# Reconstructing the Stellar Mass Distributions of Galaxies Using S⁴G IRAC 3.6 and 4.5 {$μ$}m Images. II. The Conversion from Light to Mass

 Title Reconstructing the Stellar Mass Distributions of Galaxies Using S⁴G IRAC 3.6 and 4.5 {$μ$}m Images. II. The Conversion from Light to Mass Publication Type Journal Article Year of Publication 2014 Authors Meidt, S E, Schinnerer, E, van de Ven, G, Zaritsky, D, Peletier, R, Knapen, J H, Sheth, K, Regan, M, Querejeta, M, Muñoz-Mateos, J-C, Kim, T, Hinz, J L, A. de Paz, G, Athanassoula, E, Bosma, A, Buta, R J, Cisternas, M, Ho, L C, Holwerda, B, Skibba, R, Laurikainen, E, Salo, H, Gadotti, D A, Laine, J, Erroz-Ferrer, S, Comerón, S, Menéndez-Delmestre, K, Seibert, M, Mizusawa, T Journal \apj Volume 788 Pagination 144 Keywords galaxies: stellar content, galaxies: structure, Infrared: Galaxies, supergiants Abstract {We present a new approach for estimating the 3.6 {$μ$}m stellar mass-to-light (M/L) ratio Upsilon$_{3.6}$ 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/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$_{3.6}$, which varies intrinsically less than at optical wavelengths. The uncertainty on Upsilon$_{3.6}$ 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 Upsilon$_{3.6}$ = 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 (\~{}0.1 dex) than alternatives. We expect our Upsilon$_{3.6}$ to be optimal for mapping the stellar mass distributions in S⁴G galaxies, for which we have developed an independent component analysis technique to first isolate the old stellar light at 3.6 {$μ$}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 {$μ$}m fluxes, e.g., in WISE imaging, and establishes a reliable basis for exploring variations in the stellar IMF. } DOI 10.1088/0004-637X/788/2/144