This episode's coordinates point to the galaxy 3C 324 in the constellation Serpens. 3C 324 is another object from the Third Cambridge Catalogue, which most astronomers consider to be the coolest catalog of astronomical radio sources ever compiled. It's not the best catalog of radio sources ever, but when it was compiled in 1959 [1], it was truly groundbreaking. Galaxies these days use 3C names as a sign of prestige, sort of like having a knighthood in the United Kingdom.
Like a lot of radio sources in the Third Cambridge Catalogue, people identified that 3C 324 is a very distant galaxy containing an active galactic nucleus (AGN) with a black hole millions or billions of times the mass of the Sun, a disk of gas falling into that black hole, and jets of ionized gas appearing about the poles of the AGN that are fueled by gas that doesn't make it into the black hole but instead gets deflected by its magnetic fields. The electrons in the jets will oscillate around these magnetic fields, and this oscillation will produce strong radio emission. If you listened to my last episode, this will sound very familiar, and that's because AGN come up a lot in astronomy. (Even my research is related to AGN.)
These days, 3C 324 seems sort of ordinary in terms of AGN, but back in the 1980s, it caused a lot of excitement, sort of like Cyndi Lauper [2]. First, in 1983, it was identified as an abnormally bright radio source in terms of the total radio emission as seen from Earth [3]. The next year, a redshift, as expressed by the letter z, of 1.206 was measured for the galaxy [4], which means that, because of the expansion of the universe, the light from 3C 324 is stretched out by a factor of 2.206 (or 1+z). This corresponds to a distance where the light has travelled for 8.57 billion years, but for reasons involving general relativity, this is not necessarily equivalent to a distance 8.57 billion light years. Anyway, the combination of that distance and the observed relative brightness as seen from Earth implied that 3C 324 was one of the most intrinsically bright radio sources in the universe that people knew about in the 1980s, which was rather impressive.
However, this is not the end of what made 3C 324 so exciting a few decades ago. In 1987, a group of authors led by Olivier Le Fevre published a paper in the journal Nature claiming that they had identified 3C 324 as possibly a gravitationally lensed system [5]. A gravitational lens is a situation where two galaxies line up in such a way that the light from the galaxy in the background gets gravitationally bent around the galaxy in the foreground. When this happens, the light from the background galaxy could appear in multiple locations around the foreground galaxy, or the light could be warped into the shape of an arc-like feature, or both of these things.
This might sound like a very rare phenomenon, but these days, lots of people have found gravitational lenses all over the place. For example, my last two astronomny papers were on gravitational lenses [6, 7], my next astronomy paper is also going to be on gravitational lenses, and I even have gravitational lens wallpapers that I made myself on my phone. In the 1980s, however, very few gravitational lenses had been discovered, so every discovery of a new gravitational lens was really exciting.
So, back to 3C 324. This object was identified as a gravitational lens on the basis that the radio emission originate from two different sources and that spectroscopic observations in the visible part of the electromagnetic spectrum had determined that the object actually appeared to have two redshifts: 0.845 and 1.206 [5]. The double images looked consistent with how a foreground galaxy would bend light from a background galaxy in a gravitational lens, while the two redshifts implied that 3C 324 consisted of two objects lined up with each other. As a bonus, the very strong radio emission from the galaxy could be explained by magnification effected caused by the gravitational lensing.
However, 3C 324 did not turn out to be a gravitational lens after all. Instead, it just happens to be a spiral galaxy in front of another more distant galaxy with an AGN [8]. The two separate radio sources from the background galaxy are not caused by light being warped by the foreground galaxy, but instead, the two separate radio sources correspond to two separate jets of gas that have been ejected from the AGN in the background galaxy in directions perpendicular to our line of sight. So, 3C 324 turned out to be a false positive. Nonetheless, this does not seem to have affected the career of the lead author of the paper describing that discovery, Olivier Le Fevre, who has gone on to publish several decades worth of highly cited papers about galaxies in the distant universe. As for 3C 324, people continued to look at it for a while, and a few papers werte published about a cluster of galaxies around 3C 324 circa the year 2000 [9, 10, 11, 12], but otherwise, people seem to have lost interest.