How the spectral energy distribution and galaxy morphology constrain
  each other, with application to morphological selection using galaxy colours
  We introduce an empirical methodology to study how the spectral energy
distribution (SED) and galaxy morphology constrain each other and implement
this on 8000 galaxies from the HST CANDELS survey in the GOODS-South field. We
show that the SED does constrain morphology and present a method that
quantifies the strength of the link between these two quantities. Two galaxies
with very similar SEDs are around three times more likely to also be
morphologically similar, with SED constraining morphology most strongly for
relatively massive red ellipticals. We apply our methodology to explore likely
upper bounds on the efficacy of morphological selection using colour. We show
that, under reasonable assumptions, colour selection is relatively ineffective
at separating homogeneous morphologies. Even with the use of up to six colours
for morphological selection, the average purity in the resultant morphological
classes is only around 60 per cent. While the results can be improved by using
the whole SED, the gains are not significant, with purity values remaining
around 70 per cent or below.
