Too many quiescent late-type galaxies or a matter of misclassification?

Abstract In the local Universe, the terms late-type and early-type are used to describe, respectively, disc galaxies with spiral arms and more bulge-dominated systems lacking spiral structure. These morphologies are often associated with different star formation levels, motivating the classical scenario where late-type and early-type galaxies are seen, respectively, as star-forming and “red and dead” systems. To build a comprehensive picture of galaxy evolution, we must understand the relationship between morphology and star formation, and be able to explain more exotic configurations, such as quiescence in late-type galaxies. Large morphological catalogues are valuable tools to address these questions, but they must not be used blindly. We show that selecting quiescent galaxies classified as late-type in these catalogues leads to a clear excess of edge-on systems, with unreliable morphologies, compared with the general galaxy population in the Sloan Digital Sky Survey. Consequently, the fraction of quiescent late-type galaxies in this survey could be overestimated by a factor of two. Conversely, S0 galaxies, considered early-type discs without spiral arms, are under-represented at high inclinations. We conclude that the excess of edge-on quiescent late-type galaxies can be explained if most of these systems are misclassified edge-on S0 galaxies. We make recommendations to remove this bias.