TP-AGB stars and stellar population properties of a post-starburst galaxy at z ∼ 2 through optical and near-infrared spectroscopy with JWST

¹ INAF – Osservatorio Astronomico di Brera, Via Brera 28, 20121 Milano, Italy
² DiSAT, Università degli Studi dell’Insubria, Via Valleggio 11, I-22100 Como, Italy
³ INAF – Osservatorio Astronomico di Capodimonte, Via Moiariello 16, 80131, Naples, Italy
⁴ European Space Research and Technology Centre, Keplerlaan 1, 2200 AG Noordwijk, The Netherlands
⁵ FINCA, University of Turku, Vesilinnantie 5, Turku, 20014, Finland
⁶ Department of Physics and Astronomy, Botswana International University of Science and Technology, Private Bag 16, Palapye, Botswana
⁷ Università degli studi di Milano – Bicocca, Piazza della scienza, I-20125 Milano, Italy
⁸ INAF – Osservatorio Astronomico di Arcetri, Largo Enrico Fermi 5, I-50125, Firenze, Italy
⁹ ATG Europe for the European Space Agency, European Space Research and Technology Centre, Noordwijk, The Netherlands
¹⁰ Physics and Astronomy Department, Tufts University, 574 Boston Avenue, Medford, MA 02155, USA
¹¹ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstrasse 12 – 14, 69120 Heidelberg, Germany
¹² European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
¹³ Sub-Dep. of Astrophysics, Dep. of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
¹⁴ Instituto de Astrofísica de Canarias (IAC), La Laguna, E-38200 Tenerife, Spain
¹⁵ Departamento de Astrofísica, Universidad de La Laguna, E-38205 Tenerife, Spain

^⋆ Corresponding author: davide.bevacqua@inaf.it

Received: 13 January 2025
Accepted: 21 May 2025

Abstract

We present a detailed optical and near-IR (NIR) spectral analysis of J-138717, a post-starburst galaxy at z = 1.8845 observed with JWST/NIRSpec, for which we derive a stellar mass of 3.5±0.2×10¹⁰ M_⊙ and a stellar velocity dispersion of 198±10 km s⁻¹. We estimate an age of ∼0.9 Gyr and a subsolar metallicity (between −0.4 and −0.2 dex). We find generally consistent results when we fit the optical and NIR wavelength ranges separately or with different model libraries. The reconstruction of the star formation history indicates that the galaxy assembled most of its mass quickly and then quenched rapidly, ∼0.4 Gyr before the observation. Line diagnostics suggest that the weak emission is probably powered by residual star formation (star formation rate ∼0.2 M_⊙ yr⁻¹) or a low-luminosity active galactic nucleus, without strong evidence for outflows in ionized or neutral gas. We performed a detailed study of the NIR spectral indices by comparing observations with predictions of several current stellar population models. This is unprecedented at this high redshift. In particular, the analysis of several CO and CN features argues against a strong contribution of thermally pulsating (TP) asymptotic giant branch (AGB) stars. The observations agree better with models that include very little contribution from TP-AGB stars, but they are also consistent with a mild contribution from TP-AGB stars when a younger age, consistent with the fits, is assumed. The analysis of other NIR spectral indices shows that current models struggle to reproduce the observations. This highlights the need for improved stellar population models in the NIR, especially at young ages and low metallicities. This is most relevant for studying high-redshift galaxies in the era of the JWST.