Giant black hole seen flickering on and off after galaxy snack

Active Galactic Nuclei occur when a black hole devours a cloud of gas and dust and shines really brightly. Now one has been seen doing it twice

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A supermassive black hole has been spotted snacking – twice. This is the first time astronomers have caught one in the act and it could help us better understand odd cosmic objects known as Active Galactic Nuclei (AGN).

AGN occur when an enormous black hole – often with the mass of a billion suns – finds itself in the centre of a galaxy. As large clouds of gas and dust fall into the black hole, they spin around its edge and experience extreme forces and friction. Heated to millions of degrees, the material can shine brighter than all the other stars in the galaxy combined. But once the gas and dust get used up, the black hole goes dark.

Inactive AGNs sometimes show evidence of past meals – clouds of material that were shot out of the galactic centre along with the intense radiation, lingering for hundreds of thousands of years until they dissipate.

Now Julie Comerford of the University of Colorado, Boulder, and her colleagues have spotted two separate outbursts in a galaxy called J1354+1327, located about 900 million light years away.

The earlier eruption resulted in a patch of bright dust south of the galactic center that has spread into a feathery cone approximately 30,000 light years long. A later explosion generated a compact dome of material located about 1300 light years north of the black hole.

“It solidifies the way we thought AGN flickering works,” says Comerford.

Because all they see are snapshots of activity, astronomers have wondered if AGN events occur in long blasts that last tens of millions of years, or if they are more rapid fire. Comerford and her team estimate that the two outbursts happened roughly 100,000 years apart, relatively fast for cosmic processes, confirming results seen in some computer models.

The work fits with our simulations, says Travis Fischer of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who was not part of the study. “Having an actual set of data points of what are possible reignition times is very valuable.”

J1354+1327’s double snack attack was spotted because material from the later eruption is currently pummeling into interstellar gas and dust, generating bright shockwaves. Knowing how to now find this signature, the team hopes to discover other reigniting AGN.

J1354+1327 also has a smaller companion galaxy located about 40,000 light years from it. The two galaxies appear to be connected by a bridge of stars, suggesting that they previously collided, a long-suspected trigger for AGN activity. The interaction could have thrown material at the central black hole, powering one or more of its feeding periods.

Such results have significance closer to home. Our own Milky Way possesses a central supermassive black hole and enormous clouds of gas and dust extending far above and below its plane.

Known as Fermi bubbles, these structures were likely created by the black hole feeding at some point in the past. When the neighbouring Andromeda galaxy crashes into us in about 4 billion years, “maybe we’ll get lit up as an AGN again”, says Comerford.

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