Strong CO Absorption Features in Massive ETGs
  Massive Early-Type Galaxies (ETG) in the local Universe are believed to be
the most mature stage of galaxy evolution. Their stellar population content
reveals the evolutionary history of these galaxies. However, while
state-of-the-art Stellar Population Synthesis (SPS) models provide an accurate
description of observed galaxy spectra in the optical range, the modelling in
the Near-Infrared (NIR) is still in its infancy. Here we focus on NIR CO
absorption features to show, in a systematic and comprehensive manner, that for
massive ETGs, all CO indices, from H through to K band, are significantly
stronger than currently predicted by SPS models. We explore and discuss several
possible explanations of this "CO mismatch", including the effect of
intermediate-age, AGB-dominated, stellar populations, high metallicity
populations, non-solar abundance ratios and the initial mass function. While
none of these effects is able to reconcile models and observations, we show
that ad-hoc "empirical" corrections, taking into account the effect of
CO-strong giant stars in the low-temperature regime, provide model predictions
that are closer to the observations. Our analysis points to the effect of
carbon abundance as the most likely explanation of NIR CO line-strengths,
indicating possible routes for improving the SPS models in the NIR.
