Quantifying the Poor Purity and Completeness of Morphological Samples
  Selected by Galaxy Colour
  The galaxy population is strongly bimodal in both colour and morphology, and
the two measures correlate strongly, with most blue galaxies being late-types
(spirals) and most early-types, typically ellipticals, being red. This
observation has led to the use of colour as a convenient selection criteria to
make samples which are then labelled by morphology. Such use of colour as a
proxy for morphology results in necessarily impure and incomplete samples. In
this paper, we make use of the morphological labels produced by Galaxy Zoo to
measure how incomplete and impure such samples are, considering optical
(ugriz), NUV and NIR (JHK) bands. The best single colour optical selection is
found using a threshold of g-r = 0.742, but this still results in a sample
where only 56% of red galaxies are smooth and 56% of smooth galaxies are red.
Use of the NUV gives some improvement over purely optical bands, particularly
for late-types, but still results in low purity/completeness for early-types.
No significant improvement is found by adding NIR bands. With any two bands,
including NUV, a sample of early-types with greater than two-thirds purity
cannot be constructed. Advances in quantitative galaxy morphologies have made
colour-morphology proxy selections largely unnecessary going forward; where
such assumptions are still required, we recommend studies carefully consider
the implications of sample incompleteness/impurity.
