Galactic Winds

A central question in galaxy evolution is how the formation of stars is regulated. Supernovae and massive star winds are believed to provide a source of negative 'feedback' by imparting energy and momentum to the interstellar medium out of which stars are born.  In galaxies with a high rates of star formation, the net effect is to drive gas and dust out of the galaxy disk in a dramatic 'galactic wind'.
Galactic winds contain a mixture of extremely hot metal-enriched supernova ejecta and cooler entrained gas and dust.  The amount of outflowing matter is substantial, comparable to the amount of matter that forms into stars.  Outflowing material has been observed at great distances from galaxies (10 to 100 kpc). In some cases outflows escape the galaxy potential well and pollute the intergalactic medium with heavy elements.

Galactic winds are widely recognized as important ingredients in galaxy evolution.  They cause galaxies to form stars more slowly over cosmic time, they influence the structure of galaxy disks, and they impact the chemical enrichment of galaxies and the intergalactic medium.  Winds are now routinely incorporated into theoretical models of galaxy evolution.  However, there is much about the underlying ephysics that is still poorly understood.  The key to improving our understanding is detailed observations.

Galactic winds are challenging to observe because the outflowing gas is diffuse and spans a wide range of temperatures (10 - 10^8 K).  Images of galaxies taken through special narrow-band filters that isolate the H-alpha and [NII] emission lines have revealed dramatic outflows in several local starbursts. One very famous example studied by UW astronomers is Messier 82 (see image at top).  A powerful new instrument on the SALT telescope called a Fabry-Perot will enable a host of new studies of this nature.

In more distant galaxies, the diffuse emission from the outflow becomes difficult to detect.  A better way to probe outflows in these objects is spectroscopy.  Cool gas in the wind absorbs photons produced by the galaxy's stars.  By measuring the wavelength of the absorption lines and the detailed line profile shapes astronomers can infer the velocity of the gas and its density along the line of sight.  UW researchers are using this technique to examine how the velocity of galactic winds depends on galaxy physical properties such as mass and star formation rate.

Recently, astronomers have begun to consider the impact that an active black hole could have on its host galaxy's interstellar medium.  AGN feedback [link to project under compact objects] is thought to play a role in shutting off star formation entirely in galaxies.  UW astronomers are using the Sloan Digital Sky Survey to identify and study galaxies whose star formation has been recently and abruptly quenched to gain insight into this phenomenon.

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