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Öğe 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(Wiley-V C H Verlag Gmbh, 2007) Ak, S.; Bilir, S.; Karaali, S.; Buser, R.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 degrees, and symmetric relative to the galactic plane, b = +45 degrees (north field) and b = -45 degrees (south field). The large size of each field, 60 deg(2), and the faint limiting apparent magnitude, go 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 chi(2) 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, K = 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 similar to -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 similar to -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). (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA. Weinheim.Öğe The metallicity distributions in high-latitudes with SDSS(Elsevier Science Bv, 2007) Ak, S.; Bilir, S.; Karaali, S.; Buser, R.; Cabrera-Lavers, A.We present metallicities and their variations with different parameters for 36 high-latitude fields covering Galactic longitudes < I <= 360 degrees. The metallicities for relatively short vertical distances (z < 2.5 kpc) show systematic fluctuations with Galactic longitude, similar to those of the thick-disc scaleheight, which may be interpreted as indicating a common origin, viz., the flare effect : I the disc (Bilir et al., 2007, MNRAS). This suggestion is supported by the metallicity variations which we find as functions of radial stance. The metallicity variation at larger vertical distances (6.5 < z <= 9.5 kpc) is small but monotonic. Three different vertical metallicity graents could be detected: d[M vertical bar H]dz = - 0.22( +/- 0.03), d[M vertical bar H]vertical bar dz = -0.38( +/- 0.06), and d[M vertical bar H]dz = -0.08(+/- 0.07) dex kpc(-1) for the,tervals z < 3, 3 <=, z < 5, and 5 <=, z < 10 kpc, respectively. Since our data cover the whole Galactic longitude interval, the resulting metlicity gradients can be interpreted as properties of the larger-scale Galaxy. The first gradient confirms the dissipational formation of the sc at short z-distances. The steeper gradient corresponds to the transition region between different population components of the Galcy, and finally, the lowest value provides an adequate description of the inner-halo metallicity gradient. 2007 Elsevier B.V. All rights reserved.Öğe Volume-limited dependent Galactic model parameters(Csiro Publishing, 2007) Karaali, S.; Bilir, S.; Yaz, E.; Hamzaoglu, E.; Buser, R.We estimate 34 sets of Galactic model parameters for three intermediate-latitude fields with Galactic longitudes l = 60 degrees, l = 90 degrees, and l= 180 degrees, and we discussed their dependence on the volume. Also, we confirm the variation of these parameters with absolute magnitude and Galactic longitude. The star samples in two fields are restricted with bright and unit absolute-magnitude intervals, 4 < M-g <= 5, and 5 < M-g <= 6, whereas for the third field (l= 60 degrees) a larger absolute-magnitude interval is adopted, 4 < M-g <= 10. The limiting apparent magnitudes of the star samples are g(0) = 15 and g(0) = 22.5 mag, which provide space densities within distances in the line of sight of similar to 0.9 and 25 kpc. The Galactic model parameters for the thin disc are not volume dependent. However, the ones for the thick disc and halo do show spectacular trends in their variation with volume, except for the scalelength of the thick disc. The local space density of the thick disc increases, whereas the scaleheight of the same Galactic component decreases monotonically. However, both model parameters approach asymptotic values at large distances. The relative local space density of the halo estimated by fitting the density laws to the space densities evaluated for all volumes is constant, except for the small ones. However it is absolute-magnitude and Galactic-longitude dependent. The axial ratio of the halo increases abruptly for the volumes where a thick disc is dominant, whereas it approaches an asymptotic value gradually for larger volumes, indicating a continuous transition from a disc-like structure to a spherical one at the outermost region of the Galaxy. The variation of the Galactic model parameters with absolute magnitude can be explained by their dependence on the stellar luminosity, whereas the variation with volume and Galactic longitude at short distances is a bias in analysis.
