Ca II triplet spectroscopy of RGB stars in NGC 6822: Kinematics and metallicities

Abstract

We present a detailed analysis of the chemistry and kinematics of red giants in the dwarf irregular galaxyNGC6822. Spectroscopy at ≈ 8500 Åwas acquired for 72 red giant stars across two fields using FORS2at theVLT. Line-of-sight extinctionwas individually estimated for each target star to accommodate the variable reddening across NGC 6822. The mean radial velocity was found to be 〈vrad〉 = -52.8 ± 2.2 kms-1 with dispersion σv = 24.1 kms-1, in agreement with other studies. Ca II triplet equivalent widths were converted into [Fe/H] metallicities using a V magnitude proxy for surface gravity. The average metallicity was 〈[Fe/H]〉 = -0.84 ± 0.04 with dispersion σ = 0.31 dex and interquartile range 0.48. Our assignment of individual reddening values makes our analysis more sensitive to spatial variations in metallicity than previous studies. We divide our sample into metal-rich and metal-poor stars; the former were found to cluster towards small radiiwith themetal-poor starsmore evenly distributed across the galaxy. The velocity dispersion of the metal-poor stars was found to be higher than that of the metal-rich stars (σvMP = 27.4 kms-1; σvMR = 21.1 kms-1); combined with the age-metallicity relation this indicates that the older populations have either been dynamically heated during their lifetimes or were born in a less disc-like distribution than the younger stars. The low ratio vrot/σ v suggests that within the inner 10 arcmin, NGC 6822's stars are dynamically decoupled from the H I gas, and possibly distributed in a thick disc or spheroid structure.