The Causes of the Red Sequence, the Blue Cloud, the Green Valley and the
  Green Mountain
  The galaxies found in optical surveys fall in two distinct regions of a
diagram of optical colour versus absolute magnitude: the red sequence and the
blue cloud with the green valley in between. We show that the galaxies found in
a submillimetre survey have almost the opposite distribution in this diagram,
forming a `green mountain'. We show that these distinctive distributions follow
naturally from a single, continuous, curved Galaxy Sequence in a diagram of
specific star-formation rate versus stellar mass without there being the need
for a separate star-forming galaxy Main Sequence and region of passive
galaxies. The cause of the red sequence and the blue cloud is the geometric
mapping between stellar mass/specific star-formation rate and absolute
magnitude/colour, which distorts a continuous Galaxy Sequence in the diagram of
intrinsic properties into a bimodal distribution in the diagram of observed
properties. The cause of the green mountain is Malmquist bias in the
submillimetre waveband, with submillimetre surveys tending to select galaxies
on the curve of the Galaxy Sequence, which have the highest ratios of
submillimetre-to-optical luminosity. This effect, working in reverse, causes
galaxies on the curve of the Galaxy Sequence to be underrepresented in optical
samples, deepening the green valley. The green valley is therefore not evidence
(1) for there being two distinct populations of galaxies, (2) for galaxies in
this region evolving more quickly than galaxies in the blue cloud and the red
sequence, (c) for rapid quenching processes in the galaxy population.
