This episode's coordinates point to the elliptical galaxy NGC 4291, which sits at a distance of 85.8 million light years (26.3 Mpc) within the constellation Draco as seen from Earth [1]. Note that I don't mock the constellation Draco like I mock other constellations, and that's because Draco is a cool constellation with a cool name that actually looks sort of like the dragon it's supposed to represent, even though it could be thought of as a dragon with a small body and a very long tail.
Anyway, NGC 4291 is basically a giant sphere of stars that really aren't doing a whole lot. In many ways, it may not look all that special.
However, the galaxy does contain a supermassive black hole that is much more interesting than anything else happening in the galaxy itself. Several techniques can be used to measure the masses of black holes at the centers of galaxies. In the case of NGC 4291, it was possible to use the velocities of the stars within the center of the galaxy to estimate that the black hole's mass in 960 million times the mass of the Sun give or take about 300 million [2]. Unlike a lot of supermassive black holes in a lot of the galaxies that I talk about in this podcast, the black hole at the center of NGC 4291 is not doing anything special. However, that is not important.
What is important is that that the black hole is way too large for its galaxy. Astronomers have determined that the centers of most galaxies contain supermassive black holes and that the size of the supermassive black hole at the center of a galaxy is linked to the size of the galaxy's bulge if the galaxy is a spiral or lenticular galaxy or the size of the galaxy itself if the galaxy is an elliptical galaxy, if which case the elliptical galaxy can be thought of as all-bulge and no disk. This means that whatever process leads to building up a galaxy's bulge also increased the size of the galaxy's central supermassive black hole.
A spiral galaxy's bulge forms when one or more smaller galaxies merge with it; the orbits of the smaller galaxies usually get scrambled, thus producing a spherical ball of stars within the center of the spiral galaxy. When two elliptical galaxies of the same size merge, the orbits of the stars from both galaxies get scrambled, resulting in the formation of a big ball of stars or, in other words, an elliptical galaxy. The merging process may also cause interstellar gas clouds in the two galaxies to collide and collapse to form new stars, thus making the bulges larger. Interstellar gas will fall into the centers of these galaxies and into the black holes if the galaxies have them, making the black holes larger. Additionally, if both merging galaxies contained supermassive black holes, those may also merge together to form one larger supermassive black hole.
Typically, the ratio of the mass of a galaxy's central black hole to the mass of the galaxy's bulge is around 1/500. In NGC 4291, however, the ratio is 1/50 [3]. In other words, the black hole at the center of NGC 4291 is 10 times larger than what would be expected for a galaxy of its size.
A group led by Ákos Bogdán published a paper in 2012 investigating this galaxy and another galaxy very similar to it [3]. One of their initial thoughts was that NGC 4291 might have gravitationally interacted with another galaxy and if that other galaxy had, instead of merging with NGC 4291, simply stripped away most of its outer layers of stars, the stellar mass of NGC 4291 would have decreased, but the central supermassive black hole would not have been affected. However, Ákos Bogdán's group did not find any signs that NGC 4291 had been involved in this type of gravitational interaction, so that hypothesis was eliminated [3].
This led to the conclusion that the one or more black holes that have gone on to form the black hole at the center of NGC 4291 started growing in one or more galaxies with very few stars (although potentially lots of dark matter), and after a few merger events, we ended up with an elliptical galaxy with relatively few stars but a fairly big black hole [3]. Bogdán's group also suggested that, based on the results for NGC 4291 and a few other exceptional galaxies, a galaxy's black hole mass might not actually be linked to the mass of the galaxy's bulge but actually to the mass of the galaxy's dark matter halo [3], which is basically a giant sphere surrounding the galaxy that is not made of normal matter and does not emit electromagnetic radiation. This relation between the mass of a galaxy's central supermassive black hole and the mass of a galaxy's dark matter halo has not been studied that much, but this relation is being actively investigated, and I look forward to learning more about whether more galaxies like NGC 4291 are out there and how they fit into this new relation.