Galactic Stellar Populations from Photometric Metallicity Distribution Functions

Abstract

Based on Sloan Digital Sky Survey photometric data, Gu developed a new Monte-Carlo-based method for estimating the stellar metallicity distribution functions (MDFs). This method enables a more reliable determination of MDFs compared with the conventional polynomial-based methods. In this work, MDF determined from the method are well fit by a three-Gaussian model, with peaks at [Fe/H] = −0.68, −1.38, and −1.90, associated with the thick disk, the inner halo, and the outer halo, respectively. The vertical metallicity gradient within 1 <  Z  < 5 kpc is around R  = 8.25 kpc. But the mean radial gradient is almost negligible. The density profile of the thick disk is fitted with a modified double exponential law decaying to a constant at far distance. The scale height and scale length thus estimated are H  ≈ 1.13 kpc and L  ≈ 3.63 kpc, which are consistent with the results determined from star-count methods in previous studies. The halos are described with a two-axial power-law ellipsoid, and the axis ratios of both the inner halo and the outer halo, inferred from stellar number density in the R – Z plane, are q ih  ≈ 0.49 and q oh  ≈ 0.61, respectively. It also manifests that the outer halo is more spherical than the inner halo. Moreover, the halo power-law indices estimated are n ih  ≈ 3.4 and n oh  ≈ 3.1, indicating that the stellar number density of the inner halo changes more steeply than that of outer halo.