The Stellar Masses of Disk Galaxies and the Calibration of Color-Mass to
  Light Ratio Relations
  We present new Spitzer 3.6 micron observations of a sample of disk galaxies
spanning over 10 magnitudes in luminosity and ranging in gas fraction from ~10%
to over 90%. We use these data to test population synthesis prescriptions for
computing stellar mass. Many commonly employed models fail to provide
self-consistent stellar masses in the sense that the stellar mass estimated
from the optical luminosity typically exceeds that estimated from the
near-infrared (NIR) luminosity. This problem is present in models both with and
without TP-AGB stars, but is more severe in the former. Self-consistency can be
achieved if NIR mass-to-light ratios are approximately constant with a mean
value near 0.5 Msun/Lsun at 3.6 microns. We use the Baryonic Tully-Fisher
relation calibrated by gas rich galaxies to provide an independent estimate of
the color-mass to light ratio relation. This approach also suggests that the
typical 3.6 micron mass-to-light ratio is 0.5 (0.65 in the K band) for
rotationally supported galaxies. These values are consistent with a Kroupa IMF.
