Book stuff, and a whoppingly ginormous big black hole… maybe

May 8, 2023   Issue #561

Subscribers have a supermassive impact 

My book

This is about Under Alien Skies, isn’t it? Yes. Yes it is.

Just a little… bookkeeping* here:

1) The website Next Big Idea Club asked me to write about what I think the five most important takeaways are for Under Alien Skies. That’s a pretty interesting question, so I wrote my answers for them and that’s now all live on their website! I also recorded myself reading them if you prefer to listen over reading. I gave this a lot of thought, so I hope you head over and check that out.

2) I did a fun interview about the book with David McRaney for the podcast You Are Not So Smart, and we spend a lot of it geeking out over Saturn. We also talk a bit about Not Being a Dick, UFOs, and other sciencey and non-sciencey things.

* This is one of my favorite English language words. Can you guess why?

Astronomy News

It’s a big Universe. Here’s a thing about it.

How big can a black hole get?

Technically speaking, there’s no upper limit. I mean, sure, if all the mass in the Universe were to fall into a black hole then yeah, there’s your upper limit. But realistically, they can only get so big due to three factors: They can only eat so fast (more on that anon), there’s only so much available food for them (ibid.), and the Universe is only so old (so the time for them to grow is limited to about 13+ billion years).

Despite this, though, they can get pretty danged big. Many have been found with over a billion times the Sun’s mass, and a handful in the ten-billion range.

Astronomers may have just added another to the latter: A black hole in the center of a galaxy in the cluster Abell 1201 may have a mass of about 33 billion solar masses! Yikes. [Link to paper]

Abell 1201 is about 2.1 billion light-years away from us, and has about a hundred galaxies in it. In a cluster like this there can be a single central large galaxy in the center; a big galaxy will tend to fall to the center and then grow as matter falls down into it. There are different terms for this, but Brightest Cluster Galaxy (or BCG) is a good generic one.

The BCG in Abell 1201 is an elliptical galaxy, and a beefy one, with probably over a trillion stars in it. In its center is almost certainly a supermassive black hole; every big galaxy appears to have one. But how supermassive is it?

Sometimes these black holes are feeding on material falling into them, and by measuring how much energy this material emits as it falls in can be used to get the mass. Unfortunately that’s not the case for the Abell 1201 BCG. But nature has thrown us a bone.

Gravity bends space, and very massive objects bend space a lot. This can distort the light coming from objects on the other side of them as seen from Earth, an effect called gravitational lensing. I’ve written about this topic eleventy bajillion times, so go read this article and then come back here.

Also, here’s a video explainer:

Right. As it happens, a more distant galaxy is sitting well behind the Abell 1201 BCG, at a distance from us of about 4.7 billion light-years, over twice as far away as the cluster. The mass of the BCG warps the light from that more distant galaxy, and how much it gets warped tells us about the mass of the galaxy.

We see the background galaxy sitting extremely close to the center of the BCG, so if there’s a black hole there then its gravity will really warp the light. Using that fact, and some extremely sophisticated modeling of the physics of lensing, the astronomers measure the black hole mass at 32.7 billion times that of the Sun. That’s… a lot.

There are some caveats, though. For one, there’s a big uncertainty of about 21 billion solar masses in the measurements. So it could be as little as about 11 billion or as much as 54-ish billion. Hard to say. In fact, one solution to their calculations has no black hole at all! But they argue this doesn’t make sense physically, because it involves structures in the galaxy that don’t work in real life (in a nutshell, it predicts the center of the galaxy to be in a place contraindicated by the actual observations). So they think the most likely solution is one with an ultramassive black hole.

Another possibility is that there’s a lot of dark matter around and in the galaxy, and that’s what’s causing the lensing. The authors of the paper think they have accounted for the dark matter, though, and again conclude the black hole is real and monstrous.

So how did it get that big? Assuming this calculation of its mass is correct, it must have had a lot of food. Whole galaxies can fall into the cluster center, helping that central galaxy grow. These galaxies usually have a lot of gas, which can stream to the very center where the black hole is, and it can gobble that stuff up.

As I alluded to in the first paragraph, though, black holes can only eat so quickly. The material gets very VERY hot as it falls in, and blasts out huge amounts of light. This can blow away material around it, creating a wind of matter streaming away from the black hole. That can choke off matter farther out from falling in. If that happens then the black hole goes on a diet: Less stuff falls in, the radiation decreases, the wind dies down, and then more material can fall in. This is a self-regulating process, and the upper amount of radiation the material falling into a black hole can generate is called the Eddington Limit.

That limit is higher for more massive black holes, so as this one grew it was able to continue growing as long as material was available. That’s not always the case, which is why black holes masses have a wide range. And probably why so few are ultramassive; there’s just not enough matter to keep feeding them.

Finding this one is cool, though. If this mass measurement holds up, it’ll tell astronomers a lot about how these beasts form, how they affect the galaxy around them, and how that affects the cluster around them.

Also, the method they used to get the mass is relatively new, so it’s a good test. And if it pans out (maybe confirmed through other methods) then it’s a — pardon me, but I have to — massive boon to researchers. Any tools we can bring to bear on issues like this are helpful! These objects are a long way away, and studying them difficult. I’ll be curious to see follow-up studies on this gargantuan object, and see if it holds up!

Et alia

You can email me at [email protected] (though replies can take a while), and all my social media outlets are gathered together at about.me. Also, if you don’t already, please subscribe to this newsletter! And feel free to tell a friend or nine, too. Thanks!