The current paradigm for galaxy evolution is that there are two classes of galaxy:
blue star-forming galaxies on the ‘Galaxy Main Sequence’ and a class of red passive
galaxies that contain very little gas and in which few stars are currently being formed.
It is generally assumed that some catastrophic quenching process converts the
star-forming galaxies on the Galaxy Main Sequence into the ‘red and dead’ galaxies.
I will describe how surveys with the Herschel Space Observatory are producing
a rather different view of galaxy evolution. Approximately 30% of the
galaxies detected in the Herschel ATLAS, the largest Herschel extragalactic
survey, are red galaxies that still contain a substantial reservoir of interstellar
gas and in which stars are still being formed. The existence of this population
of ‘red but not dead galaxies’ shows that more gradual evolutionary processes are
at least as important as catastrophic ones. I also combine the deepest Herschel
survey and a deep optical survey with the HST to quantify the morphological
transformation that has occurred in the galaxy population over the last 12 billion
years.