This image from NASA's Chandra X-ray Observatory shows the center of our Galaxy, with a supermassive black hole known as Sagittarius A* (Sgr A* for short) in the center. Using intermittent observations over several years, Chandra has detected X-ray flares about once a day from Sgr A*. The flares have also been seen in infrared data from ESO's Very Large Telescope in Chile. Click to embiggen.
How did I miss this story? Early in February, the folks working with the Chandra X-ray Observatory announced that they’ve been watching the black hole at the heart of the Milky Way Galaxy munching down on asteroids. According to them, this happens pretty frequently. They’ve seen the evidence: once-a-day x-ray flares from Sagittarius A* (that’s the name given to our central black hole), or Sgr A* for short. The flares last a few hours, and have also been seen in infrared light by detectors at the Very Large Telescope in Chile.
The idea is that there is a cloud around Sgr A* containing hundreds of trillions of asteroids and comets. Where did they come from? Astronomers suggest that this debris collection is made of material stripped from the parent stars by the force of gravity.
An asteroid that gets too close to another object, such as a star or planet, can get thrown into an orbit that places it on a heading for Sgr A*. If it gets too close — say within 100 million miles of the black hole (about the distance between Earth and the Sun) it would be torn into pieces as it encountered the tidal forces from the black hole. These fragments would be vaporized as they pass through the hot, thin gas that continually flows toward Sgr A*. This is very similar to what happens to a meteor when it encounters Earth’s atmosphere for example. It heats up and glows, and eventually it vaporizes, and we see a flare marking the end of the meteor’s trip. When an object does this near Sgr A*, we can see it in x-rays and infrared. Whatever’s left of the asteroid gets sucked into the black hole.
This is kind of a cool thing to see at an object so far away, and Chandra’s still on the case. Over the next year or so, the satellite will study more of these things that go “fuff” in the night near Sgr A*.
his composite image shows the central region of the spiral galaxy NGC 4151, dubbed the "Eye of Sauron" by astronomers for its similarity to the eye of the malevolent character in "The Lord of the Rings". Courtesy Chandra X-ray Telescope.
You just have to know what you’re looking at when you see something like this appear in the astronomy news of the day. Sometimes I laughingly refer to pictures like this as “blobs in space”. But, as I know — and you’re about to read– a further look at them gives you a peek at something you didn’t expect.And, you get to learn something new about things like galaxy cores and black holes, in the bargain.
This set of “blobs” is dubbed the “Eye of Sauron” because astronomers studying it decided this central region of the spiral galaxy NGC 4151 kind of looked like the object of the same name in the “Lord of the Rings” movie. The pupil of this “eye” shows x-rays (colored blue) streaming from a central object embedded in the core of the galaxy.
The yellow spots out in the corners of the eye are actually visible-light data (called “optical”) showing the location of positively charged hydrogen gas (also known as H II, which means it is energized in some way). These are regions where star formation has occurred in the recent past.
The red blobs are clouds of neutral hydrogen gas, which show up in radio wavelengths. They are part of a larger structure that is being distorted by gravitational interactions with other parts of the galaxy. And, some of it is material falling INTO the center of the galaxy, toward the pupil.
So, now that we know what the pieces and parts are, what does they mean? Recent work shows that the blue-labeled x-ray emission is probably coming from an outburst powered by a supermassive black hole at the heart of the galaxy. That black hole region is colored white in the image.
Evidence for the idea that some kind of action is tossing out x-rays from the center comes from the elongation of the x-ray emissions running from the top left to the bottom right and details of the x-ray spectrum. There are also signs of interactions between a central source and the surrounding gas, particularly the yellow arc of H II emission located above and to the left of the black hole.
Astronomers have posed a couple of different scenarios to explain the x-ray emission. One possibility is that the central black hole was growing much more quickly about 25,000 years ago (in Earth’s time frame) and the radiation from the material falling onto the black hole was so bright that it stripped electrons away from the atoms in the gas in its path. X-rays were then emitted when electrons recombined with these ionized atoms.
The second possibility also involved a hefty flow of material into the black hole in relatively recent times. The energy released by material flowing into the black hole in an accretion disk created a vigorous outflow of gas from the surface of the disk. That outflox directly heated gas in its path to temperatures hot enough to permit the emission of x-rays. Unless the gas is confined somehow, it would expand away from the region in less than 100,000 years. In both of these scenarios, the relatively short amount of time since the last episode of high activity by the black hole may imply such outbursts occupy at least about 1% of the black hole’s lifetime.
It’s amazing what you can learn when you look at blobs in space through the viewpoint of astronomers. They use not just astronomy and physics, but also chemistry and astrophysics to explore and understand the processes and events that occur in the hearts of galaxies like this one, which lies some 43 million light-years away from us. Thanks to tools like the Chandra X-ray Telescope, the Isaac Newton Group of Telescopes on La Palma, and the National Radio Astronomy Observatories, images like this can be obtained, explained, and shared with everybody. We all win when that happens. For more information on this image and to see other wavelength views of the “Eye of Sauron” visit the Chandra X-ray Telescope’s story about these observations.
