Multi-Element Abundance Measurements from Medium-Resolution Spectra. I.
  The Sculptor Dwarf Spheroidal Galaxy
  We present measurements of Fe, Mg, Si, Ca, and Ti abundances for 388 radial
velocity member stars in the Sculptor dwarf spheroidal galaxy (dSph), a
satellite of the Milky Way. This is the largest sample of individual alpha
element (Mg, Si, Ca, Ti) abundance measurements in any single dSph. The
measurements are made from Keck/DEIMOS medium-resolution spectra (6400-9000 A,
R ~ 6500). Based on comparisons to published high-resolution (R >~ 20000)
spectroscopic measurements, our measurements have uncertainties of
sigma([Fe/H]) = 0.14 and sigma([alpha/Fe]) = 0.13. The Sculptor [Fe/H]
distribution has a mean <[Fe/H]> = -1.58 and is asymmetric with a long,
metal-poor tail, indicative of a history of extended star formation. Sculptor
has a larger fraction of stars with [Fe/H] < -2 than the Milky Way halo. We
have discovered one star with [Fe/H] = -3.80 +/- 0.28, which is the most
metal-poor star known anywhere except the Milky Way halo, but high-resolution
spectroscopy is needed to measure this star's detailed abundances. As has been
previously reported based on high-resolution spectroscopy, [alpha/Fe] in
Sculptor falls as [Fe/H] increases. The metal-rich stars ([Fe/H] ~ -1.5) have
lower [alpha/Fe] than Galactic halo field stars of comparable metallicity. This
indicates that star formation proceeded more gradually in Sculptor than in the
Galactic halo. We also observe radial abundance gradients of -0.030 +/- 0.003
dex per arcmin in [Fe/H] and +0.013 +/- 0.003 dex per arcmin in [alpha/Fe] out
to 11 arcmin (275 pc). Together, these measurements cast Sculptor and possibly
other surviving dSphs as representative of the dwarf galaxies from which the
metal-poor tail of the Galactic halo formed.
