Evidence for TP‐AGB Stars in High‐Redshift Galaxies, and Their Effect on Deriving Stellar Population Parameters (original) (raw)

2006, The Astrophysical Journal

We explore the effects of stellar population models on estimating star formation histories, ages and masses of high redshift galaxies. The focus is on the Thermally-Pulsing Asymptotic Giant Branch (TP-AGB) phase of stellar evolution, whose treatment is a source of major discrepancy among different evolutionary population synthesis. In particular, besides the models usually adopted in the literature, we use models (by Maraston 2005), in which the contribution of the TP-AGB phase is calibrated with local stellar populations and is the dominant source of bolometric and near-IR energy for stellar populations in the age range 0.2 to 2 Gyr. These models also have an underlying different treatment of convective overshooting and Red Giant Branch stars. For our experiment we use a sample of high-z (1.4 ∼ < z ∼ < 2.5) galaxies in the Hubble Ultra Deep Field held to be mostly in passive evolution, with low-resolution UV-spectroscopy and spectroscopic redshifts from GRAPES, and Spitzer IRAC and MIPS photometry from the Great Observatories Origins Deep Survey. We choose these galaxies because their mid-UV spectra exhibit features typical of A-or F-type stars, therefore TP-AGB stars ought to be expected in post-Main Sequence. We find that indeed the TP-AGB phase plays a key role in the interpretation of Spitzer data for high-z galaxies, when the rest-frame near-IR is sampled. When fitting without dust reddening, the models with the empirically-calibrated TP-AGB phase always reproduce better the observed spectral energy distributions (SEDs), in terms of a considerably smaller χ 2 . Allowing for dust reddening improves the fits with literature models in some cases. In both cases, the results from Maraston models imply younger ages by factors up to 6 and lower stellar masses (by ∼ 60% on average). The observed strengths of the Mg UV spectral feature compare better to the predicted ones in the case of the Maraston models, implying a better overall consistency of SED fitting. Finally, we find that photometric redshifts improve significantly using these models on the SEDs extending over the IRAC bands. These results are primarily the consequence of the treatment of the TP-AGB phase in the Maraston models, which produces models with redder rest-frame optical to near-IR colors. This work provides the first direct evidence of TP-AGB stars in the primeval Universe.