Spectra probing the number ratio of C- to M-type AGB stars in the NGC 6822 galaxy

TL;DR

This study calibrates spectroscopic criteria to distinguish C- and M-type AGB stars in NGC 6822, analyzes their spatial distribution, and estimates the galaxy's C/M ratio, revealing insights into its metallicity and age variations.

Contribution

It provides the largest catalogue of spectroscopically confirmed AGB stars in NGC 6822 and refines photometric selection criteria for C- and M-type stars.

Findings

C/M ratio of about 0.8 with a range of 0.2 to 1.8

Metallicity index [Fe/H] between -1.2 and -1.3 dex

Significant age or metallicity variations inferred from C/M ratio trends

Abstract

Aims: We calibrate spectroscopically the C- versus (vs.) M-type asymptotic giant branch (AGB) star selection made using near-IR photometry, and investigate the spatial distribution of the C/M ratio in NGC 6822, based on low resolution spectroscopy and near-IR photometry. Methods: We obtained low resolution multi-object spectroscopy with the VIMOS instrument at the ESO VLT of ~ 800 stars in seven fields centred on NGC 6822. The spectroscopic classification of giant stars in NGC 6822 and foreground dwarf contaminants was made by comparing more than 500 good quality spectra with the spectroscopic atlas of Turnshek et al. (1985). The sample of spectroscopically confirmed AGB stars in NGC 6822 is divided into C- and M-rich giants to constrain the C vs. M AGB star selection criteria based on photometry. The larger near-IR photometric sample is then used to investigate the C/M ratio gradients across the galaxy. Results: We present the largest catalogue of near-IR photometry and spectra of AGB stars in NGC 6822 with 150 C-stars and 122 M-stars. Seventy-nine percent of the C-stars in our catalogue are redder than (J-K)0=1.2 mag, and 12% are brighter than K0=16.45 mag and bluer than (J-K)0=1.2 mag. The remaining 9% are mixed with the M-type AGB stars, 88% of which have colours J-H0>0.73 mag and (J-K)0 between 0.9 mag and 1.2 mag. The remainder are mixed with dwarfs and C-type stars. The foreground dwarfs have preferably colours (J-H)0<0.73 mag (95%). Using the proposed criteria, we estimate that the overall C/M ratio of the galaxy is around 0.8 with a spread between 0.2<C/M<1.8. These results suggest that the metallicity index [Fe/H] is between -1.2 dex and -1.3 dex according to the different calibrations and that there is a significant spread of about 0.4 - 0.6 dex. We also discuss age rather than metallicity variations that could explain the C/M ratio trends.