Home Science Strange Giant Filaments Reveal a Mystery at the Milky Way’s Heart

Strange Giant Filaments Reveal a Mystery at the Milky Way’s Heart

Strange Giant Filaments Reveal a Mystery at the Milky Way’s Heart

Strange things are afoot in the mysterious heart of the Milky Way. It’s a bustling, star-packed region that also harbors our galaxy’s supermassive black hole, which scientists call Sagittarius A*, or Sgr A*. Amid the millions of young, hot stars zipping around galactic center, astronomers have also spied a tangle of curious filamentlike structures stretching out for light-years. What exactly are the filaments? How did they come to be? And what do they tell us about the Milky Way’s heart? As of yet, these are all open questions.

The person most likely to answer them may be Farhad Yusef-Zadeh, an astrophysicist at Northwestern University, who has been studying the galactic center for decades. In the 1980s he and his colleagues discovered the first known filaments—streaks of superfast particles that stretch vertically through the galactic plane for more than 100 light-years and remain unexplained. And this month Yusef-Zadeh and his colleagues published new research in the Astrophysical Journal Letters showing the Milky Way’s heart unexpectedly hosts a second type of filament, too—so-called horizontal filaments, which are shorter and run parallel, rather than perpendicular, to the galactic plane.

Scientific American spoke with Yusef-Zadeh about these strange filaments and how they may have formed.

[An edited transcript of the interview follows.]

What’s our current understanding of the environment at the center of the galaxy?

The galactic center is a very rich environment because there is already a supermassive black hole there, and it’s got about four million times the mass of the sun. You see all kinds of weird structures, unusual ones—we still don’t understand many of them. It’s a pretty interesting region. It’s really the metropolis of the galaxy.

When you have unusual, extraordinary places, you also find very unusual structures. That’s where you find these things—not in places that are more mundane.

Can you take us back to your initial discovery of vertical filaments in this region in the 1980s?

We weren’t really looking for this kind of structure. Nobody had seen a feature like this. It was early on in the commissioning of the Very Large Array of radio telescopes. You’re finding something very odd, so you’ve got to be super careful [about determining] whether it is real or not. There were some discussions that maybe this is an artifact of imaging that we had done early on. But then it showed up at multiple wavelengths, and then other people actually found it also, so it was certainly a real structure.

Editor’s Note: Yusef-Zadeh explains that these long, vertical filaments turned out to be made of synchrotron radiation, which is produced by particles moving at nearly the speed of light through a magnetic field. But how this forest of filaments forms near the galactic center is still unclear because there’s no obvious reason why these particles should be moving so fast without apparently streaming out from a powerful source, such as Sgr A*.

The issue was: How do you accelerate these particles to such high energies? Usually you have a source—a neutron star or a black hole or a pulsar, for example—that accelerates particles. But here, they were just sitting there. How do you explain that? It just was very unusual and very mysterious.

Since then MeerKAT [a radio telescope] has also discovered long filaments in other active galaxies. They look very, very similar to the ones that we see in the Milky Way. They’re much more large-scale in terms of their lengths, but their underlying physical properties are very similar—that’s what we argue. They may complement each other in terms of understanding the origins of these structures. It’s not just unique only to our own galaxy. It is also in other places.

How did you and your colleagues find the horizontal filaments described in the new paper?

For the last few years or so, we’ve been studying the statistical properties of the filaments. We had never really quantified them.

We found, surprisingly, a distribution of filaments parallel to the galactic plane. It didn’t really start out with a statistical measurement. We were looking at the images, and one night I just realized, Why are all these filaments pointing radially? Could it be a random thing? Then we started doing statistical tests to see if it actually pans out, and we learned that this is for real. It’s significant.

Then we did a bunch of other plots to indicate that these are radial. That was another element to the surprise. Why are they pointing toward the nucleus of the galaxy? That was actually really exciting for us, simply because it gave us some clue as to how they may have originated, whereas the vertical filaments are still very mysterious. We still don’t know how they formed.

So what’s your theory for how the horizontal filaments formed?

We think that Sgr A*, the black hole, has a jet-driven outflow. I’m simplifying it, because we still don’t know exactly how jets are created from accreting black holes. But when you accrete energy onto a supermassive black hole, a fraction of that energy actually goes into an outflow as a jet. Our galaxy is a bit dormant right now, but we think that this jet has been active and should actually still be there. We’re talking about an outflow that has been going for about six million years, we think.

It’s really just like wind blowing. For anything that has a lower density, or the pressure is not sufficiently high, the high pressure from this outflow is going to stretch it out.

Editor’s note: Yusef-Zadeh notes that the horizontal filaments appear to come in two different flavors. One is made of similar material to the vertical filaments. He and his colleagues think this flavor forms when the black hole’s outflow slams into a vertical filament, snipping it and aligning it to point toward Sgr A*. The other flavor, he says, likely forms when the outflow blasts through what scientists call H II regions, which are clouds of ionized gas around hot stars.

We think that because the center of the galaxy has a lot of massive stars, their atmospheres could be affected by this outflow and that this pressure interacts with the atmosphere and stretches out. It’s a mechanism that we’re hoping to test with higher-resolution observations. To see, basically, where these filaments are connected to and which star they’re connected to, we need higher-resolution observations. That’s one of the plans that we have to test this idea of the stretching and elongation and alignment of these filamentary structures.

Is there an observatory now that could get those higher-resolution views?

Radio telescopes could do that, and in some cases, the James Webb Space Telescope can also. Hopefully we’ll see, basically, a connection or a linkage between the filaments and [the stars]. But we need higher resolution because there are so many stars along the line of sight that confusion is always a big issue, and that’s what the problem we have is. We cannot identify which star is associated with one end of the filament. But if we go to a much higher resolution, we should be able to see not only the star but also the atmosphere of the star being basically elongated in the direction of the filament itself. If we can do that and we can measure also the velocities, then I think that’s one way to really test this picture.

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