Title: A New Class of Changing-look LINERs
Authors: Sara Frederick, Suvi Gezari, Matthew J. Graham, S. Bradley Cenko, Sjoert van Velzen, Daniel Stern, Nadejda Blagorodnova, Shrinivas R. Kulkarni, Lin Yan, Kishalay De, U. Christoffer Fremling, Tiara Hung, Erin Kara, David L. Shupe, Charlotte Ward, Eric C. Bellm, Richard Dekany, Dmitry A. Duev, Ulrich Feindt, Matteo Giomi, Thomas Kupfer, Russ R. Laher, Frank J. Masci, Adam A. Miller, James D. Neill, Chow-Choong Ngeow, Maria T. Patterson, Michael Porter, Ben Rusholme, Jesper Sollerman, Richard Walters
First Author’s Institution: Department of Astronomy, University of Maryland; Department of Physics and Astronomy, Vanderbilt University (Current)
Status: Published in The Astrophysical Journal [open]
Most massive galaxies in the local Universe host supermassive black holes in their centers. Sometimes, massive clouds of gas and dust fall onto the black hole. This gas and dust forms a disk around the black hole, which gets super hot and starts to glow. When a supermassive black hole has a glowing disk of gas and dust, we call it an “active black hole” or “active galactic nucleus” – or “AGN” for short.
AGN are only “active” as long as there’s gas and dust to fall onto them. Once they’ve eaten through their gas supply, they fade back into a regular, inactive supermassive black hole. One class of galaxies – “Low Ionization Nuclear Emission Line Region” galaxies, or LINERs – is thought to be either the remains of an old AGN which has faded or the signature of a “weak” AGN. These galaxies have emission lines in their spectra that come from warm gas in the center of the galaxy. This gas is too energized to be heated fully by stars, but it’s less energized than the gas around typical AGN. LINERs are therefore a galaxy showing features somewhere in the middle of purely star-forming and AGN-dominated galaxies.
This paper finds six objects that were previously classified as LINERs but now show properties typical of AGN. They argue that they have witnessed AGN “turning on,” or starting to feed on nearby gas and dust. The authors’ biggest challenge is distinguishing this theory from other short-lived transient events that make galaxies suddenly light up. Let’s take a look at their methods.
Methods
Like many transient event searches in astronomy, this study begins with the Zwicky Transient Facility, which tracks the colors and brightnesses of millions of objects across the sky to detect transient events like supernovae and tidal disruption events (or the violent disruption of stars by supermassive black holes). Since supermassive black holes (and therefore AGN) exist in the centers of galaxies, the authors start by detecting sudden brightening, or “flares,” in the centers of known massive galaxies that were previously classified as LINERs. They identify six objects for further study .
“Changing-Look” LINERs in this study
The objects in this study show a wide variety of behavior. Three objects had slow, months-long increases in brightness that eventually plateaued. One of the three showed two separate rises, while another one got redder over time. About half of the objects brightened within a few months, whereas the rest took a few years to change states. All six objects transitioned from stable brightness to significant variability after the flare (Figure 1), a common property of AGN.
While all six hosts were originally classified as LINERs, the team’s re-analysis of the original spectra revealed that three of them required at least some level of AGN activity before the flare to explain their emission lines. The other three objects could have been classified as either LINER or AGN before the flare, but all definitely showed AGN-like properties after the flare.
What are they?
One of the biggest challenges of transient science is distinguishing between similar-looking transients. A flare in the center of a galaxy could be many things: a supernova, a tidal disruption event, or even just an issue with your telescope. This team narrowed down the options a few different ways. First, they looked at the spectral changes before versus after the flare (Figure 2). A spectrum encodes information about the physical properties of a system, including its temperature, chemistry, and geometry. By taking many spectra, you can track changes in these properties over time. This team does just that and finds that the spectral changes look more similar to so-called “changing-look AGN,” or AGN switching between high- and low-energy states, than other transient events.
Specifically, this team looks for the evolution and development of “broad” emission lines. These broad lines tell you about the velocities involved in a system. As emitting gas moves toward or away from you at high speeds, the Doppler effect shifts the emission to bluer or redder wavelengths, respectively. You expect to see very broad emission lines from an explosive transient (like a supernova) because there is material exploding outward at high speeds. AGN, on the other hand, only have a few broad emission lines, since the material involved is moving more slowly. The team finds that these galaxies are lacking certain broad emission lines commonly associated with tidal disruption events and supernovae, like helium, which is typically ejected from the disrupted star at high speeds. The only broad emission lines detected are from hydrogen, which is commonly present in AGN.
What does this mean?
The authors theorize that these new AGN are tied to their LINER host galaxies. The spectral properties in the LINERs may have come from patches of gas that were heated the last time the AGN was on, and this could be part of a larger cycle of gas fueling, AGN feeding, gas exhaustion, and dormancy. These bursts of AGN fueling are key to understanding how black holes and galaxies grow together throughout cosmic history.
Astrobite edited by Niloofar Sharei and William Lamb.
Featured image credit: Figure 3, Frederick et al. (2019)
