Near Infrared spectroscopy of post-starburst galaxies: a limited impact
  of TP-AGB stars on galaxy SEDs
  We present VLT-ISAAC NIR spectro-photometric observations of 16
post-starburst (PSB) galaxies aimed at constraining the debated influence of
TP-AGB stars on the SED of galaxies with stellar ages of 0.5-2 Gyr, critical
for high-redshift studies. PSB galaxies have negligible on-going star formation
and a SED dominated by the stars formed in a recent (<2 Gyr) burst. By
spectroscopically selecting PSB galaxies with mean luminosity-weighted ages
between 0.5 and 1.5 Gyr and a broad range of metallicities, we explore the
parameter space around the peak of the relative energy output of TP-AGB stars.
Crucially, we target galaxies at z~0.2, so that two main spectral features of
TP-AGB stars (C-molecule band-head drops at 1.41 and 1.77mum), which are
heavily absorbed by our atmosphere for targets at z~0, move inside the H and K
atmospheric windows and can be constrained for the first time to high accuracy.
Our findings provide key constraints to stellar population synthesis models: i)
The NIR regions around 1.41 and 1.77mum (rest-frame) are featureless for all
galaxies in our sample over the whole range of relevant ages and metallicities
at variance with the Maraston (2005) TP-AGB heavy models, which exhibit marked
drops there; ii) No flux boosting is observed in the NIR. The optical-NIR SEDs
of most of our PSB galaxies can be consistently reproduced with Bruzual &
Charlot (2003) models, using either simple stellar populations of corresponding
light-weighted ages and metallicities, or a more realistic burst plus an old
population containing <~60% of the total stellar mass. In contrast, all
combinations of this kind based on the Maraston (2005) models are unable to
simultaneously reproduce the smoothness of the NIR spectra and the relatively
blue optical-NIR colours in the observations. The data collected in this study
appear to disfavour TP-AGB heavy models with respect to TP-AGB light ones.
