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 spirals and most early-types, (typically ellipticals) being red. This observation has led to the use of colour as a convenient selection criterion to make samples that are then labelled by morphology. Such use of colour as a proxy for morphology results in necessarily impure and incomplete samples. We make use of the morphological labels produced by Galaxy Zoo to measure how incomplete and impure such samples are, considering optical (ugriz), near-ultraviolet (NUV), and near-infrared (NIR; JHK) bands. Use of the NUV gives some improvement over purely optical bands, particularly for late-types, but results in low purity/completeness for early-types. Advances in quantitative galaxy morphologies, including those provided by Galaxy Zoo have made colour-morphology proxy selections largely unnecessary going forward; where such assumptions are still required, we recommend careful consideration of the implications of this sample selection on any conclusions about galaxy evolution. This poster is based on work published as Smethurst, Masters et al. 2022, MNRAS 510, 4126.