgorges after glutting itself.

"Although we have seen these outflows before, this observation was the first time we were able to see the launching of the gases being connected with changes in the luminosity of black holes," said University of Maryland postdoctoral astronomy researcher Erin Kara, a member of the IRAS research team and co-author of the recent study published in Nature.

X-ray emissions can reveal what a black hole spews out in gory detail since they pass through outflows on their way towards Earth. Using NASA's NuSTAR (Nuclear Spectroscopic Telescope Array) telescope and the European Space Agency's (ESA) XMM-Newton telescope, the researchers studied X-ray emissions originating at the edge of the IRAS black hole to determine both the temperature of the wind and which elements were being belched out with it. Absorption features, or irregularities in the X-ray signal that show up as telltale dips, indicate the presence of elements that absorb particular parts of the X-ray spectrum, which tells scientists what is floating around in an outflow.

What really opened the team's eyes (besides bad manners in space) was how the absorption features vanished and then materialized again within hours. It meant X-ray emissions were causing wind temperatures to skyrocket so high that no more X-rays could possibly be absorbed by the winds. The astronomers therefore inferred that extremely variable outflows such as that of IRAS 13224-3809 seem to be linked with X-rays. Such erratic behavior also suggests that both the winds and emissions emerge from somewhere near the black hole, and may help future studies identify the exact origin of both the X-rays and outflows. Even so, there is still plenty of dissection to do.

"We need to observe this black hole with better and more spectrometers, so we can get more details about these outflows," noted UMD Professor of Astronomy and co-project lead Chris Reynolds. "All that information will be crucial to understanding how these outflows are connected to galaxy formation."