Markarian 231 can best be described as a disorganized mess of stars and gas. The galaxy is the remnant of the merging of two other galaxies, and the merging process has not finished yet, so the galaxy has all sorts of weird clumpy and arc-like structures, but the nucleus is quite bright.
The galaxy was first discovered by Armenia's most notable astronomer ever, Beniamin Markarian, in his surveys of really bright ultraviolet objects in the sky. Markarian 231 made it into Markarian's third catalog of these sources, which was published in 1969 [1]. A lot of the objects from Markarian's catalogs have turned out to be rather unusual or interesting galaxies, and they are so notable that I have spent a couple of previous podcast episodes talking about some of the other objects with Markarian designations. However, it was clear soon after it was discovered that Markarian 231 specifically was even more unusual and more interesting than most of the other galaxies from Markarian's catalogs. Visible light spectroscopic observations published in the 1970s showed that the spectrum of Markarian 231 looked quite different from the average Markarian galaxy. The combination of bright and dark lines in the spectrum of the galaxy indicated that it was either like an abnormal Seyfert galaxy or like a quasar [1]. In fact, people today now recognize that Markarian 231 is indeed a quasar.
As a quick review (because I have mentioned this in so many past episodes), both Seyfert galaxies and quasars contain what is called an active galactic nucleus (AGN). An AGN contains a supermassive black hole millions or billions of times the mass of the Sun, a disc of gas and dust falling into that black hole, and jets of ionized gas travelling at fractions of the speed of light that originating from infalling material deflected away from the black hole's event horizon by the AGN's magnetic fields. While the black holes themselves do not produce any electromagnetic radiation, the compressed gas falling into the black hole as well as the ionized jets of gas flying away from the black hole will produce huge amounts of radiation and will therefore look very bright.
Generally, the AGN in Seyfert galaxies and quasars are the same except that Seyfert galaxy AGN are fainter and more nearby while the AGN in quasars are brighter and further away. Typically, quasars are located billions of light years from Earth. However, Markarian 231 is only located 616 million light years (189 Mpc) from Earth [2], so it's actually relatively nearby, but it's so bright that people treat it like a quasar instead of a Seyfert galaxy, and in fact people usually describe Markarian 231 as the closest quasar to Earth.
The galaxy is extraordinarily bright not only in ultraviolet light but also across almost all other parts of the electromagnetic spectrum (but not in X-rays for some reason [3], so if you want to see an X-ray bright quasar, go look somewhere else). One particular thing that stood out to me is that Markarian 231 is technically an ultraluminous infrared galaxy [4]. These are galaxies where the amount of energy that is emitted in the infrared part of the electromagnetic spectrum is equivalent to over one trillion times the amount of light emitted by the Sun.
In general, infrared light from other galaxies is produced by interstellar dust that absorbs light in the ultraviolet or visible parts of the spectrum and re-radiates that energy in the form of infrared light. Within our galaxy and through most parts of other galaxies, the interstellar dust absorbs ultraviolet and visible light from stars. When new stars form in star forming regions and in particular when very large numbers of stars form in events called starbursts, some of the new stars will be very hot, very bright, and very blue, but they will also have very short lifespans before they explode as supernovae, so they will only be found in places close to where lots of stars are forming. These stars produce huge amounts of ultraviolet and visible light, so if they are surrounded by interstellar dust, the regions will produce huge amounts of infrared light. Consequently, in places where astronomers see lots of infrared light, they typically expect to see lots of stars forming.
However, in the centers of galaxies with AGN like Markarian 231, it is possible that the interstellar dust is also absorbing ultraviolet and visible light from the much hotter inner parts of the AGN and re-radiating that huge amount of energy in the form of infrared light. With many galaxies in general but with ultraluminous infrared galaxies specifically, people have debated whether the infrared emission comes from dust heated by very young stars in starbursts or from the AGN, and in fact, my PhD advisor, Bob Joseph, was a very strong advocate for ultraluminous infrared galaxies being powered entirely by starbursts [5].
These days, however, people have converged on a middle ground where they think that starbursts and AGN may both be present within ultraluminous infrared galaxies, and Markarian 231 has been used to demonstrate this. The galaxy contains not only an extraordinarily bright AGN in its center but also a very extended starburst region where huge numbers of stars are forming, and in the latest images from the James Web Space Telescope, people have been able to see that the infrared light comes from a complex region with multiple shells that is thousands of light years in diameter [6]. This starburst was powered by the collision of the two galaxies that merged together to form Markarian 231. Both of the galaxies would have contained large numbers of interstellar gas clouds that would have collided with each other when the galaxies merged, and when interstellar gas clouds collide, they will collapse, and when interstellar gas clouds collapse, they form stars, and the huge numbers of newly formed stars in Markarian 231 will produce huge amounts of energy that gets absorbed by interstellar dust and re-radiated in the infrared. The exact ratio of the ratio of infrared energy from starburst to infrared energy from the AGN seems to vary, but it's in the neighborhood of a 50/50 split [7, 8, 9, 10].
So both the extremely bright AGN and the extreme star formation in Markarian 231 have attracted a lot of attention from astronomers, and people end up looking at the galaxy to understand all sorts of things about how extreme quasar / ultraluminous infrared galaxies work in general. All of this extreme stuff has actually led to some of the interstellar gas getting blown out of the center of the galaxy [11, 12, 13, 14, 15, 16], and this could even potentially lead to star formation shutting off (because no more gas would be around to form new stars) and to the AGN turning dark (because no more gas would be falling into the supermassive black hole, and because that gas is needed to produce the electromagnetic emission from AGN). So, wait a few hundred million more years, and Markarian 231 will look like a relatively faint and boring galaxy.
As a final note, it's worth mentioning that some astronomers suspect that Markarian 231 might actually contain two superamssive black holes in its center rather than just one. This is primarily based on the fact that its spectrum in ultraviolet and visible light looks a bit peculiar for a scenario where only one black hole is present but would make sense if a smaller supermassive black hole was orbiting the one at the center of the AGN [17, 18, 19]. However, other people have debated this interpretation [20], and quite honestly, I would feel more likely to believe another black hole was present if the observational evidence included something like the periodic Doppler shifting of light from something in the AGN consistent with two objects orbiting each other or some sort of weird twisting in the jet emerging above the pole of the larger black hole that could be caused by it being gravitationally pulled byh a smaller black hole.