In Brief
Active Galactic Nuclei, or AGN, glow brightly in the universe because of space gas. It was previously believed that they stayed "on" at all times, but researchers have observed flickering.

If you want to save energy on a large cosmic scale, you would probably get yourself a set of flickering space lamps. According to new research, these cosmic lamps already exist, and they are among the brightest objects in the universe. Called active galactic nuclei (AGN), these objects are not new, but researchers worked under the assumption that their luminosities are, for all intents and purposes, fixed (of course, small periods of variability were to be expected). Now, it looks like these active galaxy centers “switch on and off” every couple of hundred thousand years.

Centaurus A (pictured) is one example of an active galactic nuclei. Image Credit: Rolf Olsen

AGN are formed when supermassive black holes forcefully siphon so much gas from their surroundings that the gas eventually forms into a disk that rotates around the black hole. The disk then becomes progressively hotter through friction and begins to radiate. Once formed, they shine brighter than all of the stars in our galaxy combined. However, the visible light does not come from active galactic nuclei themselves, but from the gas that fills the space between stars.

Active Galactic Nuclei (AGN)
Artist rendering of an active galactic nucleus. Image Credit: NASA/JPL

“When the massive black holes in galaxy centers accrete, they become visible as quasars or active galactic nuclei (AGN). The energy liberated by accretion episodes is thought to be a critical regulatory mechanism in galaxy evolution,” the researchers write in the paper.

AGN emit radiation at all wavelengths, from X-rays to radio, but with a certain delay. You can liken this to a gas lamp, which does not light up immediately when switched on. This delay is a consequence of the amount of time it takes for the light to reach the edge of the galaxy and turn on the “galactic gas-lamp.” Before this occurs, however, some AGN (the research suggests approximately 5 percent of them) remain in a “switched-off” state, at least in terms of their apparent brightness (this has no affect on their x-ray emission).

This state can be compared with adolescence, lasting approximately 10,000 years, with a complete AGN phase lasting 200,000 years on average, researcher Kevin Schawinski noted. However, compared to the hundreds of millions of years in which a galaxy’s nucleus remains active, 200,000 years qualifies as a pretty short period of time.

"Hanny's Voorwerp" (green, below) is an astronomical object that has been turned off around 200,000 year ago. Visible in the upper part is the spiral galaxy IC 2497
“Hanny’s Voorwerp” (green, below) is an astronomical object that was turned off 200,000 years ago. IC 2497 is the galaxy on the right. Image Credit: NASA, ESA, W. Keel, Galaxy Zoo Team

Commenting further, Schawinski remarked, “Now we know that light emitted by an active galactic nucleus resembles an energy-saving lamp that flickers on and off every 20 milliseconds.” The paper expands on this, saying, “This short lifetime implies that black holes grow via many such short bursts and that AGN therefore ‘flicker’ on and off.”

The new findings are also important because some studies assume that our home galaxy, the Milky Way, was once a host to an AGN. A supermassive black hole at the center of our galaxy probably shone as an active galactic nucleus a few millions years ago, and it may once more when we merge with the Andromeda Galaxy billions of years from now.