What do radio galaxies, quasars, blazars, Seyfert galaxies and active galactic nuclei all have in common? Everything, it turns out: These are all different names for the same celestial objects viewed from different angles. Now, for the first time astronomers have directly imaged the last piece of the puzzle that ties them all together – a dusty donut of material that surrounds a supermassive black hole.
Most galaxies have supermassive black holes at their hearts, and in many cases the intense gravity pulls in material, which swirls around it as an accretion disk. As this debris falls into the black hole, it heats up and shines bright. At the same time, many of these objects are also throwing out jets of material at almost the speed of light.
These objects are all known broadly as active galactic nuclei (AGN), but somewhat confusingly they go by many different names, depending on how they look from here on Earth. The brightest AGN are called quasars, while blazars are those with a jet that just happens to be pointing directly at us. Seyfert galaxies are basically quasars with visible host galaxies, while radio galaxies are particularly loud at radio frequencies.
But for this "unified model" to work, astronomers long ago proposed that a key component of AGN was a torus – a thick, donut-shaped cloud of dust and gas that surrounds the whole structure. This explains why looking at AGN from different angles makes them appear to be different types of objects. If we're looking through the middle of the torus, for example, we see the bright light as a quasar, while if we're looking side-on the dusty donut obscures the light and only the radio frequencies get through, creating a "radio galaxy."
The problem is, no one's ever imaged a torus directly. They have been indirectly spotted in galaxies like M77, but now researchers at the National Radio Astronomy Observatory (NRAO) have been able to see one properly.
"The torus is an essential part of the AGN phenomenon, and evidence exists for such structures in nearby AGN of lower luminosity, but we've never before directly seen one in such a brightly-emitting radio galaxy," says Chris Carilli, lead author of the study. "The torus helps explain why objects known by different names actually are the same thing, just observed from a different perspective."
To capture the image of a torus, the team looked to a bright AGN at the heart of the galaxy Cygnus A. Located 760 million light-years from Earth, this galaxy hosts a supermassive black hole with the mass of 2.5 billion Suns – and, it turns out, an absolutely massive torus.
The team used the Very Large Array (VLA) telescope to peer at the center of Cygnus A. Lo and behold, the torus came into focus as a clumpy cloud of dust with a radius of a whopping 900 light-years.
"Cygnus A is the closest example of a powerful radio-emitting galaxy – 10 times closer than any other with comparably powerful radio emission," says Rick Perley, co-author of the study. "That proximity allowed us to find the torus in a high-resolution VLA image of the galaxy's core. 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."
An intriguing side note is the other bright spot visible in the lower-right corner of the images. This is a second supermassive black hole, according to the team, which has just recently fired up as it encounters material to chow down on. This suggests that Cygnus A collided with another galaxy in the astronomically recent past.
The research was published in the Astrophysical Journal Letters. The animation below demonstrates how the torus affects the viewing angle.
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