At the center of most galaxies there’s a supermassive black hole, although not all supermassive black holes look the same. Some end up hidden, shrouded by a thick ring of dust, and astronomers have just discovered there is more than one way for these obscured AGNs (active galactic nuclei) to form.
Supermassive black holes can be obscured by a compact donut of material a few light-years across, or by a large disk of dust thousands of light-years across. The latter scenario was observed in a nearby galaxy called NGC 7582 and galaxy mergers are thought to be responsible for the larger dust rings. Researchers are presenting their observations at the 230th Meeting of the American Astronomical Society in Austin, Texas, this week.
NGC 7582 is a spiral galaxy 70 million light-years from Earth. It hosts an extreme and highly obscured AGN. Researchers have seen the AGN emit powerful winds, surrounded by a ring of dust 2,000 light-years in diameter. The ring is not just hiding the supermassive black hole from view, it’s also protecting the galaxies by focusing the black hole winds away from the galactic plane.
“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,” team leader Stephanie Juneau, from the National Optical Astronomy Observatory, said in a statement. “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.”
AGNs are often activated by galaxy mergers and this balancing act between supermassive black holes, winds, dust, and galaxies themselves suggest a complex feedback mechanism during these cosmic encounters. Astronomers think that NGC 7582 has been busy snacking on smaller companions and that formed the ring.
“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,” co-author Lisa Kewley added.
Obscured AGNs are also found in major mergers, when two galaxies of the same size collide, but minor collisions are more common so they might be responsible for a higher fraction of AGNs.
The international team of astronomers has now started a more ambitious project based on these results. They’ll look for more obscured AGNs, hoping to find out if the one likely produced by galaxy mergers are rare or common.