A galactic wind is a large-scale outflow of gas from a galaxy. The winds are powered by the supernovae and stellar winds of massive stars, as well as the accretion of matter onto the supermassive black hole at the center of the galaxy. The winds can have a significant impact on the evolution of galaxies, by ejecting material from them and preventing further star formation.
Galactic winds were first observed in ultraviolet light coming from nearby starburst galaxies. These galaxies are undergoing intense periods of star formation, with new stars being born at a rate 10–100 times higher than in normal galaxies like our own Milky Way. The UV light from young, hot stars is absorbed by interstellar gas and re-emitted at longer wavelengths, which we see as infrared light. Infrared observations showed that starbursts also have strong outflows of gas, with velocities up to 1000 km/s.
The winds are thought to be powered by several mechanisms. First, the radiation pressure from massive stars can push on interstellar gas and create an outflow. Second, stellar winds from these same massive stars can also drive an outflow. Finally, when matter falls towards a supermassive black hole it releases energy that can also power an outflow (this process is known as accretion). All of these mechanisms can work together to create powerful galactic winds.
The effect of galactic winds on their host galaxies is two-fold. First, the outflowing gas removes material that could otherwise be used to form new stars (this is known as “feedback”). This feedback can help regulate the growth of a galaxy and prevent it from becoming too large too quickly. Second, when galactic winds collide with clouds of cold gas they heat up this gas and prevent it from collapsing to form new stars (this is known as “prevention”). Together feedback and prevention can have a significant impact on how galaxies evolve over time.