This is a very interesting case that is reminiscent of Hanny’s Voorwerp galaxy. Both cases exhibit extended emission line regions, which are very large swaths of gas that are ionized. Ionization occurs when diffuse gas is illuminated by very energetic photons. The “ionizing photons” interact with the gas, give energy to electrons, and kick the electrons off of their atoms. Subsequently, an electron freed in this manner occasionally falls back down onto an atom. When it does so, it gives off energy by emitting another photon. These are the photons that we see when we observe the gas.
If you take a spectrum of the gas, you’ll be able to identify discrete bright spots - what we call emission lines. Some of those emission lines show up stronger than others. This is the result of various physical conditions such as the density of the gas, and the light that is causing it to be ionized. What that means is that if you observe the emission line intensities and their ratios, you can infer things about the ionizing source. In the Teacup AGN, that ionizing source is an active galactic nucleus. I can write more about them later.
In the paper I read recently, the investigators (Gagne, et al.) measured the spectrum at discrete locations along the extended emission line region (the ionized gas). Then, they predicted properties of the ionizing source based on all those individual data points. Specifically, they calculated the luminosity (or brightness) of the active nucleus that would be needed to create all the emission line strengths and ratios that they observed at the specific locations in the galaxy. And because light has a finite travel speed, this allowed them to track the luminosity of the source over time.
By analogy, the analysis worked, because each spot in the gas acted like a photograph of the active nucleus. The "photographs" from the outer edges of the galaxy showed a very bright source and those from the inner regions showed a dim source. Because the "photos" from the inner region are more recent than those from the outer regions, it can be inferred that the active nucleus dimmed over that time period.
The authors’ main result is that the illuminating source decreased its brightness by a factor of 100 over the course of 46,000 years. That is not that long relative to the lifetime of stars and galaxies. The result, suggests that active nuclei can turn on and off rather quickly.
Kyle D Hiner