This episode's coordinates point to an object named 3C 186 in the constellation Lynx. Yes, that is a lynx as in the wild cat. It's a bunch of really faint stars in the northern sky lying between Ursa Major and Gemini. If you live in the northern hemisphere, you've probably looked up at this part of the sky and never realized that the constellation Lynx is located here.
Anyway, let's move on to 3C 186. If you live in the northern hemisphere, you've probably looked up at this part of the sky and never realized that the object 3C 186 is located here unless you like looking at the sky with a radio telescope. 3C 186 is indeed is a very strong radio source. It and many other really bright radio sources were discovered in the 1950s, with many of those discoveries first published in the Third Cambridge Catalogue [1], which is where the designation 3C comes from. 3C 186 just happens to be the 186th object in the catalog.
Like many other objects in the Third Cambridge Catalogue, 3C 186 is a distant galaxy containing a quasar. Quasars are a class of active galactic nuclei. These objects consist of supermassive black holes millions or billions of times the mass of the Sun, disks of gas falling into the black holes, and jets of gas emerging from the poles of these disks that are produced when hot infalling gas is deflected away from the black holes by the systems' magnetic fields. Those jets of gas produce very strong radio emission. What distinguishes quasars from other types of active galactic nuclei is, first of all, they are really far away, and second, the jets from the quasars are aligned in such a way that one of them points roughly (but not exactly) towards the Earth.
3C 186 specifically is a particularly massive galaxy forming from the merger of two smaller but still relatively large galaxies. The object is located within a cluster of galaxies at a distance of about 8 billion light years [2]. No one has produced especially sharp images of the galaxy yet, but the Hubble Space Telescope images indicate that the galaxy is rather distorted as would be expected for something formed from when two galaxies merge together [3]. The supermassive black hole has a mass of somewhere between 3 and 6 billion times the mass of the Sun [3], which may actually sound quite large but fairly typical for an active galactic nucleus.
What is peculiar, though, is that the quasar is not in the galaxy's nucleus but actually in a location about 36 thousand light years (11 kpc) away from the center of the galaxy forming in this merger process [3]. It's also moving at a speed of 2140 km/s relative to the center of the galaxy [3]. That is highly unusual. First of all, we can't exactly continue calling this an active galactic nucleus. It's more like an active galactic off-center type of thing. Much more importantly, though, this poses a major question. How did an object with a mass that is billions of times the mass of the Sun escape from the center of a galaxy?
This is actually a phenomenon that involves general relativity and gravitational waves, which are things that are usually relevant in extreme situations involving merging black holes. So, the two galaxies that have merged together must have each started out with their own supermassive black holes in their nuclei. However, it is likely that one of the black holes was larger than the other. As the galaxies merged, the supermassive black holes themselves would have not merged right away but instead would have gotten close enough that they could orbit each other. When black holes get really close to each other, they start radiating energy in the form of gravitational waves, but since one black hole is larger than the other, the gravitational waves would have been emitted more strongly in one direction in the black holes' rotating frame of reference [4]. Once the black holes merged, the asymmetric gravitational wave emission towards one direction would have resulted in the final black hole being given a kick out of the center of the galaxy in the opposite direction [4]. This phenomenon, which is called recoil, is the most likely explanation for what has happened to the supermassive black hole that used to be in the center of 3C 186 [3].
This type of recoil is quite rare. For decades, researchers had been working on the trying to predict the weird effects of what would happen if black holes merge, but potential (but not necessarily positively confirmed) recoiling supermassive black holes that have been ejected from their host galaxies have only been identified since 2010, and the one in 3C 186 was only announced in a paper published in 2017 that was led by Marco Chiaberge [3]. So, for astrophysicist who want to understand how general relativity and gravitational waves work during black hole mergers, 3C 186 is one of the very rare but one of the very best objects in the sky to study.