Being WISE I: Validating Stellar Population Models and M/L ratios at 3.4 and 4.6 microns

TL;DR

This study validates stellar population models using WISE infrared data across diverse dust-free stellar systems, revealing metallicity effects on W1-W2 colors and establishing W1 and W2 bands as reliable stellar mass indicators for old populations.

Contribution

It broadens the metallicity range for WISE stellar population analysis and confirms the robustness of W1 and W2 bands as stellar mass tracers for dust-free older populations.

Findings

W1 - W2 colors are insensitive to age but become bluer with metallicity.

Most SPS models do not fully reproduce the metallicity trend in W1 - W2 colors.

W1 and W2 bands provide robust stellar mass estimates for dust-free older stellar populations.

Abstract

Using data from the WISE mission, we have measured near infra-red (NIR) photometry of a diverse sample of dust-free stellar systems (globular clusters, dwarf and giant early-type galaxies) which have metallicities that span the range -2.2 < [Fe/H] (dex) < 0.3. This dramatically increases the sample size and broadens the metallicity regime over which the 3.4 (W1) and 4.6 micron (W2) photometry of stellar populations have been examined. We find that the W1 - W2 colors of intermediate and old (> 2 Gyr) stellar populations are insensitive to the age of the stellar population, but that the W1 - W2 colors become bluer with increasing metallicity, a trend not well reproduced by most stellar population synthesis (SPS) models. In common with previous studies, we attribute this behavior to the increasing strength of the CO absorption feature located in the 4.6 micron bandpass with metallicity. Having used our sample to validate the efficacy of some of the SPS models, we use these models to derive stellar mass-to-light ratios in the W1 and W2 bands. Utilizing observational data from the SAURON and ATLAS3D surveys, we demonstrate that these bands provide extremely simple, yet robust stellar mass tracers for dust free older stellar populations that are freed from many of the uncertainties common among optical estimators.