Category Archives: Sagittarius A*

astronomy, black holes, Sagittarius A*

A Snake on a Galactic Plane

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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

stars near SGR A*
Stars and gas at the heart of the Milky Way, as seen through repeated observations by telescopes at the European Southern Observatory

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

snake in the galaxy
A radio image from the Karl G. Jansky Very Large Array showing the center of our galaxy and a curious radio filament (the curved red line). It is located near the center of the image, & the supermassive black hole Sagittarius A* (Sgr A*, the bright source near the bottom).
NSF/VLA/UCLA/M. Morris et al.

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.

alma, radio astronomy, Sagittarius A*

Could Stars Form Near Black Holes?

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Apparently They Can in the Milky Way

While I was gone a lot of really good astronomy  news came out.  It’s tough to stay on touch onboard ship, what with access being expensive and not very fast, so I stockpiled stories until I could get home and read more about them. One of the tales that caught my eye was a study made by the newly commissioned Atacama Large Millimeter Array, an international collaboration in millimeter and submillimeter astronomy between North America, Europe, and East Asia. In the U.S. it falls under the wing of the National Radio Astronomy Observatory and is funded by the National Science Foundation. ALMA is a single instrument composed of 66 high-precision antennas that function as one telescope.

19ALMAGarnier (1)
The Atacama Large Millimeter/submillimeter Array at its 16,500 ft elevation site in northern Chile. ALMA is becoming the most powerful telescope of its kind in the world. At the time of this photo, 19 radio telescopes were in the array. The initial array of 66 radio telescopes is now complete and stretches over a nearly 100 square mile area. CREDIT: W. Garnier, ALMA (ESO/NAOJ/NRAO)

As part of its scientific study of the universe, ALMA focused its attention on the center of our Milky Way, looking for tracers of star formation in the region. The core of our galaxy has at least one massive black hole at its heart, and as most folks now know, a black hole’s neighborhood is not usually considered a great place to raise little stars to become big ones. For one thing, the gravity of a black hole produces tidal forces that would disrupt any nearby clouds of gas and dust that could be sites for star formation.  The whole region of the black hole is not very hospitable, and of course, anything that gets TOO close to the accretion disk of a black hole runs the very real risk of getting caught up in the strong gravitational pull of the singularity.

A combined ALMA and Very Large Array (VLA) image of the galactic center. The supermassive black hole is marked by its traditional symbol Sgr A*. The red and blue areas, taken with ALMA, map the presence of silicon monoxide, an indicator of star formation. The blue areas have the highest velocities, blasting out at 150-200 kilometers per second. The green region, imaged with the VLA, traces hot gas around the black hole and corresponds to an area 3.5 by 4.5 light-years. Credit: Yusef-Zadeh et al., ALMA (ESO, NAOJ, NRAO), NRAO/AUI/NSF.
A combined ALMA and Very Large Array (VLA) image of the galactic center. The supermassive black hole is marked by its traditional symbol Sgr A*. The red and blue areas, taken with ALMA, map the presence of silicon monoxide, an indicator of star formation. The blue areas have the highest velocities, blasting out at 150-200 kilometers per second. The green region, imaged with the VLA, traces hot gas around the black hole and corresponds to an area 3.5 by 4.5 light-years.
Credit: Yusef-Zadeh et al., ALMA (ESO, NAOJ, NRAO), NRAO/AUI/NSF.

Yet, over the past decade or so, astronomers have observed massive youngish stars moving rapidly in the vicinity of the black hole (which is called Sagittarius A*) and that prompted them wonder about where those stars came from. Did they form somewhere else and migrate to the bustling neighborhood of the black hole?  Or, did they somehow form in clouds of gas and dust despite the odds of their stellar birth creches being torn apart by the gravity of the black hole?

ALMA took a look at the region, trying to spy out radio emissions from molecules of silicon monoxide (SiO).  This stuff is found in most molecular clouds where stars form, and when the process of star birth reaches a certain stage, SiO becomes excited. That means it is heated and gives off emissions in millimeter and the microwave wavelengths that ALMA can detect. The SiO becomes part of a river of superheated material that flows away from a newborn star in a jet-like structure, and that makes these molecules tracers of star formation in a cloud.

When ALMA studied the Sagittarius A* neighborhood, it found telltale jets of material flowing away from extremely dense cocoons of gas and dust not all that far from the black hole. Those jets likely indicate the presence of star formation inside the cocoons.  If so, it means that the clouds have enough material and self gravity to somehow resist the gravitational pull of the black hole next door. And, because of that, such clouds could have been the birthplaces of the hot young stars we already see whizzing around in the core of the galaxy. It’s a neat finding, and just the sort of result that ALMA will study in the universe at millimeter and submillimeter wavelengths of radiation.

ALMA had its first light earlier this year and is now in what’s called “science verification”. This is a period where the instruments in the array get tested on real targets. In addition, as new parts of the array come on line throughout the year, they will be added to the observing power of the full array and tested as well. Eventually 66 antennas will be able to focus on the sky, giving astronomers 71,000 square feet of radio light collecting area. This will allow them to look farther out through space, and look at dim, distant, and small objects and processes in the universe.