Flexible Models for Galaxy Star Formation Histories Both Shift and Scramble the Optical Color-Mass-to-light Ratio (M/L) Relationship

The remarkably tight relationship between galaxy optical color and stellar mass-to-light ratio (M (*)/L) is widely used for efficient stellar mass estimates. However, it remains unclear whether this low scatter comes from a natural order in the galaxy population, or whether it is driven by simple relationships in the models used to describe them. In this work, we investigate the origins of the relationship by contrasting the derived relationship from a simple 4 parameter spectral energy distribution (SED) model with a more sophisticated 14D Prospector-alpha model including nonparametric star formation histories (SFHs). We apply these models to 63,430 galaxies at 0.5 < z < 3 and fit a hierarchical Bayesian model (HBM) to the population distribution in the (g - r)- log(M/Lg) Prospector-alpha infers systematically higher M (*)/L by 0.12 dex, a result of nonparametric SFHs producing older ages, and also systematically redder rest frame (g - r) by 0.06 mag owing to the contribution from nebular emission. Surprisingly, the combined effects of the M (*)/L and (g - r) offsets produce a similar average relationship for the two models; though Prospector-alpha produces a higher scatter of 0.28 dex compared to the simple model of 0.12 dex. Also, unlike the simple model, the Prospector-alpha relationship shows substantial redshift evolution due to stellar aging. These expected and testable effects produce overall older and redder galaxies; though the color-M (*)/L relationship is measured only at 0.5 < z < 3. Finally, we demonstrate that the HBM produces substantial shrinkage in the individual posteriors of faint galaxies, an important first step toward using the observed galaxy population directly to inform the SED-fitting priors.

Automated summary

There is a close relationship between galaxy optical color and stellar mass-to-light ratio which is used for efficient stellar mass estimates. The origins of this relationship are unclear, and it is important to investigate whether it comes from the natural order in the galaxy population or from the models used. By comparing the results of a simple model and a more sophisticated model, it is found that the complex model has a similar relationship but with higher scatter and is affected by stellar aging and redshift evolution.