OOOoooo, I love it when I can talk about two of my favorite things: monster black holes, and STIS, the Hubble camera I worked on!

A team of astronomers just published their research [link to journal paper] showing details in the relatively nearby quasar 3C 273 that have never been seen before. The image is super cool, the science is also super cool, and, as it so happens, the way they got the observations are super cool. So let’s dive in.

All big galaxies, including our own, have a supermassive black hole in their centers, usually with millions or billions of times the mass of the Sun. For most galaxies the black holes are quiescent, meaning they’re not feeding on material around them. But some have material falling into them. This stuff — usually loads of gas, dust, and sometimes even whole stars — collects itself into a huge, flattened disk called an accretion disk that sits just above the event horizon, the Point Of No Return. Stuff closer to the black hole orbits madly, at very nearly the speed of light, while stuff farther out moves more slowly. That means the material is rubbing together, which heats the gas to extremely high temperatures, millions of degrees. Matter that hot emits a lot of light, like a lot a lot. In some cases the disk can outshine all the stars in the galaxy combined!

We call such galaxies active galaxies, and they come in a lot of different flavors. For some we only see them in radio waves, others in high-energy like X-rays, and some blast out infrared light. It mostly depends on the structure of the disk around the black hole, which can also have a surrounding torus of dust that can obscure the light from the disk. Different geometries can dictate how much and what kind of light we see.

3C 273 is a quasar, which originally meant a quasi-stellar radio source, an object that was blasting out radio waves but looked like a star through telescopes. I wrote a fun history of 3C 273 and the “quasar” moniker on The Old Blog™ if you want to check it out.

3C 273 is about 2 billion light-years from us, yet so bright it can be seen in amateur telescopes (it’s a bucket-list time for me to see with my own eyes). The luminosity of the central source is mind-vaporizing; it’s about 3 trillion times more powerful than the Sun. Three. Trillion. That’s brighter than our entire galaxy, and we’re talking about an object just a few light-years across. 

In fact it’s so bright that studying it is hard. We’d like to know more about the galaxy around the central quasar, since the two interact, and their relationship will help us understand active galaxies as well as just how galaxies behave. Also, most quasars are much farther away than 3C 273, so studying it could yield details otherwise too small to spot.

The problem is, seeing details in it is like trying to stare into a spotlight and seeing a firefly next to it. So what do you do? You block the light! 

This is where STIS comes in. The Space Telescope Imaging Spectrograph is a camera on board Hubble Space Telescope, and it has a special device in it called a coronographic mask. It’s literally a thin metal plate that’s mostly empty in the middle, but has two intersecting parallel metal wedges in it, and two smaller bars called “fingers”. If you put a bright source behind one of these structures, the central part can be blocked and fainter material around it can be seen.

I worked on STIS before and after it was placed on Hubble to characterize how it works (in fact, the 3C 273 paper references a paper I’m on in which we described how the mask worked; I’m the last author which in this case means I did all the grunt work of processing the dozens of test images we took after launch to make sure the mask worked correctly), so seeing how they got their observations was fun. They placed 3C 273 behind two different spots in the mask, blocking the innermost glare. Even then, though the faint material was tough to spot. So they observed two stars that were near 3C 273 in the sky, and that also had similar colors (the quasar is quite blue); the way light scatters inside the telescope depends strongly on the color of the source, so if they used a red star they’d see a very different pattern.

They then used those stars to subtract the remaining bright glow from the quasar, revealing details in the underlying galaxy. As you can see, the parts of the coronograph they used are near the camera edge, so they waited a couple of months and repeated the observations; at that time the natural motion of Hubble rotated the observatory by nearly 90°, so parts of the galaxy that were out of frame in the first observations were rotated into view in the later ones. They could also use the later data to fill in the blanks of the first ones, and vice-versa, letting them see quite close in to the quasar itself. Clever!

I wrote about this technique (and how I used it on STIS) in BAN Issue #725 if you’d like to read more.

In the end, what they got was the closest-in view of 3C 273 ever obtained!

What they found is pretty cool. The line of material you see at some distance from the center is called a jet, material blasted out by the intense radiation and intense magnetic fields in the central part of the accretion disk. That’s been known for decades, but Hubble observations taken over the past 20 years actually show its motion; what they found is that material farther out in the jet appears to be moving faster than stuff closer in to the quasar, which is not what I would have expected (while there’s not much stuff in intergalactic space, there’s some, so it would slow down the jet via drag). Apparently the material still accelerates even when it’s over 100,000 light-years from the center!

In the central galaxy they found that overall the structure is symmetric around the center, though there are some interesting blobs and doodads near the core. They see, for example, an inner jet just a few thousand light-years from the quasar, lining up with the outer jet. Details are difficult that close in, but just knowing it exists will excite astronomers who study such things. There’s also a small fuzzy spot they call a “core blob”, for lack of a better term, and it’s not clear what it is. I don’t think it’s a background galaxy, but instead something in the galaxy itself. Possibly a huge star cluster…? The astronomers carefully say further observations will be needed to nail anything down.

All in all, reading this paper made me happy. I do love a ferociously erupting gourmand black hole, especially the one in 3C 273, and it makes my heart sing to know this was done not just with a camera I worked on but using techniques I and the team I was a part of worked on so assiduously two decades ago. I don’t do any science these days — I enjoy sharing it with y’all more — but it’s nice to know that a new generation is still able to make use of what I did do once upon a time.

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