Raging storms sweep away galactic gas
The gas outflows are robbing galaxies of the raw material they need to make new stars.
May 9, 2011
The European Space Agency’s (ESA) Herschel infrared space observatory has detected raging winds of molecular gas streaming away from galaxies. Suspected for years, these outflows may have the power to strip galaxies of gas and halt star formation in its tracks.
An artist’s impression showing a galaxy with a molecular outflow. Herschel has discovered that such outflows can travel at 620 miles (1,000 kilometers) per second, which could deplete the galaxy of the gas needed for further star formation within one million to 100 million years. ESA/AOES Medialab
The winds that Herschel has detected are extraordinary. The fastest is blowing at a speed of more than 620 miles (1,000 kilometers) per second, or about 10,000 times faster than the wind in a terrestrial hurricane.
This is the first time that such molecular gas outflows have been unequivocally observed in a sample of galaxies. This is an important discovery because stars form from molecular gas, and these outflows are robbing the galaxy of the raw material it needs to make new stars. If the outflows are powerful enough, they could even halt star formation altogether.
“With Herschel, we now have the chance to really study what these outflows mean for galactic evolution,” said Eckhard Sturm from the Max Planck Institute for Extraterrestrial Physics in Germany. Sturm and colleagues used Herschel’s Photoconductor Array Camera and Spectrometer to study 50 galaxies.
They infer that 1,200 times the mass of our Sun is being lost each year from the galaxies with the most vigorous outflows. That is enough to strip them of their entire reserves of star-forming gas within 1 million to 100 million years. In other words, some galaxies could completely expel their star-forming gas in as little as a million years. Inhibiting star formation in a galaxy is known as negative feedback.
These winds could be generated by the intense emission of light and particles from young stars or by shock waves from the explosion of old stars. Alternatively, they may be triggered by the radiation given off as matter swirls around a black hole at the center of the galaxy.
The fastest winds appear to be coming from the galaxies that contain the brightest “active galactic nuclei,” in which a giant black hole is feeding from its surroundings. Sturm and his colleagues are now testing this idea with the other galaxies in their sample. The results could be a step toward explaining how some elliptical galaxies are formed.
Elliptical galaxies are vast islands of stars that have now stopped producing appreciable numbers of new stars because they have exhausted their gas supplies.
As smaller galaxies interact and merge with each other, more food is supplied to the central black hole in the combined nucleus, making it larger and more active. This could result in a more powerful wind, which removes the molecular gas and prevents any further star formation from taking place, thus leading to an elliptical galaxy.
“By catching molecular outflows in the act, Herschel has finally yielded long-sought-after evidence that powerful processes with negative feedback do take place in galaxies and dramatically affect their evolution,” said Göran Pilbratt from ESA.