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Caught in the act: Herschel detects gigantic storms sweeping entire galaxies clean
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 is the first
conclusive evidence for the importance of galactic winds in the evolution of galaxies.
This illustration shows an Ultra-Luminous InfraRed Galaxy (ULIRG) that exhibits massive outflows of
Image: ESA/AOES Medialab
In the distant - and therfore younger - 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 as little as 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.
Schematic diagram indicating how outflows of molecular gas can be detected in the spectra of
galaxies using ESA's Herschel Space Observatory. The astronomers use a particular spectral line
of the hydroxyl molecule (OH), which exhibits a very characteristic shape resulting from a
combination of emission by the central black hole and by the gas cloud itself: the emission
from the accretion disk around the galactic centre has to pass through the gas clouds along
the line of sight, in which OH molecules absorb the light - and since these gas clouds are
moving towards us, the absorption lines are blue-shifted. At the same time, all gas clouds
emit the OH line, especially those which 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.
Image: ESA/AOES Medialab
"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.
Notes to editors:
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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