The mass-to-light ratios and the star formation histories of disc galaxies

We combine new data from the main sequence (M-* versus SFR) of star-forming galaxies and galaxy colours (from GALEX to Spitzer) with a flexible stellar population scheme to deduce the mass-to-light ratio ((sic)*) of star-forming galaxies from the SPARC and S(4)G samples. We find that the main sequence for galaxies, particularly the low-mass end, combined with the locus of galaxy colours, constrains the possible star formation histories of disc and dwarf galaxies to a similar shape found by Speagle et al. Combining the deduced star formation history with stellar population models in the literature produces reliable (sic)* values as a function of galaxy colour with an uncertainty of only 0.05 dex. We provide prescriptions to deduce (sic)* for optical and near-IR bandpasses, with near-IR bandpasses having the least uncertainty ((sic)* from 0.40 to 0.55). We also provide the community with a webtool, with flexible stellar population parameters, to generate their own (sic)* values over the wavelength range for most galaxy surveys.