The late-type stellar content of the Fornax and Sculptor dwarf galaxies

Abstract

A field of area 0.13 square degrees has been surveyed for late-type stars in each of the Fornax and Sculptor dwarf elliptical galaxies. JHK photometric data have been obtained for most of the stars found. In Fornax, we have positively identified 25 C stars and one M giant. In Sculptor, two relatively blue C stars and a small number of possible M giants have been identified. In contrast to the Magellanic Clouds, there are no M6-M9 giants in Fornax or Sculptor. The mean value and the variance of the bolometric luminosity function for the Fornax C stars are — 4.66 ± 0.47, quite similar to the values for the Magellanic Cloud C stars. The colors of the Fornax C stars overlap those of the C stars in the Magellanic Clouds but are bluer in the mean. The large dispersion in the color-magnitude diagram of the Fornax C stars is interpreted as arising from a significant spread in age and/or metallicity in the stellar population of Fornax. The C stars found in Sculptor are quite similar in color and luminosity to the C stars in the globular cluster ω Centauri and are at the faint end of the luminosity distribution of C stars found in the Magellanic Clouds and Fornax. The ratio of cool C stars to M stars, as determined from identical survey techniques, increases dramatically along the sequence Milky Way, LMC, SMC, and Fornax. This increase, together with systematic changes in the colors of the C and M stars, can be understood as arising from a systematic decrease in the mean metallicity of the galaxies on this sequence. The lack of concurrent significant changes in M_(bol)(mean) or M_(bol)(max) of the carbon stars may not be consistent with current theories of C star formation and evolution. The new data for Sculptor, as well as those previously published, point to a stellar population, and possibly a star-formation history, qualitatively similar to that of ω Cen. A sharp discontinuity between Fornax and Sculptor in some of the properties which characterize the late-type stellar population of these two systems stands in contrast to a rather smooth gradation in the same properties for Fornax and galaxies more massive than it. We speculate that this discontinuity could have arisen if Sculptor were stripped of its gas component at a much earlier time than Fornax.

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