About 750 million light-years from Earth is a cluster of ~500 galaxies called Abell 85. Galaxy clusters are already immense objects — they have hundreds or even thousands of galaxies in them like our own Milky Way — and A85, as it’s called, is big even among them.

Like many such clusters there is a swollen elliptical galaxy in its very center. That’s because the cluster core is a kind of gravitational trap: other galaxies can pass through there, and any material lost from galaxies tends to fall down to there. Once a galaxy there starts to grow from all this material, its gravity gets big enough to really draw in the material as well — including other galaxies — ensuring it grows huge.

The central galaxy in A85 is called Holmberg 15A. It’s been understood for some time it’s a big boy, though estimates of its mass have varied a lot. At the same time, we know that every big galaxy has a supermassive black hole in its heart, and in general the mass of that black hole scales with the mass of the galaxy. In other words, a bigger galaxy tends to have a bigger central black hole. That’s not always the case (the Milky Way is huge but we have a relatively small central black hole) but it’s a good assumption to start with.

If Holmberg 15A is so big, how big is its black hole? It’s been estimated at 3 to as much as several hundred billion times the mass of the Sun, which means it’s either really big or OHMYGOD big. The top end of that scale would make it by far the heaviest black hole known. However, that range is pretty wide, and it’s not clear where it actually falls.

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To find out, a team of astronomers took a look at Holmberg 15A with the Keck Telescope in Hawai’i [link to journal paper]. They used a nifty instrument called an integral field spectrograph, which is very powerful: it can take spectra of multiple objects over an area of the sky simultaneously. Usually you can get spectra of one object at a time, or maybe a few, but this does it by the hundreds.

A spectrum is what you get when you break light up into individual colors, like a prism or a raindrop does for a rainbow. When astronomers do it, though, we break up the light into hundreds or even thousands of individual colors, very narrow wavelength slices. This is important because a spectrum of an object can tell us a huge amount about it: its temperature, chemical composition, rotation speed, and much more.

When you point such an instrument at the center of a galaxy, it can take simultaneous spectra of the billions of stars there. We can’t say anything about individual stars in such an observation, but only what they’re doing en masse, and more importantly, at different places around the galaxy core.

As stars orbit the galaxy center, half are headed toward us and half away. This induces a Doppler shift in their spectra, moving the colors slightly blue or red, respectively, and that shift can be measured. So what the astronomers got from Holmberg 15A was a map of how rapidly stars are moving around the galaxy core versus their position around the core.

This information is a gold mine. We can make physical models of what galaxies look like when observed this way — say, the shape, mass, mass distribution (how the stars are spread out), and more. These can be compared to the observations to see what the conditions in the galaxy are like.

For one thing, these observations get an estimate of the total mass of all the stars in the galaxy: they find it’s nearly three trillion times the mass of the Sun! Three. TRILLION. That’s sixty times the mass of all the stars in our Milky Way, which I remind you, is considered a big galaxy. Holmberg 15A is immense.

And so is its central supermassive black hole. The astronomers get a mass for it of a staggering 22 billion solar masses (± about 200 million). That’s just mind blowing. It’s one of the two beefiest black holes known in the local Universe (a volume of space very roughly 1 billion light-years in radius centered on us).

There are bigger black holes out there in the more distant Universe, but a lot of them don’t have well constrained masses, because the techniques used to “weigh” them aren’t as accurate. But either way, Holmberg 15A’s central monster is indeed a beast.

I’ll note that for a while it was thought that this object might actually be a binary black hole; two massive ones orbiting each other. However, a paper came out in 2020 refuting that. Coincidentally there is a much more distant galaxy seen superposed near the center of Holmberg 15A, and again some astronomers wondered if it might be a third black hole. However, that galaxy is 9.5 billion light-years away, so way way in the background.

I was surprised to learn that Holmberg 15A’s black hole isn’t very active; that is, gobbling down tons of matter. A black hole that big can eat a lot of stuff very rapidly! And while it is active, it’s only mildly so. There is likely material piled up in a disk around the black hole, because radio observations show a pair of beams of material blasting away from the galactic center; those disks of hot matter can focus these beams (astronomers actually call them jets) that blast material away at high speed. In this case, the jets are a few thousand light-years long. On a human scale that’s a really long way, but for black holes on this scale it’s kindof meh. Not bad, but a lot less than I would have expected.

A final thought: we’re still not sure how black holes can get this big. There’s an upper limit to how rapidly they can grow; as they feed the material in the disk gets so hot it starts to blow a wind of subatomic particles that can be so fierce it actually prevents material from falling into the black hole. This is called the Eddington Limit, and there are ways to circumvent it a bit, but even so it’s hard to get black holes that are really big. Worse, we see billion-solar-mass black holes in galaxies not long after the Universe itself formed, and that’s a big mystery astronomers are working to solve right now.

There’s a lot left to learn about black holes, including how they came to be in the first place. But we love a good mystery, don’t we? With science, and more and better observations, we’re well on our way to solving this one.

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