Gigantic storms sweep galaxies empty

Possible cause identified for disappearance of molecular gas and stop of star formation

Bright infrared galaxy (ULIRG) with massive winds of molecular gas. © ESA / AOES Medialab
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In the centers of many active galaxies, giant storm clouds rage from molecular gas. They can blow away the entire gas supply of a galaxy and stop further star formation and black hole growth in the center. This is shown by new observations with the Herschel Space Telescope, which have now been published in the Astrophysical Journal. For the first time, they systematically demonstrate the importance of galactic winds for the development of galaxies.

Faraway galaxies in the early Universe show much more activity than our Milky Way today. This is explained in current development models by the fact that gas-rich galaxies merge, which not only leads to increased star formation in so-called "starburst" galaxies, but also increases the black hole in the center. Suddenly, however, this increased activity ceases; in just a few million years, the star formation rate drops rapidly and the black hole does not continue to grow. In this - for cosmic proportions - short period of time enormous amounts of raw material, about a billion solar masses, must be removed from the galaxy. But which physical processes are responsible for this?

Storm blows gas supply out of galaxy

A solution to this puzzle has now been found by an international team of scientists headed by Eckhard Sturm from the Max Planck Institute for Extraterrestrial Physics (MPE). Astronomers used the PACS instrument to observe some particularly bright infrared galaxies aboard the Herschel space observatory, discovering strong, massive winds of cold, molecular gas.

They reach wind speeds of sometimes more than 1, 000 kilometers per second, many thousands of times higher than with hurricanes on earth. Driven by newly formed stars, the shock of starburst explosions, or even the black hole in the center of a galaxy, they could almost completely remove the gas supply from a galaxy and thus bring to a standstill the activities that created it.

"That galaxies literally blow away molecular gas - and thus the raw material for star formation - in huge winds is an essential part of galactic genesis and evolution models, but there was no clear evidence to support our observations, " explains Sturm. "With this evidence of strong galactic winds removing the cold, molecular gas from the galaxy, we can finally directly observe their influence on the formation of stars. The replenishment for further star formation comes to a halt very quickly - the winds blow up to a thousand solar masses per year from the centers of the galaxies. "Display

Schematic representation of how the winds from molecular gas can be detected in the spectra of galaxies with Herschel PACS. Here, in particular, the spectral line of the hydroxyl molecule (OH) is used, which has a very characteristic "fingerprint". At the same time, the emission of the black hole accretion disk and the gas clouds themselves overlap: the radiation from the galactic center shines through the gas clouds along the line of sight, where the OH molecule absorbs the light - and there these clouds moving towards us, these absorption lines are blue-shifted. At the same time, all gas clouds emit the OH line, especially those that are not exactly on the line of sight to the black hole, and move away from us so that their light reaches us red-shifted. ESA / AOES Medialab

Declaration for black hole growth stop

The observations thus show not only an intermediate step in galaxy evolution, from disk galaxies with many young stars and a high proportion of gas to elliptical galaxies with old stellar populations and little gas. They also explain another empirical observation: The mass of the black hole in the center of a galaxy and the mass of the stars in the inner part of the galaxy seem to be correlated. Such a correlation would be a natural consequence of the now found galactic winds as they remove the common gas reservoir and thus prevent both the star formation and the growth of the black hole.

"The sensitivity of Herschel allows us to prove for the first time these huge galactic storms, " ​​says co-author Albrecht Poglitsch, who led the development of the PACS instrument at MPE. "Now we can show that they could be strong enough to completely stop further star production."

Cause of the winds still unclear

The observations are not enough to definitively determine the driving force behind these winds. However, there seem to be two categories: galaxies with strong star formation ("starburst" galaxies) lose up to several hundred solar masses of gas per year, an amount roughly equivalent to their star formation rate. At speeds of a few hundred kilometers per second, these winds are likely to be driven by the radiation pressure of stars and stellar explosions.

Galaxies, which are dominated by the black hole in their center, lose much more material, up to a thousand solar masses per year and more; Their winds, at speeds of about a thousand kilometers per second, are probably mainly due to the radiation pressure of the active galaxy nucleus. In order to confirm these first results and to clarify other properties of the winds, the Herschel-PACS observations are continued in a larger number of galaxies. (The Astrophysical Journal, 2011; DOI: 10.1088 / 2041-8205 / 733/1 / L16)

(Max Planck Institute for Extraterrestrial Physics (MPE), 11.05.2011 - NPO)