Near Infrared spectroscopy of post-starburst galaxies: a limited impact of TP-AGB stars on galaxy SEDs

TL;DR

This study uses near-infrared spectroscopy of post-starburst galaxies to test the impact of TP-AGB stars on galaxy spectral energy distributions, finding limited evidence for their significant influence as predicted by some models.

Contribution

The paper provides observational constraints that challenge TP-AGB heavy stellar population models, favoring models with a lighter TP-AGB contribution.

Findings

NIR spectra are featureless at key TP-AGB features, contrary to heavy models.

No flux boosting observed in NIR, supporting lighter TP-AGB models.

Most observed SEDs are well reproduced by models with minimal TP-AGB impact.

Abstract

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.