A new method for classifying galaxy SEDs from multi-wavelength
  photometry
  We present a new method to classify the broad band optical-NIR spectral
energy distributions (SEDs) of galaxies using three shape parameters
(super-colours) based on a Principal Component Analysis of model SEDs. As well
as providing a compact representation of the wide variety of SED shapes, the
method allows for easy visualisation of information loss and biases caused by
the incomplete sampling of the rest-frame SED as a function of redshift. We
apply the method to galaxies in the UKIDSS Ultra Deep Survey with 0.9<z<1.2,
and confirm our classifications by stacking rest-frame optical spectra for a
fraction of objects in each class. As well as cleanly separating a tight
red-sequence from star-forming galaxies, three unusual populations are
identifiable by their unique colours: very dusty star-forming galaxies with
high metallicity and old mean stellar age; post-starburst galaxies which have
formed greater than around 10% of their mass in a recent unsustained starburst
event; and metal-poor quiescent dwarf galaxies. We find that quiescent galaxies
account for 45% of galaxies with log(M*/Msol)>11, declining steadily to 13% at
log(M*/Msol)=10. The properties and mass-function of the post-starburst
galaxies are consistent with a scenario in which gas-rich mergers contribute to
the growth of the low and intermediate mass range of the red sequence.
