The Spectral Energy Distribution of Post-Starburst Galaxies in the
  NEWFIRM Medium-Band Survey: A Low Contribution from TP-AGB Stars
  Stellar population synthesis (SPS) models are a key ingredient of many galaxy
evolution studies. Unfortunately, the models are still poorly calibrated for
certain stellar evolution stages. Of particular concern is the treatment of the
thermally-pulsing asymptotic giant branch (TP-AGB) phase, as different
implementations lead to systematic differences in derived galaxy properties.
Post-starburst galaxies are a promising calibration sample, as TP-AGB stars are
thought to be most prominently visible during this phase. Here, we use
post-starburst galaxies in the NEWFIRM medium-band survey to assess different
SPS models. The available photometry allows the selection of a homogeneous and
well-defined sample of 62 post-starburst galaxies at 0.7<z<2.0, from which we
construct a well-sampled composite spectral energy distribution (SED) over the
range 1200-40 000 Angstrom. The SED is well-fit by the Bruzual & Charlot SPS
models, while the Maraston models do not reproduce the rest-frame optical and
near-infrared parts of the SED simultaneously. When the fitting is restricted
to lambda < 6000 Angstrom, the Maraston models overpredict the near-infrared
luminosity, implying that these models give too much weight to TP-AGB stars.
Using the flexible SPS models by Conroy et al, and assuming solar metallicity,
we find that the contribution of TP-AGB stars to the integrated SED is a factor
of ~3 lower than predicted by the latest Padova TP-AGB models. Whether this is
due to lower bolometric luminosities, shorter lifetimes, and/or heavy dust
obscuration of TP-AGB stars remains to be addressed. Altogether, our data
demand a low contribution from TP-AGB stars to the SED of post-starburst
galaxies.
