Reconstructing the stellar mass distributions of galaxies using S 4G IRAC 3.6 and 4.5 μm images. II. the conversion from light to mass

Abstract

<p> We present a new approach for estimating the 3.6 &mu;m stellar mass-to- light (M/L) ratio &Upsilon; 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/near-IR colors normally leveraged to assign the mass-to-light ratio &Upsilon;. By calibrating a new relation between near-IR 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 &Upsilon;, which varies intrinsically less than at optical wavelengths. The uncertainty on &Upsilon; of &sim;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 &Upsilon; = 0.6 (assuming a Chabrier initial mass function (IMF)), independent of [3.6]-[4.5] color, is also feasible because it can be applied simultaneously to old, metal-rich and young, metal-poor populations, and still with comparable (or better) accuracy (&sim;0.1 dex) than alternatives. We expect our &Upsilon; to be optimal for mapping the stellar mass distributions in SG galaxies, for which we have developed an independent component analysis technique to first isolate the old stellar light at 3.6 &mu;m from nonstellar emission (e.g., hot dust and the 3.3 polycyclic aromatic hydrocarbon feature). Our estimate can also be used to determine the fractional contribution of nonstellar emission to global (rest-frame) 3.6 &mu;m fluxes, e.g., in WISE imaging, and establishes a reliable basis for exploring variations in the stellar IMF. &copy; 2014. The American Astronomical Society. All rights reserved..</p>