Object 45: Mrk 590
Podcast release date: 19 April 2021
Right ascension: 02:14:33.6
Corresponding Earth location: Lake Victoria in Africa
Markarian 590 (also written as Mrk 590) is one of a series of galaxies cataloged by Benjamin Markarian in the 1960s, 1970s, and 1980s . The special thing about all of these galaxies was that they were brighter than usual in the ultraviolet part of the electromagnetic spectrum. I think I'll talk more about Benjamin Markarian and his galaxies in a future episode, but, today, I'll just focus on Mrk 590. For reference, this galaxy is listed three times in the New General Catalogue (NGC) by accident, so it is also known as NGC 863, NGC 866, and NGC 885 [2,3]. This is as confusing as heck, so we're going to stick with Mrk 590.
Mrk 590 is a spiral galaxy that contains an active galactic nucleus (AGN). An AGN consists of a supermassive black hole that is millions of times more massive than the Sun, a disk of gas and dust that is slowly falling towards the black hole, and jets of gas that appear above the poles of the black hole. These jets form when the infalling gas gets too hot from being gravitationally compressed by the black hole and flows away from the center of the disk instad of falling all of the way into the black hole itself. In the case of Mrk 590, the mass of the central black hole is about 50 million times the mass of the Sun .
AGN like the one in Mrk 590 are typically labelled as type 1 or type 2 based on differences in how they look. Type 1 AGN typically produce light from hydrogen gas that is very Doppler shifted, which means that we see the gas in the jets moving very quickly and that the gas moving towards us looks bluer than usual while the gas moving away from us looks redder than usual. Type 2 AGN produce light from hydrogen gas that is not Doppler shifted that much, which means that we don't see the gas moving towards or away from us very quickly and that the light from the hydrogen gas all looks like the same color. Type 1 AGN are also generally brighter than Type 2 AGN.
The general theoretical model that astronomers have been using states that type 1 AGN are viewed from above the disks where it is possible to see the jets of gas from above the poles of the black holes moving towards or away from us, which is what makes the hydrogen gas look like it's moving so fast, while type 2 AGN are seen from the sides where the disks of gas and dust falling into the black holes also hide the jets near the black hole, so we don't see the hydrogen gas moving that fast and we end up seeing less light from the AGN. However, Mrk 590 completely messes up this theory because its AGN has switched between type 1 and type 2 a couple of times in the past 50 years.
When the light from hydrogen gas in Mrk 590 was measured in the 1970s, it was relatively faint and didn't seem to be moving very fast, which would have made this a type 2 AGN . In the 1980s and 1990s, though, the center of Mrk 590 looked much brighter, and the gas appeared to be moving very fast, which made this a type 1 AGN . After 2000, though, the center of the galaxy got fainter, and the hydrogen gas stopped moving as quickly, which means that it switched back to type 2 . Then, in 2014, it looked like the center of the galaxy was beginning to get brighter again, so it looked like it was switching back to type 1 .
So, astronomers don't think that the standard model of the differences between type 1 and type 2 AGN apply to Mrk 590 because it's very difficult for AGN to change their orientation that quickly. Instead, astronomers think that what is actually happening in Mrk 590 is that the AGN is viewed from one angle (probably from above the disk) but that the varaiations in brightness and the variations in the jets are related to how quickly material is falling into the central black hole [5,6]. In the 1980s and 1990s, we seemed to be seeing more gas falling into the black hole, and as more gas was falling inwards, it would have emitted more light. This also meant that more gas would have escaped from the disk in the jets above and below the black hole, thus causing the appearance of the high-velocity hydrogen gas we saw in the center of this galaxy. Once that gas stopped falling inwards after the year 2000, the center of Mrk 590 got fainter, and since no gas was falling towards the black hole, we weren't going to see high velocity gas escaping in the jets, either. In 2014, though, it looks like gas stared falling towards the black hole again, which made the center of the galaxy brighter, and we might start seeing the high velocity gas in the jets sometime soon.
Mrk 590 is one of a few galaxies with AGN called "changing-look" AGN, and like I indicated above, these galaxies are forcing astronomers to re-examine their theories about how AGN works. Because Mrk 590 is a very good example of this phenomenon, astronomers are going to spend a lot more time observing the changes in this galaxy in the future.
 Browse Software Development Team (High Energy Astrophysics Science Archive Research Center), MARKARIAN - Markarian Galaxies Catalog, 2004
 NASA/IPAC Extragalactic Database, 2021
 Peterson, B. M. et al., Central Masses and Broad-Line Region Sizes of Active Galactic Nuclei. II. A Homogeneous Analysis of a Large Reverberation-Mapping Database, 2004, Astrophysical Journal, 613, 682
 Denney, K. D. et al., The Typecasting of Active Galactic Nuclei: Mrk 590 no Longer Fits the Role, 2014, Astrophysical Journal, 796, 134
 Mathur, S. et al., The Changing-look Quasar Mrk 590 Is Awakening, 2018, Astrophysical Journal, 866, 123