Tantalo, M, Bono, G, Salaris, M, Pietrinferni, A, Monelli, M, Fabrizio, M, Braga, VF, Calamida, A, Dall'Ora, M, D'Orazi, V, Ferraro, I, Fiorentino, G, Iannicola, G, Marengo, M, Matsunaga, N, Mullen, JP and Stetson, PB (2025) The Dwarf Irregular Galaxy NGC 6822. II. Young, Intermediate and Old Stellar Populations: Comparison between Theory and Observations. The Astrophysical Journal, 983 (2). pp. 1-22. ISSN 0004-637X

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Abstract

This paper presents a quantitative analysis of the stellar content in the Local Group dwarf irregular galaxy NGC 6822 by comparing stellar evolution models and observations in color–magnitude diagrams (CMDs) and color–color diagrams (CC-Ds). Our analysis is based on optical ground-based g, r, i photometry, and deep archival Hubble Space Telescope photometry of two fields in the galactic disk. We compared young, intermediate-age, and old stellar populations with isochrones from the BaSTI-IAC library and found that NGC 6822 hosts a quite metal-rich ([Fe/H] = −0.7 to −0.4) young component with an age ranging from 20–100 Myr. The intermediate-age population experienced a modest chemical enrichment between 4 and 8 Gyr ago, while stars older than 11 Gyr have a low metal abundance ([Fe/H] ∼ −1.70). We also identified the asymptotic giant branch (AGB) clump population with a luminosity peak at i ∼ 23.35 mag. Our analysis of both the CMD and the optical–near-IR (NIR)–mid-IR (MIR) CC-Ds of AGB oxygen- and carbon-rich stars, using the PARSEC+COLIBRI isochrones with and without circumstellar dust, reveals that this stellar component exhibits a spread in age from 1–2 Gyr and in metallicity between [Fe/H] = −1.30 and −1.70. The stellar models we used reproduce very well the two distinct color sequences defined by AGB O- and C-rich stars in the various optical–NIR–MIR CC-Ds, suggesting that they are reliable diagnostics to identify and characterize intermediate-age stellar populations. However, we also find that evolutionary prescriptions in the optical i-(r − i) CMDs predict, at fixed color, systematically lower luminosities than observed AGB stars.

Item Type:	Article
Uncontrolled Keywords:	0201 Astronomical and Space Sciences; 0202 Atomic, Molecular, Nuclear, Particle and Plasma Physics; 0306 Physical Chemistry (incl. Structural); Astronomy & Astrophysics; 5101 Astronomical sciences; 5107 Particle and high energy physics; 5109 Space sciences
Subjects:	Q Science > QB Astronomy Q Science > QC Physics
Divisions:	Astrophysics Research Institute
Publisher:	American Astronomical Society
SWORD Depositor:	A Symplectic
Date Deposited:	10 Apr 2025 12:40
Last Modified:	10 Apr 2025 12:45
DOI or ID number:	10.3847/1538-4357/adbf0e
URI:	https://researchonline.ljmu.ac.uk/id/eprint/26161

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