Action at the Heart of the Milky Way

The center of the Milky Way galaxy is a busy place. While we can’t see everything that’s there using optical light due to intervening clouds of gas and dust, astronomers do look at it using infrared-enabled telescopes as well as x-ray telescopes. The wavelengths of light they see reveal some interesting details about the stars and masses of gas and dust that lie at the core. Astronomers using radio telescopes are studying the supermassive black hole at the galaxy’s heart and have found clouds of hot gas and a gas streamer there. In the not-too-distant future, we’ll see the first “image” of that object, called Sagittarius A*.

Stars at the Galaxy’s Heart

One of my favorite images of the stars at the Milky Way’s heart was made over a period of 16 years by astronomers at the European Southern Observatory in Chile. What they wanted to do was track the motions of stars in the region of Sag A*. As they watched the motions of 30 stars near the black hole and tracked their orbits. Knowing the orbits of the stars also reveals information about the mass of the black hole, plus knowledge about other stellar motions and formation. There is a great deal of star-forming material in the region, and it’s useful to know if stars can form in such a busy environment.

Sag A* in Radio Emissions

Radio emissions from the center of the galaxy also tell an interesting tale. Those come from superheated material near the black hole. In 2016, a researcher named Farhad Yushuf-Zaden spotted a very odd-looking filament—a gas streamer—near the region of the black hole. The data came from the Karl G. Jansky Very Large Array in New Mexico, and it showed a 2.3 light-year-long “snake” of gas. New observations show that the hot gas originated from the area of the accretion disk around the black hole. It’s not possible for something to actually escape FROM the black hole itself because its gravity is too strong. However, activity in the accretion disk kicks stuff away before it gets swallowed up. This generally happens through energetic jets of hot material escaping the region of the black hole. In the case of this stream, astronomers are still speculating on its cause.

Building a Snake of Hot Gas

So, how would such a lengthy hot gas streamer make its way across space from the region of Sag A*? Nobody’s quite sure, but astronomers have some good ideas. In an accretion disk environment, particles can get kicked away at very high speed by the spinning of the accretion disk. These particles get sped up as they circle around lines of magnetic force generated by actions in the disk. That could cause them to be ejected from the disk at very high speeds. If there are enough of them, they would form a constant, fast-moving stream and that could be what the VLA “saw”.

The gas streamer might be something called a cosmic string. It’s a bit more farfetched, but not entirely out of the question. Nobody’s actually SEEN a cosmic string, so they remain theoretical until one is detected. Scientists think of them as very long, thin objects with some amount of mass and carry an electric current. If they do exist, astronomers suspect they might gravitate to the centers of galaxies, and they could be “captured” if they get too close to any lurking supermassive black holes. It’s an “out there” kind of idea. If it’s true, finding it at the heart of our galaxy would prove a great deal of theoretical work. To prove it, however, is going to take more observations.

The gas streamer might just be superimposed over the region and not connected to the black hole at all. It just “looks” like it’s connected to Sag A* because of our point of view. However, there’s one kink in the snake that implies something in the region is affecting it. What that could be remains to be figured out. The jury’s still out on all three ideas.