Hi everyone! 👋

Today I'm going to talk a little bit about black holes and what I do as a postdoctoral fellow at Stanford. I can't cover all black hole physics, or all of my work, but I'll try my best to highlight some of the most interesting things! Join me for a wild ride through spacetime!

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Let's start at the basics: black holes are areas in space where gravity is so strong that not even light can escape. One way that this can happen is if matter has been squeezed into a tiny, tiny area. So, if you have the ability to compress something into a small ball, you can make your own black hole!

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It's not *that* easy to do, however. If we wanted to make Earth a black hole, you would have to compress it into ball about ~9 mm across. Good luck!

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There are a few flavors of black holes, the most well-studied being: stellar-mass (have masses on the order of 3-10s Suns) and supermassive (have masses on the order of millions of Suns!). I love all kinds of black holes, but my expertise is in *supermassive black holes*. We use "SMBH" for short. We think that most galaxies have a SMBH sitting right in its center (this includes our very own Milky Way galaxy! Look up "Sgr A*" to learn more about the SMBH sitting in our backyard).

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So I told you these are objects where light can't even escape their gravitational pull. How do we study them, then?! We study the stars and gas that are affected by the strong gravity around the black hole. Specifically, many supermassive black holes have large accretion disks, full of gas and dust, that orbit SMBHs and emits light in many different wavebands.

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I mostly study the X-ray emission around SMBHs. There are a handful of X-ray telescopes in space that are used to further our understanding of black holes. Some of my favorites are Chandra, NuSTAR, XMM-Newton, and Swift. Chandra (image below) is a fantastic telescope, due to its superb resolution. It's able to resolve the smallest spatial scales out of all the X-ray telescopes, allowing for a lot of interesting science to be done!

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There are a lot of unanswered questions about SMBHs like: how are they created, how do they grow and evolve, and how do they affect their surroundings? There's no straightforward answer to any of these questions -- we're learning that the evolution of one SMBH may look different than the evolution of another. But, one way to probe some of these questions is to look at SMBHs that are in the process of merging ...

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AH! Merging SMBHs?! BUT, we know that galaxies merge with one another over time. We see galaxies merging in most places in the sky (see below for one observed by the Hubble Space Telescope). Remember when I said that most galaxies have a central SMBH? SMBHs will also interact and merge with other SMBHs over time.

Merging galaxies are interesting systems where gas and stars can get sloshed around near the two SMBHs, affecting their accretion disks, and thus their growth and evolution.

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So, I study systems where two supermassive black holes are in the process of merging, because their host galaxies are interacting with one another.

We want to find more of these systems to better understand how supermassive black holes grow and evolve, as well as understand the timescales for them to merge.

Because as they merge, they'll emit some of the loudest gravitational waves in our Universe ... (video shows example of gravitational waves as black holes merge)

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I've shown you pretty images and drawing of galaxies and black holes, but do you want o see what these actually look like in X-rays?

Pixelated blobs! But beautiful pixelated blobs (images taken from Foord+2019)!

And when the separation between the two SMBHs is small, it's actually hard sometimes to tell if you have one pixelated blob (a single SMBH), or two pixelated blobs (a SMBH pair)

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I developed a code that helps better identify when you have 1 pixelated blob vs. 2.

And I'm running it on many Chandra observations of SMBHs to find possible pairs that we can't identify by eye!

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We hope that by finding more interacting SMBHs, we can start to figure out if their growth depends on different environmental parameters (do they like dusty mergers? do they not care?), and how the frequency of SMBH mergers evolves over time (are there more in the past, when the Universe was different than it is today?)

In the near future, I'll share more of our discoveries in attempting to answer these questions.

But for now, I need to start work and find more SMBH pairs!

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@AdiFoord omg I’ve seen so many cool space people today and I’m just absolutely floored at how neat this stuff is 👽


It's fun to talk about it, it helps you remember how cool your job is! :)

@AdiFoord This is some quality content that I was wanting for here in the fediverse. Thank you!

So, if we get hit by a gravitational wave loud enough so a human can feel it. How would it feel by human senses?

@AdiFoord thank you for this🧵! As a complete layman, I was wondering what you think about the Planck star theory (e.g. max energy density reached way before smallest possible scale of object; if I am not mistaken), especially as a sort of solution for the information loss paradox (which may or may not have been solved by Hawking radiation already...?).

I found that theory to sound quite elegant. Although I don't get the math behind it, unfortunately.

@AdiFoord thank you for writing about this, I was always interested in black holes and more in general in astrophysics but I'm too lazy and maybe too dumb to learn something about it

@jwuphysics nope! Bayesian statistics can be quite powerful on it own in the photon-counting regime that is X-rays 😃 but ML rules!!

@AdiFoord Yay for Bayesian methods! It's great to hear that statistical methods are so powerful.

@AdiFoord I'd like to see more like the pixelated images; showing what your work and research looks like. Very interesting to see!

@michaelseraph yes, in the future I’ll show more data + data analysis!

@AdiFoord That sounds incredibly cool! And that video, damn!
That is surely sped up, though, isn't it? How different is the speed in real life?

@michaelseraph the total time for two galaxies to merge could be on the order of 100s of Myrs! Once the two black holes are at smaller separations, it can still take millions of years (or more) to eventually merge.

@AdiFoord Is there a "maximum loudness"? If so, would that constrain what could happen when two really huge black holes merge?

@AdiFoord Is the James Webb telescope equipped to study black holes by any chance?
I suspect it's not?

@michaelseraph yes JWST will definitely study black holes! Black holes can emit in radio, Infrared, optical, UV and X-rays; so JWST will be look at infrared emission from black holes and their jets, and finding some of the earliest black holes in our Universe!

@AdiFoord That's pretty impressive ... 👍

Shouldn't one day all BHs fall together in ONE big BH ? 🤔

And then, maybe, TBBT begins again ? 🤔

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