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.

Schematic fingerprint: to trace the galactic wind, Herschel-PACS uses a particular spectral line of the hydroxyl molecule (OH). The emission from the galactic centre passes through the gas clouds along the line of sight, in which OH molecules absorb the light – and since these gas clouds are speeding towards us, the absorption lines are blue-shifted.  At the same time, all gas clouds emit the OH line, especially those who are not on a direct line of sight to the black hole – and as they are moving away from us, this light is red-shifted.

“Herschel's sensitivity enabled us to detect these gigantic galactic storms, and to demonstrate, for the first time, that they may be strong enough to shut down stellar production entirely,” says co-author Albrecht Poglitsch, also from MPE and the Principal Investigator of PACS.

The sample of galaxies observed is still too small to pin down the driving force behind these outflows. The first results seem to indicate that the galaxies fall in two categories: starburst-dominated objects loose material of up to a few hundred solar masses per year which is similar to their star formation rate; with velocities of a few hundred kilometres per second these outflows are probably driven by radiation pressure from starbursts or supernovae explosions. Galaxies dominated by the activity of the black hole in their centre loose material at much higher rates, up to a thousand solar masses per year or more; with velocities around 1000 kilometres per second these outflows are probably powered mostly by radiation pressure from the active galactic nucleus. To confirm these first conclusions and study potential trends in the outflow characteristics, the Herschel-PACS observations will continue to cover a much larger sample of galaxies.


PACS has been developed by a consortium of institutes led by MPE (Germany) and including UVIE (Austria); KU Leuven, CSL, IMEC (Belgium); CEA, LAM (France); MPIA (Germany); INAF-IFSI/OAA/OAP/OAT, LENS, SISSA (Italy); IAC (Spain). This development has been supported by the funding agencies BMVIT (Austria), ESA-PRODEX (Belgium), CEA/CNES (France), DLR (Germany), ASI/INAF (Italy), and CICYT/MCYT (Spain).

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