From ScienceNews: “Here’s what really happened to Hanny’s Voorwerp”

ScienceNews bloc


November 27, 2017
Lisa Grossman

Astronomers can finally explain a gas cloud’s strange glow.

GLOWING GAS Hanny’s Voorwerp, the greenish smudge at the bottom of this image, is glowing thanks to photons from a feasting black hole in the galaxy above. NASA, ESA, W. Keel (Univ. Alabama), et al., Galaxy Zoo Team.

The weird glowing blob of gas known as Hanny’s Voorwerp was a 10-year-old mystery. Now, Lia Sartori of ETH Zürich and colleagues have come to a two-pronged solution.

Hanny van Arkel, then a teacher in the Netherlands, discovered the strange bluish-green voorwerp, Dutch for “object,” in 2008 as she was categorizing pictures of galaxies as part of the Galaxy Zoo citizen science project.

Further observations showed that the voorwerp was a glowing cloud of gas that stretched some 100,000 light-years from the core of a massive nearby galaxy called IC 2497. The glow came from radiation emitted by an actively feeding black hole in the galaxy.

To excite the voorwerp’s glow, the black hole should have had the brightness of about 2.5 trillion suns; its radio emission, however, suggested the black hole emitted the equivalent of a relatively paltry 25,000 suns. Either the black hole was obscured by dust, or it stopped eating around 100,000 years ago, causing its brightness to plunge.

Sartori and colleagues made the first direct measurement of the black hole’s intrinsic brightness using NASA’s NuSTAR telescope, which observed IC 2497 in high-energy X-rays that cut through the dust.

NASA NuSTAR X-ray telescope

They found that, yes, the black hole is obscured by dust, and yes, it is dimmer than expected. The team reported on on November 20 that IC 2497’s heart is as bright as 50 billion to 100 billion suns, meaning it dropped in brightness by a factor of 50 in the past 100,000 years — a less dramatic drop than previously thought.

“Both hypotheses that we thought before are true,” Sartori says.

Sartori plans to analyze NuSTAR observations of other voorwerpjes to see if their galaxies’ black holes are also in the process of shutting down — or even booting up.

“If you look at these clouds, you get information on how the black hole was in the past,” she says. “So we have a way to study how the activity of supermassive black holes varies on superhuman time scales.”

See the full article here .

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