Blowing the Cover of a Hidden Black Hole

5 June 2017

A close look at the center of a nearby galaxy has revealed a surprising connection between the galaxy and the supermassive black hole at its center. The galaxy is an extreme example of an “obscured AGN”, an accreting supermassive black hole that is deeply enshrouded in gas and dust.

Observations made by an international team of astronomers show that the black hole launches a powerful wind, which is confined by a component of the galaxy, a rotating ring of gas and dust 2000 light years in diameter. The team, led by NOAO astronomer Stephanie Juneau, suspects that the ring also contributes to the extreme obscuration of the black hole. The results — which Juneau is presenting at this week’s meeting of the American Astronomical Society in Austin, Texas — lend new insights into the interaction between black holes and their host galaxies.

Supermassive black holes are believed to lurk at the centers of essentially all massive galaxies, growing in mass as they feed on gas and stars from their surroundings. Analyses of the cosmic X-ray background, which records the integrated feeding history of black holes, point to the existence of a mysterious population of “obscured AGN”, actively feeding black holes that are deeply enshrouded in gas and dust.

Why are these black holes so obscured? Is the obscuration related to the evolution of the black hole itself or to the evolution of its host galaxy? Understanding the nature of the obscuring material is one way to find out.

New observations of the nearby galaxy NGC 7582, which hosts an extreme, highly obscured AGN, reveal the existence of a powerful wind (seen in green in the image) launched by its active black hole, which is hidden behind a rotating ring of gas and dust 2000 light years in diameter at the center of the galaxy. In addition to obscuring the black hole, the ring also appears to play a role in focusing or collimating the wind from the black hole. The observations were made with the Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope (VLT).

The surprising results show that highly obscured AGN can arise in more than one way. While it is commonly believed that extreme obscuration can only be produced by a compact torus (less than a few light years across) in the immediate vicinity of the black hole, the new results show that large amounts of dust on a much larger scale, in the host galaxy itself, can also help to blot out the black hole.

The existence of thick, dusty structures on a galaxy scale is not all that difficult to understand. Coauthor Lisa Kewley explains, “The obscuring ring may result from the merger of the galaxy with a much smaller companion, perhaps 10 times less massive. Simulations of such ‘minor mergers’ can produce rings embedded in an otherwise normal-looking galaxy.”

Obscured AGN have also been found in galaxies that have experienced a major merger (the merger of two Milky Way-like galaxies) and which appear highly disturbed as a result. With minor mergers occurring much more commonly than major mergers in the evolutionary histories of galaxies, it seems plausible that they could be responsible for a fair fraction of highly obscured AGN.

The results also show that galaxies can play a role in focusing the winds from black holes. Juneau muses, “Here we have a hungry black hole at the center of this galaxy that is also blasting out a powerful, potentially destructive wind into its surroundings. Luckily the galaxy is able to contain the outflow and direct much of the energy out of the galaxy. It’s a clever survival strategy, a way for the galaxy to protect itself.”

With this important path-finding observation under their collective belt, the research team is now investigating other obscured AGN, to study the nature of their obscuring material and to determine whether the results found here are common or rare.

More information

National Optical Astronomy Observatory is operated by the Association of Universities for Research in Astronomy Inc. (AURA) under a cooperative agreement with the National Science Foundation.

The Very Large Telescope (VLT) is operated by the European Southern Observatory (ESO).

Update February 2022: The findings are published in the Astrophysical Journal.

Contacts

Dr. Stephanie Juneau
National Optical Astronomy Observatory
950 N Cherry Ave, Tucson AZ 85719, USA
Email: stephanie.juneau@noirlab.edu