NGC 4261 is a big elliptical galaxy located at a distance of roughly 106 million light years (32.4 Mpc) [1] in the constellation Virgo. It's also on the outskirts of the Virgo Cluster, a very large gravitationally-bound collection of over a thousand galaxies that, as seen from Earth, appears in the constellation of the same name. The galaxy basically looks like a giant sphere of hundreds of billions of reddish-looking stars. The most interesting thing about NGC 4261 is that it contains a supermassive black hole in its center. However, it took people a few decades to figure out exactly what was going on within the galaxy's nucleus. So let's cover the history of the astronomical observations of NGC 4261.
The first potential sign that something strange was happening in the center of NGC 4261 was when radio emission was detected from the region in the 1950s. One of the most notable catalogs of astronomical sources of radio emission produced in that decade was the Third Cambridge Catalog of Radio Sources (3C) [2]. You may ask what happened to the 1C and 2C catalogs, but I learned a couple months ago when visiting Cambridge that you should never mention the first or second catalogs to anyone at any of the research organizations there or else. Actually, that's not true; the Third Cambridge Catalog was just much better than the first two, sort of like how the best Lord of the Rings movie is The Return of the King (maybe). Anyway, the radio source in the center of NGC 4261 was given the designation 3C 270 in the 3C catalog [2]. Additional observations in the 1980s revealed that the radio emission originated from two jets of ionized gas emerging from the center of the galaxy [3].
The next sign that something strange was happening in NGC 4261 was when people using some very early astronomical digital cameras in the 1980s were able to determine that the center of NGC 4261 contained a small disk of dust [4]. At that point in time, most people thought that the interstellar medium within elliptical galaxies only contained very thin but very hot gas that was primarily visible in X-ray emission. Only truly weird and unusual elliptical galaxies, which were technically classified as "peculiar", could be expected to contain any cold interstellar gas or dust. What was extra unusual about this dust disk was that it was perpendicular to the jets of gas seen in radio wavelength emission [4].
So, by the time 1990 rolled around, astronomers had developed the idea that an active galactic nucleus (AGN) like the center of NGC 4261 should contain a supermassive black hole. The black hole would be fed by the surrounding disk of gas and dust, but some of the gas would get deflected by the magnetic fields around the black hole and would be channeled into jets of gas emerging perpendicularly from the disk. However, people did not have any data indicating that the center of NGC 4261 had a black hole. Then the Hubble Space Telescope was launched into space, and in 1994, it pointed at the center of NGC 4261 and found that the dust disk had a spiral shape with a small gap in the center, which was indicative of gas falling into a central object, and measurements of the rotational velocity of the gas showed that the galaxy probably contained a compact supermassive object that was most likely a black hole [5]. Current estimates place the mass of that black hole at 1.6 billion times the mass of the Sun [6, 7, 8].
If you have listened to previous episodes of my podcast, you would have heard me describe active galactic nuclei with these types of structures around supermassive black holes in many other galaxies. However, what is special about NGC 4261 is that it is one of the first objects where astronomers were actually able to clearly identify the different parts of the active galactic nucleus and were able to put all together, especially the supermassive black hole at the center. That is what makes this object truly important in professional astronomy.