A spectroscopic study of the ancient Milky Way: F- and G-type stars in the third data release of the Sloan Digital Sky Survey

We perform an analysis of spectra and photometry for 22,770 stars included in the third data release (DR3) of the Sloan Digital Sky Survey ( SDSS). We derive atmospheric parameters and distances. Our analysis procedures are throughly checked using three recently published spectroscopic libraries of nearby stars and alternative methods. The SDSS sample covers a range in stellar brightness of 14 < V < 22, primarily at intermediate Galactic latitudes, and comprises large numbers of F- and G-type stars from the thick-disk and halo populations, therefore including some of the oldest stars in the Milky Way. We find that halo stars exhibit a broad range of iron abundances, with a peak at [Fe/ H ]similar or equal to - 1.4. This population exhibits essentially no Galactic rotation. Thick-disk G-dwarf stars at distances from the Galactic plane in the range 1 < vertical bar z vertical bar < 3 kpc show a much more compact metallicity distribution, with a maximum at [Fe/H] similar or equal to - 0.7, and a median Galactic rotation lagging the local standard of rest by 63 km s(-1). A comparison of color indices and metal abundances with isochrones indicates that no significant star formation has taken place in the halo in the last similar to 11 Gyr, but there are thick-disk stars that are at least 2 Gyr younger. We find the metallicities of thick disk stars to be nearly independent of Galactocentric distance between 5 and 14 kpc from the Galactic center, in contrast with the marked gradients found in the literature for the thin disk. No vertical metallicity gradient is apparent for the thick disk, but we detect a gradient in its rotational velocity of -16 +/- 4 km s(-1) kpc(-1) between 1 and 3 kpc from the plane. We estimate that among the stars in our sample there are over 2000 with an iron abundance [Fe/ H] <-2, and over 150 stars with an iron abundance [Fe/ H] < -3.