A tale of two galaxies

The colour and shape of a galaxy is largely controlled by gas flowing through an extended halo around it. All modern simulations of galaxy formation find that they cannot explain the observed properties of galaxies without modeling the complex accretion and "feedback" processes by which galaxies acquire gas and then later expel it after chemical processing by stars. Hubble spectroscopic observations show that galaxies like our Milky Way recycle gas while galaxies undergoing a rapid starburst of activity will lose gas into intergalactic space and become "red and dead."