The Milky Way's Central Black Hole Doesn't Have an Appetite It Should Have, Astronomers Found Out Why

Sagittarius A* - Milky Way's central black hole. X-ray and infrared combined.

Sagittarius A* - Milky Way's central black hole. X-ray and infrared combined.

September 1, 2013

Black holes are usually very active, consuming large quantities of gas surrounding them. This is, however, not the case when it comes to the Milky Way's central black hole. Scientists finally found out that this is because the gas around our galaxy's central black hole is too hot.

Black holes are massive objects with the gravity pull so strong, even the light can't escape their event horizon, hence "black" in their name. We can't observe them directly, but we can observe the effect they have on their surroundings. One example is the effect a black hole can have on the gas around it as it approaches the event horizon – the point of no return. The cloud of gas rotates around the black hole and the inner region of this cloud gets so heated while approaching the black hole, it emits radiation in form of X-rays, which we can detect with our telescopes. Some of that gas gets "eaten" by the black hole, the rest is ejected.

Observing the black hole in the center of our galaxy, 26,000 light-years from Earth, astronomers have made some new discoveries which will help us better understand these objects. Images of Sagittarius A* (Sgr A*) region taken with Chandra X-ray Observatory, a space telescope detecting X-rays, show that Milky Way's black hole eats less than 1% of the gas within its gravitational grasp and that the majority of the radiation comes from the gas that is ejected, not the black hole itself.

The Chandra space telescope observed Sgr A* for five weeks in 2012, obtaining very detailed X-ray images and energy signatures of super-heated gas around the black hole. The study was published in Science last Thursday. "We think most large galaxies have a super-massive black hole at their center, but they are too far away for us to study how matter flows near it," said Q. Daniel Wang, University of Massachusetts, Amherst, USA, who led the study. "Sgr A* is one of very few black holes close enough for us to actually witness this process."

The black hole in the center of the Milky Way is obscured by clouds of gas and dust, so we can't see it with optical telescopes, but we can with X-ray telescopes and we know there are stars orbiting this region, Sgr A*. It has 4 million solar masses, meaning it's 4 million times heavier than our Sun, so it should come to you as no surprise that there are stars orbiting it. Some theoretical models predict that X-rays are emitted from small stars around the black hole, but these new data favour models that suggested that the gas near the black hole emits the majority of X-ray radiation.

They analyzed the X-ray data and focused on the observed emission lines, zeroing in on the light given off by iron atoms. Iron can be found in both the stars and the gas surrounding the black hole, but the temperature of the iron from stars is many times cooler. The light wavelengths observed in the said data suggests that the iron was very hot, meaning the X-rays came from the gas around the black hole. The gas, however, is made from stellar winds coming from massive young stars in the area.

"This new Chandra image is one of the coolest I've ever seen," said co-author Sera Markoff, University of Amsterdam, the Netherlands. "We're watching Sgr A* capture hot gas ejected by nearby stars, and funnel it in towards its event horizon."

In order for gas to be captured by the black hole, it mustn't be too hot. By ejecting matter, the black hole makes the gas lose heat and momentum. "Most of the gas must be thrown out so that a small amount can reach the black hole", said Feng Yuan of Shanghai Astronomical Observatory, China. He is the study's co-author. "Contrary to what some people think, black holes do not actually devour everything that's pulled towards them. Sgr A* is apparently finding much of its food hard to swallow."

"We think most of the energy, or a lot of it, is going toward pushing the gas away from the black hole and not letting it fall in," says Mike Nowak, at MIT's Kavli Institute for Astrophysics and Space Research, Cambridge, USA. "Now we have a better understanding of what parts are active, and what aren't."

"The black hole doesn't have a chance to do its meat-grinder thing and turn that matter into energy," says Joey Neilsen, also from MIT Kavli. "All of that stuff basically escapes before the black hole can destroy it."

The gas surrounding a quasar is dense and cool. Quasars are regions of space surrounding super-massive black holes which can be found in distant i.e. old, massive galaxies, which "eat" large quantities of gas. The gas around Srg A*, however, is diffuse and super-hot, so the black hole has trouble consuming larger quantities of it. Good news for us is that the event horizon of Sgr A* casts a shadow against this glowing gas in low-energy part of electromagnetic spectrum, meaning we could use radio telescopes to observe and understand this shadow.

Many of the same researchers published another, related study in The Astrophysical Journal, where they looked at the radiation coming from the black hole itself, not the surrounding gas. It turns out that the energy comes from continuous flares in the black hole's area close to the event horizon. "It's this sort of constant burn — this little sizzle of flares that is always happening," Neilsen said. "It's doing all this flaring and popping, and all sorts of little activity on a fairly faint scale."

By further analyzing data, they were able to detect large and medium-sized flares occurring once every few days, but they also predicted smaller flares too faint to be seen. So, based on the frequency of observed flares at various luminosities and on the luminosity distribution they estimated the frequency of these smaller flares. The results returned a near constant "buzz" of small flares from the black hole that matched X-ray activity in galactic center in accordance with the group's earlier estimates.

"Rarely do we have a chance to answer these questions in detail," Neilsen says. "This is a really great chance to actually dive in and say, "How do we understand what normal galaxies are doing, as opposed to gigantic luminous quasars and active galaxies?' This is the first time we could really work on answering a question like this with high-quality data."

Sources: NASA.gov and mit.edu

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