Stellar Populations and Abundances in Sculptor and Fornax

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Session 48 -- Spiral, Dwarf and Irregular Galaxies
Display presentation, Wednesday, 1, 1994, 9:20-6:30

[48.10] Stellar Populations and Abundances in Sculptor and Fornax

Eva K.\ Grebel (Sternwarte Bonn), Wm J.\ Roberts (STScI)

We present Washington photometry of the dwarf spheroidal galaxies Fornax and Sculptor and new methods for obtaining reddening, metallicity, age, and distance modulus. Our method is based on simultaneous isochrone fits in three independent colours, and on the analysis of metallicity distribution functions of cool stars.

For Sculptor, we find a reddening of E$_{\rm B-V,0}=0.045 \pm 0.008$ mag and a distance modulus of $19.15 \pm 0.05$ mag. We get the best fits for isochrones on the assumption of two main populations, one with an age of $17.3 \pm 0.7$ Gyr and a second population with an age of $15.0 \pm 1.4$ Gyr. The metallicity distribution shows three abundance populations at [Fe/H] = $-1.64 \pm 0.06$ dex, at $-2.1 \pm 0.1$ dex, and at $-2.5 \pm 0.5$ dex. The first population encompasses $\simeq$ 90\% of all red giants in our field, while $\simeq$ 10\% of the red giants belong to the subpopulations.

The weighted means we compute for the four main vital statistics of the Fornax field are (1) a reddening of E$_{\rm (B-V),0}=0.045 \pm 0.015$ mag, (2) a distance modulus of DM = $20.26 \pm 0.17$ mag, (3) abundance of [Fe/H] = --1.32 $\pm$ 0.11 dex, and (4) an age of t = 13.0 $\pm$ 1.6 Gyr. The GC\#3 population fits an age of $13.8 \pm 1.1$ Gyr, and [Fe/H] = -1.87 $\pm$ 0.11 dex. The metallicity distribution function is in general agreement with these isochrone results, but also reveals a more metal rich subpopulation of stars in the Fornax field. There is evidence of an intermediate age population in the colour magnitude diagrams that is fit by an isochrone of 3 Gyr and an abundance of --1.3 dex. The metallicity of the few stars so identified does not seem metal rich.

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