Black Holes Behaving Badly

The 245th meeting of the American Astronomical Society (AAS) kicked off today, and what better way to start the largest meeting of astronomers ever with deep discussions about black holes?

In particular, the focus was on supermassive black holes, which reside at the centers of most large galaxies. But even a supermassive black hole is just one itty-bitty object inside a collection of hundreds of billions (or perhaps even a trillion) stars that make up a galaxy. As astronomer Peter Boorman from Caltech pointed out, the size of one of these black holes compared to the galaxy itself is the same as a pea relative to Earth—yet these objects have an outsized influence on their host galaxies.

We often imagine black holes gobbling up everything around them, but please don’t picture a pea eating up our entire home world! Instead, the observable presence of particularly active black holes typically takes the form of dramatic jets. These jets can stream away from the galaxy at remarkable speeds, creating features larger than the distribution of the billions of stars that make up the galaxy.

In the first presentation at this morning‘s opening press conference, Eileen Meyer from the University of Maryland described a new result about a very unusual galaxy with a typically unfortunate name, 1ES 1927+654. This galaxy caught astronomers’ collective eyes back in 2018, when it flared brightly in x-rays, and it continues to attract attention, quieting down then becoming more active over the intervening years.

From 2023 to 2024, in addition to growing brighter in crazy high-energy x-rays, the galaxy showed significant brightening in low-energy radio emission as well. What emits in radio wavelengths? Jets!

Meyer’s team was able to observe jets forming in real time, with vast quantities of material moving away from the galaxy at a third the speed of light! This is a first. And although the image is something only an astronomer could love, when you realize what you’re looking at, it’s pretty mind boggling.

According to Meyer, “The launch of a black hole jet has never been observed before in real time. We think the outflow began earlier, when the x-rays increased prior to the radio flare, and the jet was screened from our view by hot gas until it broke out early last year.”

What’s fueling the jet? That was the topic of the next press release, with an equally impressive announcement by Megan Masterson from the Massachusetts Institute of Technology.

Detailed observations of the radio emission from the center of the galaxy revealed a steady but accelerating rhythm. Masterson and her team propose that the most likely explanation is a white dwarf orbiting near the inner edge of the accretion disk around the black hole.

(So, yeah, black holes do a bit of gobbling after all. But their appetite is suppressed by the physics of matter flowing into them. This forms a disk of material swirling around the black hole, which is what we call an accretion disk. Basically you have the black hole itself, defined by its infamous “point of no return” event horizon, then the inner edge of the disk is as close as you can get to the black hole without falling in.)

And that white dwarf is whipping around the black hole once every seven minutes!

Why a white dwarf? Because they are compact objects, not big fluffy balls of gas like stars. As Masterson said, “You can’t push a star that close to the event horizon,” Masterson said, “It would rip a normal star to shreds.” Ouch.

Excitingly, NASA’s proposed Laser Interferometer Space Antenna (LISA) mission, set to launch in the 2030s, might be able to confirm the presence of a white dwarf! LISA will detect gravitational waves, much like the ground-based Laser Interferometer Gravitational-Wave Observatory (LIGO), but with much greater sensitivity.

While 1ES 1927+654 is getting a lot of attention because it’s such an unusual object, there’s a lot to be learned by studying the cores of numerous galaxies in search of these supermassive black holes. Some black holes don’t raise much of a stink (like the one at the center of the Milky Way, which is pretty mellow, as black holes go), but others fuel what are called Active Galactic Nuclei (AGNs for short).

Boorman (whom I quoted above) talked about a survey he conducted to characterize the relative numbers of different types of AGNs. If you can imagine the accretion disk around a supermassive black hole being observed from different angles, you can imagine that a dense disk could block light from the central region of a galaxy. The press release describing Boorman’s results has some images and videos to help you imagine what this might look like.

Basically, a disk can end up blocking (or obscuring, if you want to sound like an astronomer) most of the light, some of the light, or very little of the light. We refer to these as being (drumroll, please) heavily obscured, obscured, or unobscured.

Interestingly, light comes in different energies (or wavelengths) that interact differently with matter. So some wavelengths of light will be blocked (or, yes, obscured) more than others. Capitalizing on this different behavior at different wavelengths, Boorman and his team were able to create a small but thorough catalog that allowed them to determine what percentage of AGNs fell into each of the three categories.

This is important for astronomers to understand in order to develop a more complete census of AGNs.

And that in turn helps us understand how galaxies and their black holes evolved over time.

Right before the lunch break, AAS attendees got to hear from Alexandra Pope at the University of Massachusetts Amherst, who also talked about supermassive black holes, paying attention to how they grow in tandem with their host galaxies.

Basically, the bigger a galaxy (the more stars it has), the bigger the black hole at its center. That seems perfectly reasonable, but any good scientist’s job is to ask why… And as Pope noted, the processes for forming stars and black holes are very different, so why should they be related?

That’s a tricky question, and there are different ways we can get to the same result, with supermassive black holes growing quickly or slowly, and Pope pointed to another exciting future space mission that could help find the answers…

The PRobe far-Infrared Mission for Astrophysics (PRIMA) is certainly eligible for the most tortured acronym of any proposed space mission, but it would also observe specific wavelengths of light with unprecedented sensitivity and fidelity. If it moves forward, it might offer exciting answers to questions about supermassive black holes, AGNs, and galaxies.

By the way, you might wonder, how big is the largest astronomy gathering ever? More than 3,600 astronomers are attending the meeting in National Harbor, Maryland, this week! Expect more stories as the week unfolds.