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

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.

A&A, in press

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• Dusty Stellar Birth and Death in the Metal-Poor Galaxy NGC 6822
• Ca II triplet spectroscopy of RGB stars in NGC 6822: kinematics and metallicities
• The AGB population in IC 1613 using JHK photometry
• Massive young stellar objects in the Local Group irregular galaxy NGC6822 identified using machine learning
• Distance and reddening of the Local Group dwarf irregular galaxy NGC 6822
• Investigating episodic mass loss in evolved massive stars: III. Spectroscopy of dusty massive stars in three northern galaxies
• On the dwarf irregular galaxy NGC 6822. I. Young, intermediate and old stellar populations
• The dwarf irregular galaxy NGC 6822. II. Young, intermediate and old stellar populations: comparison between theory and observations