The Mid-Infrared Luminosities of Normal Galaxies over Cosmic Time
  Modern population synthesis models estimate that 50% of the restframe K-band
light is produced by TP-AGB stars during the first Gyr of a stellar population,
with a substantial fraction continuing to be produced by the TP-AGB over a
Hubble time. Between 0.2 and 1.5 Gyr, intermediate mass stars evolve into
TP-AGB C stars which, due to significant amounts of circumstellar dust, emit
half their energy in the mid-IR. We combine these results using published
mid-IR colors of Galactic TP-AGB M and C stars to construct simple models for
exploring the contribution of the TP-AGB to 24micron data as a function of
stellar population age. We compare these empirical models with an ensemble of
galaxies in the CDFS from z=0 to z=2, and with high quality imaging in M81.
Within the uncertainties, the TP-AGB appears responsible for a substantial
fraction of the mid-IR luminosities of galaxies from z=0 to z=2, the maximum
redshift to which we can test our hypothesis, while, at the same time, our
models reproduce much of the detailed structure observed in mid-IR imaging of
M81. The mid-IR is a good diagnostic of star formation over timescales of ~1.5
Gyr, but this implies that on-going star formation rates at z=1 may be
overestimated by factors of ~1.5-6, depending on the nature of star formation
events. Our results, if confirmed through subsequent work, have strong
implications for the star formation rate density of the universe and the growth
of stellar mass over time.
