Open AccessUsing the Ca ii triplet to trace abundance variations in individual red giant branch stars in three nearby galaxies
Author(s) -
Tolstoy Eline,
Irwin Michael J.,
Cole Andrew A.,
Pasquini L.,
Gilmozzi R.,
Gallagher J.S.
Publication year - 2001
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1046/j.1365-8711.2001.04785.x
Subject(s) - physics , astrophysics , metallicity , galaxy , red giant branch , stars , local group , astronomy , red giant , dwarf galaxy , giant star , photometry (optics) , dwarf spheroidal galaxy , interacting galaxy
Spectroscopic abundance determinations for stars spanning a Hubble time in age are necessary in order to determine unambiguously the evolutionary histories of galaxies. Using FORS1 in multi‐object spectroscopy mode on ANTU (UT1) at the ESO VLT on Paranal, we have obtained near‐infrared spectra from which we have measured the equivalent widths of the two strongest Ca ii triplet lines to determine metal abundances for a sample of red giant branch stars, selected from ESO NTT optical ( I , photometry of three nearby Local Group galaxies: the Sculptor dwarf spheroidal, the Fornax dwarf spheroidal and the dwarf irregular NGC 6822. The summed equivalent width of the two strongest lines in the Ca ii triplet absorption‐line feature, centred at 8500 Å, can be readily converted into an [Fe/H] abundance using the previously established calibrations by Armandroff & Da Costa and Rutledge, Hesser & Stetson. We have measured metallicities for 37 stars in Sculptor, 32 stars in Fornax and 23 stars in NGC 6822, yielding more precise estimates of the metallicity distribution functions for these galaxies than it is possible to obtain photometrically. In the case of NGC 6822, this is the first direct measurement of the abundances of the intermediate‐age and old stellar populations. We find metallicity spreads in each galaxy which are broadly consistent with the photometric width of the red giant branch, although the abundances of individual stars do not always appear to correspond to their colour. This is almost certainly predominantly due to a highly variable star formation rate with time in these galaxies, which results in a non‐uniform, non‐globular‐cluster‐like evolution of the Ca/Fe ratio.