The Mass-to-light Ratios and the Star Formation Histories of Disk
  Galaxies
  We combine new data from the main sequence (M_* versus SFR) of star-forming
galaxies and galaxy colors (from GALEX to Spitzer) with a flexible stellar
population scheme to deduce the mass-to-light ratio (\Upsilon_*) of
star-forming galaxies from the SPARC and S^4G samples. We find that the main
sequence for galaxies, particular the low-mass end, combined with the locus of
galaxy colors, constrains the possible star formation histories of disk and
dwarf galaxies to a similar shape found by Speagle et al. (2014). Combining the
deduced star formation history with stellar population models in the literature
produces reliable \Upsilon_* values as a function of galaxy color with an
uncertainty of only 0.05 dex. We provide prescriptions to deduce \Upsilon_* for
optical and near-IR bandpasses, with near-IR bandpasses having the least
uncertainty (\Upsilon_* from 0.40 to 0.55). We also provide the community with
a webtool, with flexible stellar population parameters, to generate their own
\Upsilon_* values over the wavelength range for most galaxy surveys.
