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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 therange -2.2 < [Fe/H] (dex) < 0.3. This dramatically increases the sample sizeand 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) stellarpopulations are insensitive to the age of the stellar population, but that theW1 - W2 colors become bluer with increasing metallicity, a trend not wellreproduced by most stellar population synthesis (SPS) models. In common withprevious studies, we attribute this behavior to the increasing strength of theCO 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, weuse these models to derive stellar mass-to-light ratios in the W1 and W2 bands.Utilizing observational data from the SAURON and ATLAS3D surveys, wedemonstrate that these bands provide extremely simple, yet robust stellar masstracers for dust free older stellar populations that are freed from many of theuncertainties common among optical estimators.

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