First Evidence That Active Galaxies Could All Be Powered In Same Way

Artist's impression of the dusty torus around an active supermassive black hole. Bill Saxton, NRAO/AUI/NSF

Certain galaxies are known to have an AGN, an active galactic nucleus. The supermassive black holes at their cores gobble up matter at such a rate that it begins to radiate in all the wavelengths of the electromagnetic spectrum.

Astronomers have seen several different classes of AGNs, however, they have long believed that there is actually just a single type, with the differences being down to observing the systems from different angles. One important characteristic of this unified model is the presence of a “dusty torus”, a thick donut of material that surrounds the black hole and stops part of the radiation.

In a study soon to be published in The Astrophysical Journal Letters, the team observed a dusty torus in Cygnus A, a galaxy some 760 million light-years from Earth. The AGN in Cygnus A is extremely bright so the presence of a torus suggests that we would see the galaxy very differently if we weren’t placed where we currently are.

“The torus is an essential part of the AGN phenomenon, and evidence exists for such structures in nearby AGNs of lower luminosity, but we’ve never before directly seen one in such a brightly emitting radio galaxy,” lead author Chris Carilli, of the National Radio Astronomy Observatory (NRAO), said in a statement. “The torus helps explain why objects known by different names actually are the same thing, just observed from a different perspective.”

VLA image of Cygnus A's central region, with labels. Carilli et al., NRAO/AUI/NSF

The dusty torus is 900 light-years in radius and the supermassive black hole is a whopping 2.5 billion times the mass of the Sun. The team discovered the torus using the Very Large Array (VLA), an observatory in New Mexico, while they were obtaining more data about a second supermassive black hole discovered in the core of Cygnus A a few years ago. The galaxy must be the result of a recent galactic merger.

“Cygnus A is the closest example of a powerful radio-emitting galaxy – 10 times closer than any other with comparably powerful radio emission. That proximity allowed us to find the torus in a high-resolution VLA image of the galaxy’s core,” said co-author Rick Perley, also of the NRAO. “Doing more work of this type on weaker and more distant objects will almost certainly need the order-of-magnitude improvement in sensitivity and resolution that the proposed Next Generation Very Large Array (ngVLA) would bring.”

The idea of a dusty torus being a common feature around every AGN was first proposed in 1982 and since then it has been a key element in all unified models. These observations and follow-up studies will certainly improve the AGN model and our general understanding of active galaxies.

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