Nikolay Kacharov, Marina Rejkuba, Maria-Rosa L. Cioni
Aims: Our aims are to calibrate spectroscopically the C vs. M-type Asymptotic
Giant Branch (AGB) star selection made using near-IR photometry, and to
investigate the spatial distribution of C/M ratio in NGC 6822, based on low
resolution spectroscopy and near-IR photometry. Methods: We have obtained low
resolution multi-object spectroscopy with the VIMOS instrument at the ESO VLT
for ~800 stars in seven fields centered on NGC 6822. The spectroscopic
classification of giant stars in NGC 6822 and foreground dwarf contaminants has
been made by comparison of more than 500 spectra with good quality with the
spectroscopic atlas of Turnshek et al. (1985) The sample of spectroscopically
confirmed AGB stars in NGC 6822 is split 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 151 C-stars and 123 M-stars. 79% of the
C-stars in our catalogue are redder than (J-K)_0=1.2 mag and 12% are brighter
than K_0=16.45 mag and bluer than (J-K)_0=1.2 mag. The rest 9% are mixed with
the M-type AGB stars. 88% of the latter have colours (J-H)_0>0.73 mag and
(J-K)_0 between 0.9 mag and 1.2 mag. The rest 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 the overall C/M ratio of the galaxy to
be around 0.8 with spread between 0.2View original: http://arxiv.org/abs/1110.4116
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