Reconstructing the stellar mass distributions of galaxies using S4G IRAC 3.6 and 4.5 μm images: the conversion from light to mass

We present a new approach for estimating the 3.6 μm stellar mass-to-light ratio Υ3.6 in terms of the [3.6]-[4.5] colors of old stellar populations. Our approach avoids several of the largest sources of uncertainty in existing techniques using population synthesis models. By focusing on mid-IR wavelengths, we gain a virtually dust extinction-free tracer of the old stars, avoiding the need to adopt a dust model to correctly interpret optical or optical/NIR colors normally leveraged to assign the mass-to-light ratio Υ. By calibrating a new relation between NIR and mid-IR colors of giant stars observed in GLIMPSE we also avoid the discrepancies in model predictions for the [3.6]-[4.5] colors of old stellar populations due to uncertainties in the molecular line opacities assumed in template spectra. We find that the [3.6]-[4.5] color, which is driven primarily by metallicity, provides a tight constraint on Υ3.6, which varies intrinsically less than at optical wavelengths. The uncertainty on Υ3.6 of ~0.07 dex due to unconstrained age variations marks a significant improvement on existing techniques for estimating the stellar M/L with shorter wavelength data. A single Υ3.6=0.6 (assuming a Chabrier IMF), independent of [3.6]-[4.5] color, is also feasible as it can be applied simultaneously to old, metal-rich and young, metal-poor populations, and still with comparable (or better) accuracy 0.1 dex) as alternatives. Our Υ3.6 is optimal for mapping stellar mass distributions in S4G/DAGAL, for which we are first constructing a new catalog of images using an Independent Component Analysis technique to isolate the old stellar light at 3.6 μm from non-stellar emission (e.g. hot dust and the 3.3 μm PAH feature). Our estimate should also be useful for determining the fractional contribution of non-stellar emission to global (rest-frame) 3.6 μm fluxes, e.g., in WISE imaging and establishes a reliable basis for exploring variations in the stellar IMF.