Huge storms sweep entire galaxies clean

The infred space observatory Herschel discovers how Milky Way systems lose substance 

May 11, 2011

With observations from the PACS instrument on board the ESA Herschel space observatory, an international team of scientists led by the Max Planck Institute for Extraterrestrial Physics have found gigantic storms of molecular gas gusting in the centres of many galaxies. Some of these massive outflows reach velocities of more than 1000 kilometres per second, i.e. thousands of times faster than in terrestrial hurricanes. The observations show that the more active galaxies contain stronger winds, which can blow away the entire gas reservoir in a galaxy, thereby inhibiting both further star formation and the growth of the central black hole. This finding confirms accepted theories of galaxy evolution and is the first conclusive evidence for the importance of galactic winds in this episode of the history of elliptical galaxies.

Looking far away in the Universe, many galaxies show much more activity than our Milky Way today. In commonly accepted evolutionary scenarios gas-rich galaxies merge, which triggers increased star formation (“starburst” galaxies) as well as the growth of supermassive black holes at their centres. This increased activity, however, seems to cease fairly suddenly, effectively stalling star formation and further growth of the black hole in a few million years’ time. What processes could be responsible for removing all the raw material powering this activity – around a billion solar masses – in such a (cosmologically) short timespan?

The solution to this riddle could be powerful winds that blow gas outwards from the centre of the galaxy. Powered by newly formed stars, shocks from stellar explosions or by the Black Hole in the galaxy’s centre, these storms would remove all the gas supply from the galaxy thereby halting the same mechanisms that produced them in the first place.

“Outflows are key features in models of galactic formation and evolution, but prior to our work no decisive evidence of their active role in such processes had been gathered,” explains Eckhard Sturm from the Max Planck Institute for Extraterrestrial Physics (MPE). Sturm led a study of ultra-luminous infrared galaxies with the PACS instrument on board the Herschel space observatory, which revealed massive outflows of molecular gas. Almost all previous observations dealt mainly with neutral and ionised gas, which does not contribute to the formation of stars.

“By detecting outflows in cold molecular gas from which stars are born, we can finally witness their direct impact on star formation,” Sturm adds. “Star formation stalls as the gas supply is blown out of the centres of the galaxies with a rate of up to a thousand solar masses per year.”

However, the observations not only reveal an intermediate stage of galaxy evolution, from disc galaxies with many young stars and a large gas fraction to elliptical galaxies with old stellar populations and little gas. In addition, they can explain another empirical property: The mass of the Black Hole in the centre and the mass of stars in the inner regions of a galaxy seem to correlate. Such a correlation is a natural consequence of the newly found galactic outflows as they remove the common gas reservoir thus inhibiting both star formation and the growth of the Black Hole.

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