Estimation of galactic model parameters with the Sloan Digital Sky Survey and the metallicity distribution in two fields in the anti-centre direction of the Galaxy

We estimated the galactic model parameters from the Sloan Digital Sky Survey (SDSS) data reduced for two fields in the anti-centre direction of the Galaxy, l = 180°, and symmetric relative to the galactic plane, b = +45° (north field) and b = –45° (south field). The large size of each field, 60 deg2, and the faint limiting apparent magnitude, g0 = 22, give us the chance to determine reliable parameters for three components, thin and thick discs and halo, in the north and south hemispheres of the Galaxy, except the scale lengths for two discs which are adopted from Juric et al. (2005). Metallicities were evaluated by a recent calibration for SDSS, and absolute magnitudes of stars with 4 < M (g) ≤ 8 were derived as a function of (g – r)0 colour and metallicity. A χ2 method was employed to fit the analytical density laws to the observational-based space densities with the addition constraint of producing local densities consistent with those derived from Hipparcos. Conspicuous differences could not be detected between the corresponding galactic model parameters for the thin disc of north and south fields, and our results are consistent with the ones in the literature. The same case is valid for the halo, especially the axis ratios for two fields are exactly equal, κ = 0.45, and close to the one of Juric et al. (2005). However, we revealed differences between the scale heights and local space densities for the thick disc of the north and south fields. The metallicity distribution for unevolved G type stars with 5 < M (g) ≤ 6 shows three substructures relative to the distance from the galactic plane: for z * < 3 kpc, the metallicity gradient for two fields is d[M/H]/dz ∼ –0.20(±0.02) dex kpc–1, consistent with the formation scenario of the thin disc. For 5 < z * < 10 kpc, d[M/H]/dz ∼ –0.03(±0.001) dex kpc–1 for two fields, confirming that the metallicity gradient for the halo component is close to zero. However, the tendency of the metallicity for stars with 3 < z * < 5 kpc, corresponding to the transition region from thick disc to halo is different. For the north field d[M/H]/dz = –0.36(±0.12) dex kpc–1, whereas it is half of this value for the south field, d[M/H]/dz = –0.18(±0.01) dex kpc–1. The origin of this conflict is probably due to the different structure of the thick disc in opposite latitudes of the Galaxy. When we combine these substructures, however, we find a smooth metallicity gradient for two fields, –0.30(±0.04) dex kpc–1. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)