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 galaxies ‘switch on and off’ every couple of hundred thousand years.
AGN are formed when supermassive black holes forcefully siphon so much gas from their surroundings, 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.
“When the massive black holes in galaxy centers accrete, they become visible as quasars or 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 AGN) 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, Kevin Schawinski (lead author) 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.
Commenting further, Schawinski (lead author) remarks that “Now we know that light emitted by an active galactic nucleus resembles an energy-saving lamp that flickers on and off every 20 milliseconds." He continues, “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, 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 be one once more when we merge with the Andromeda Galaxy billions of years from now.