The New Galaxy Evolution Paradigm Revealed by the Herschel Surveys
  The Herschel Space Observatory has revealed a very different galaxyscape from
that shown by optical surveys which presents a challenge for galaxy-evolution
models. The Herschel surveys reveal (1) that there was rapid galaxy evolution
in the very recent past and (2) that galaxies lie on a a single Galaxy Sequence
(GS) rather than a star-forming `main sequence' and a separate region of
`passive' or `red-and-dead' galaxies. The form of the GS is now clearer because
far-infrared surveys such as the Herschel ATLAS pick up a population of
optically-red star-forming galaxies that would have been classified as passive
using most optical criteria. The space-density of this population is at least
as high as the traditional star-forming population. By stacking spectra of
H-ATLAS galaxies over the redshift range 0.001 < z < 0.4, we show that the
galaxies responsible for the rapid low-redshift evolution have high stellar
masses, high star-formation rates but, even several billion years in the past,
old stellar populations - they are thus likely to be relatively recent
ancestors of early-type galaxies in the Universe today. The form of the GS is
inconsistent with rapid quenching models and neither the analytic bathtub model
nor the hydrodynamical EAGLE simulation can reproduce the rapid cosmic
evolution. We propose a new gentler model of galaxy evolution that can explain
the new Herschel results and other key properties of the galaxy population.
