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Ca ɪɪ triplet spectroscopy of RGB stars in NGC 6822: kinematics and metallicities
journal contributionposted on 2023-05-18, 18:05 authored by Swan, J, Andrew ColeAndrew Cole, Tolstoy, E, Irwin, MJ
We present a detailed analysis of the chemistry and kinematics of red giants in the dwarf irregular galaxy NGC 6822. Spectroscopy at ≈8500 Å was acquired for 72 red giant stars across two fields using FORS2 at the VLT. Line-of-sight extinction was individually estimated for each target star to accommodate the variable reddening across NGC 6822. The mean radial velocity was found to be 〈υrad〉 = −52.8 ± 2.2 km s−1 with dispersion συ = 24.1 km s−1, in agreement with other studies. Ca ɪɪ 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 radii with the metal-poor stars more 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 (συMP = 27.4 km s−1; συMR = 21.1 km s−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 υrot/συ suggests that within the inner 10 arcmin, NGC 6822's stars are dynamically decoupled from the H ɪ gas, and possibly distributed in a thick disc or spheroid structure.
Publication titleMonthly Notices of the Royal Astronomical Society
Department/SchoolSchool of Natural Sciences
PublisherBlackwell Publishing Ltd
Place of publication9600 Garsington Rd, Oxford, England, Oxon, Ox4 2Dg
Rights statementCopyright 2016 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.