Reconstructing the stellar mass distributions of galaxies using S4G IRAC 3.6 and 4.5 micron images: II. The conversion from light to mass

Abstract: We present a new approach for estimating the 3.6 micron stellar mass-to-light ratio 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. 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 M/L. By calibrating a new relation between NIR and mid-IR colors of GLIMPSE giant stars we also avoid discrepancies in model predictions for the [3.6]-[4.5] colors of old stellar populations due to uncertainties in molecular line opacities. We find that the [3.6]-[4.5] color, which is driven primarily by metallicity, provides a tight constraint on M/L_3.6, which varies intrinsically less than at optical wavelengths. The uncertainty on M/L_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 M/L_3.6=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. We expect our M/L_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 micron from non-stellar emission (e.g. hot dust and the 3.3 PAH feature). Our estimate can also be used to determine the fractional contribution of non-stellar emission to global (rest-frame) 3.6 micron fluxes, e.g. in WISE imaging, and establishes a reliable basis for exploring variations in the stellar IMF.

Estimating Stellar Mass Distributions in Galaxies with Mid-IR Imaging

The research presented in the paper focuses on a methodology for reconstructing the stellar mass distribution of galaxies using InfraRed Array Camera (IRAC) 3.6 and 4.5 μm images from the Spitzer Survey of Stellar Structure in Nearby Galaxies (S^4G). The novel approach primarily aims to calibrate the 3.6 μm stellar mass-to-light ratio ((\Upsilon_{3.6})) using the [3.6]-[4.5] color indices, derived from old stellar populations. By leveraging mid-infrared wavelengths, the methodology effectively minimizes the interference from dust extinction, which is a significant concern in optical and near-infrared (NIR) observations where traditional techniques face challenges.

Methodological Advances

• Independent Component Analysis (ICA): The study utilizes ICA to distinguish old stellar light from other emissions such as hot dust and PAH features at 3.3 μm. ICA allows the separation of non-stellar contaminants, thus enabling a more accurate estimation of stellar mass.

• Calibration of (\Upsilon_{3.6}): A new empirical relationship was established between NIR colors and the [3.6]-[4.5] color using observed colors of giant stars from the GLIMPSE survey. This allows for a re-assessment of the modeled [3.6]-[4.5] colors towards more realistic values and improves the predictive capability of population synthesis models at mid-IR wavelengths.

Quantitative Results and Findings

• Tight Constraints on (\Upsilon_{3.6}): The study presents a tight relationship between [3.6]-[4.5] color and (\Upsilon_{3.6}), with the latter exhibiting an uncertainty of approximately 0.07 dex due to age variations. A potential single value of (\Upsilon_{3.6} = 0.6) for a Chabrier IMF is proposed, offering applicability across different stellar populations with an uncertainty comparable to other techniques.

• Improvement Over Existing Methods: The proposed method demonstrates better accuracy than conventional techniques that use shorter wavelength data, achieving stellar mass estimations with uncertainties comparable to two-color approaches without additionally requiring dust models.

Implications and Future Directions

The findings have significant implications for galaxy modeling efforts, particularly for understanding the baryonic mass distribution essential in studying galactic dynamics and evolution over cosmic time. The ability to accurately measure stellar masses aids in delineating the dark matter distribution by minimizing uncertainties in the baryonic mass component.

Additionally, the work sets a foundation for exploring the variance in the initial mass function (IMF) across different galactic environments based on accurately mapped stellar masses. The methodology’s robustness and non-dependence on ancillary datasets enhance its applicability to large galaxy samples in other major surveys.

Future developments could refine this approach through enhanced modeling of stellar evolutionary phases or incorporating observational advances capturing star formation variations over cosmic scales. The inclusion of datasets spanning other infrared wavelengths could further strengthen the constraints on stellar mass mapping and deepen insights into the complex interplay between star formation history, metallicity, and stellar mass distributions.

This study marks a step forward in the accurate mass modeling of galactic structures using mid-infrared data, offering a systematic pathway to disentangle the intricacies of galaxy formation and evolution.