Black Holes Threaten Star Formation

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Astronomers studying the supermassive black hole at the centre of the galaxy IRAS F11119+3257 have found proof that the winds blowing from the black hole are sweeping away the host galaxy’s reservoir of raw star-building material. ESA/ATG medialab

Black holes are getting a bad rap in the hearts of galaxies. Not only do they swallow up material that happens to wander too close to their event horizons, but now it appears that winds from a central supermassive black hole have the power to sweep away clouds of star-forming gases between the stars, which ultimately blocks the births of stars. 

If the black hole is active enough—that is, if it is sending out high-speed winds across light-years of space—that's enough to slow down, or even stop, the process of star formation throughout a galaxy. Astronomers have long thought that such winds could play some role in draining galaxies of their interstellar gas, in particular the gas molecules from which stars are born. The big challenge was to a) find the winds, and b) find evidence of the gases being pushed away. This doesn't happen in an easy-to-spot way; you have to search for energetic winds (which generally aren't visible-light objects), and also the clouds of gas and dust being shoved around.

To  see this kind of galactic activity, a team of observers used the European Space Agency's Herschel space observatory to look at a galaxy called IRAS F11119+3257 to see if they could detect the effect of fast-moving winds on clouds of gas. Herschel is sensitive to infrared light, which is given off as clouds of gas and dust are heated by nearby stars or other energetic objects. 

The astronomers combined their Herschel observations with data from the Japanese/U.S. Suzaku satellite, which is sensitive to x-ray radiation given off by very energetic objects and activities, such as the high-speed winds rushing away from black holes. One instrument would be used to spot the action of the winds and the other would see the heating of the gas clouds. Between the two sets of observations, astronomers had a chance to figure out what was happening at the heart of the galaxy as its black hole jets sped out to space. 

In the data, astronomers see that the winds start small near the black hole, and they move pretty fast—gusting up to about 25% the speed of light near the black hole. At that speed, the winds blow away about the equivalent of one solar mass of gas every year. As they progress outwards, the winds slow but sweep up an additional few hundred solar masses of gas molecules per year and push it out of the galaxy. The regions where the gas existed are essentially stripped bare, and that stops the star formation process in its tracks.

So, now it seems that black holes are more than just a curiosity at the hearts of galaxies. They're also destroyers of star formation, and without that activity, galaxies cannot easily grow. 

Some supermassive black holes are pretty active (such as in the galaxy the astronomers observed) while others are more quiescent. Our own Milky Way has a black hole in its heart, but it's a fairly quiet one, and there's not a lot of evidence of the types of high-speed winds that are disrupting the star-forming in IRAS F11119+3257.  The nearby Andromeda Galaxy has at least one black hole that may be affecting it, too. The next step will be to study other galaxies with active black holes and see if their actions are similar to this one. If so, then astronomers will have another hook to understanding the complex (and still largely unknown) relationship between galaxies and the black holes embedded in their hearts. 

The next step will be to study other galaxies with active black holes and see if their actions are similar to this one. If so, then astronomers will have another hook to understanding the complex (and still largely unknown) relationship between galaxies and the black holes embedded in their hearts.