Abundances of Red Giants in the Andromeda II Dwarf Spheroidal Galaxy

Abstract

We have obtained spectra for 50 candidate red giants in Andromeda II, a dwarf spheroidal companion of M31, using the Low Resolution Imaging Spectrometer on the Keck II Telescope. After eliminating background galaxies and Galactic foreground stars, we are left with a sample of 42 red giants for which membership in Andromeda II can be established unambiguously from radial velocities. Line indexes measured on the Lick/IDS system are combined with VI photometry obtained with the Keck II and Palomar 5 m telescopes to investigate the age and metallicity distribution of these stars. Based on a comparison of the measured line indexes with those of Lick/IDS standard stars in globular and open clusters, we derive a mean metallicity of 〈[Fe/H]〉 = -1.47 ± 0.19 dex. This confirms the earlier conclusion, based on Thuan-Gunn gr photometry, that Andromeda II obeys the familiar relation between mean stellar metallicity and galaxy luminosity. There is also evidence for a dispersion in metallicity of σ([Fe/H]) = 0.35 ± 0.10 dex, based on the scatter in the measured Mg b line indexes and the observed width of the galaxy's giant branch. We note that, while existing observations of Local Group dwarf galaxies indicate that their mean metallicity depends rather sensitively on total luminosity, the internal spread in metallicity appears to be relatively independent of galaxy luminosity. Our spectroscopic sample contains one carbon star. We measure MI ≃-3.8 for this star, which places it below the tip of the red giant branch and suggests a common origin with the CH stars found in the Galactic halo. Although this carbon star alone does not provide evidence of an intermediate-age component in Andromeda II, two other stars in our spectroscopic sample have MI ≃-4.7 and -4.5. Membership in Andromeda II is unambiguous in both cases, indicating that these stars fall along an extended asymptotic giant branch and pointing to the presence of a modest intermediate-age population in this galaxy.

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