Cosmic ray influences on star formation in galaxies
The triggering of star formation, and also its quenching, is regulated by young massive stars in galaxies which inject energy and momentum into the interstellar medium. Feedback from the supermassive black holes at galaxies’ nuclei plays a similarly important role. These processes drive the massive gas outflows observed in galaxies, for example. However the details including how they work and the relative roles of the different feedback processes are actively debated. Cosmic rays in particular are accelerated in strong shocks formed by supernova explosions and stellar winds (both aspects of star formation), and generate considerable pressure in the interstellar medium. They play a central role in regulating thermal balance in dense molecular clouds where most stars form and may play an important role in regulating star formation, driving galactic winds, and even in determining the character of the intergalactic medium. Astronomers believe that a key property limiting cosmic ray influence is the ability to propagate out of the sites where they are produced into the interstellar medium and beyond the disk, but the details are not very well understood.
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