The Rest-Frame Near-Infrared Colors and M/L of Early-Type Galaxies at z = 1

We present the results of a study of the evolution of the optical and near-IR colors and mass-to-light ratios (M/L) of early-type galaxies from z~ 1 to the present. This exercise is relevant for studies that use photometry of galaxies to infer properties such as stellar masses and star formation histories through comparison with stellar population models. We have dynamical M/L for a sample of 20 early types at z~ 1 with velocity dispersions from deep optical spectroscopy and structural parameters (size and surface brightness) from high-resolution HST imaging. We compare those with 23 early-type galaxies in the Coma Cluster at z=0.02. For both samples rest-frame optical/near-IR photometry is available, from HST and Spitzer in the case of the z~ 1 sample. We find that the M/L evolves faster in the near-IR than expected from most stellar population models, and also that predictions from different models can differ significantly in the near-IR. As a consequence, there is a systematic uncertainty of a factor of two in stellar mass estimates from near-IR photometry for evolved, high-z galaxies. Optical colors provide a less biased indicator of the M/L. Agreement among the models is required before near-IR photometry can be used as a robust tool to estimate galaxy masses without systematic uncertainties.