Deep SDSS optical spectroscopy of distant halo stars II. Iron, calcium, and magnesium abundances

Aims. We analyze a sample of 3944 low-resolution (R similar to 2000) optical spectra from the Sloan Digital Sky Survey (SDSS), focusing on stars with effective temperatures 5800 <= T-eff <= 6300 K, and distances from the Milky Way plane in excess of 5 kpc, and determine their abundances of Fe, Ca, and Mg. Methods. We followed the same methodology as in the previous paper in this series, deriving atmospheric parameters by chi(2) minimization, but this time we obtained the abundances of individual elements by fitting their associated spectral lines. Distances were calculated from absolute magnitudes obtained by a statistical comparison of our stellar parameters with stellar-evolution models. Results. The observations reveal a decrease in the abundances of iron, calcium, and magnesium at large distances from the Galactic center. The median abundances for the halo stars analyzed are fairly constant up to a Galactocentric distance r similar to 20 kpc, rapidly decrease between r similar to 20 and r similar to 40 kpc, and flatten out to significantly lower values at larger distances, consistent with previous studies. In addition, we examine [Ca/Fe] and [Mg/Fe] as a function of [Fe/H] and Galactocentric distance. Our results show that the most distant parts of the halo show a steeper variation of [Ca/Fe] and [Mg/Fe] with iron. We found that at the range -1.6 < [Fe/H] < -0.4, [Ca/Fe] decreases with distance, in agreement with earlier results based on local stars. However, the opposite trend is apparent for [Mg/Fe]. Our conclusion that the outer regions of the halo are more metal-poor than the inner regions, based on in situ observations of distant stars, agrees with recent results based on inferences from the kinematics of more local stars, and with predictions of recent galaxy formation simulations for galaxies similar to the Milky Way.