ESA’s Herschel Space Observatory has found gigantic storms of molecular gas gusting in the centers of many galaxies. These powerful storms, driven by star formation and central black holes, are strong enough to halt the mechanisms that produced them in the first place.
Some of these massive outflows can reach extremely high velocities, up to 1,000 kilometers per second, thousands of times faster than in terrestrial hurricanes. Star formation and supermassive black holes at the centers of galaxies are thought to be triggered by the merger of gas-rich galaxies; however, this increased activity seems to stop relatively quickly, after a few million years only.
An international team of scientists led by the Max Planck Institute for Extraterrestrial Physics suggests this could be explained by powerful winds that blow gas outwards from the centre of the galaxy. These storms, powered by newly formed stars, shocks from stellar explosions or the central black hole, would sweep away the galaxy’s entire reservoir of gas. Eventually, it would halt star formation and the growth of the black hole, which ironically initiated the phenomenon.
“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). The PACS instrument on board Herschel revealed massive outflows of molecular gas, which contributes to star formation, in ultra-luminous infrared galaxies (ULIRGs). These galaxies, enshrouded in gas and dust, shine brightly in the infrared.
“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.”
These observations could also explain another empirical property: the central black hole’s mass and the mass of stars in the inner regions of a galaxy seem to correlate. As these newly found galactic outflows remove the galaxy’s entire reservoir of gas, inhibiting star formation and the growth of the black hole, this correlation appears natural.
It is not clear yet how the outflows are produced: it appears that slower outflows may be initiated by star forming regions, whereas those with higher velocity appear to be related to the activity of Active Galactic Nuclei (AGN) powered by central black holes. The brighter the AGN is, the faster it seems to sweep gas away.
Anyway, it will be necessary to analyze a much larger sample of galaxies in order to confirm this claim and identify the driving force behind these outflows.
E. Sturm, E. González-Alfonso, S. Veilleux, J. Fischer, J. Graciá-Carpio, S. Hailey-Dunsheath, A. Contursi, A. Poglitsch, A. Sternberg, R. Davies, R. Genzel, D. Lutz, L. Tacconi, A. Verma, R. Maiolino, J. A. de Jong. Massive molecular outflows and negative feedback in ULIRGs observed by Herschel-PACS. The Astrophysical Journal, 2011; 733 (1): L16 DOI: 10.1088/2041-8205/733/1/L16