What do we mean by stellar mass? The impact of the pre-main sequence on the mass to light ratio of young and intermediate age stellar populations

Stellar population synthesis models are an essential tool with which galaxy physical parameters are extracted from observations. However they are built on assumptions designed for use in the local Universe, and not always appropriate to high redshift galaxies. Here we consider the impact of including the hitherto-neglected stellar pre-main sequence delay timescale on the interpretation of composite stellar populations at ages of <1 Gyr. We find that doing so has an impact on the optical luminosity of very young stellar populations of up to ~10 per cent, although smaller changes in observed light (<5 per cent) are expected in most use cases. However the impact on the inferred stellar mass and mass-to-light ratios is significant (a factor of 2 or more), depending on how those properties are defined. We find that the short time scales for star formation in the distant Universe require a clearer definition for the stellar mass in a population, and will impact assumptions about the inferred shape of the stellar initial mass function from observations.