Astronomy and Astrophysics – Astrophysics – Cosmology and Extragalactic Astrophysics
Scientific paper
2011-10-18
Astronomy and Astrophysics
Astrophysics
Cosmology and Extragalactic Astrophysics
A&A, in press
Scientific paper
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
Cioni Maria-Rosa L.
Kacharov Nikolay
Rejkuba Marina
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