2025MNRAS.543...51S

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 time-scale 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.