In the present paper, we introduce a two-component model of the Galactic disk to investigate its chemical evolution. The formation of the thick and thin disks occur in two main accretion episodes with both infall rates to be Gaussian. Both the pre-thin and post-thin scenarios for the formation of the Galactic disk are considered.The best-fitting is obtained through χ 2 -test between the models and the new observed metallicity distribution function of G dwarfs in the solar neighbourhood (Hou et al 1998).Our results show that post-thin disk scenario for the formation of the Galactic disk should be preferred. Still, other comparison between model predictions and observations are given.

Current theory of disc galaxy formation is used to study fundamental-plane (FP) type of relations for disc galaxies. We examine how the changes in model parameters affect these relations and explore the possibility of using such relations to constrain theoretical models. The distribution of galaxy discs in the space of their fundamental properties are predicted to be concentrated in a plane, with the Tully-Fisher (TF) relation (a relation between luminosity L and maximum rotation velocity V-m) being an almost edge-on view. Using rotation velocities at larger radii generally leads to larger TF scatter. In searching for a third parameter, we find that both the disc scalelength R-d (or surface brightness) and the rotation-curve shape are correlated with the TF scatter. The FP relation in the (log L, log V-m, log R-d)-space obtained from the theory is L proportional to R-d(alpha') V-m(beta'), with alpha' similar to 0.50 and beta' similar to 2.60, consistent with the preliminary result we obtain from observational data. Using the isophotal radius instead of R-d leads to higher alpha', lower beta' and smaller scatter. Among the model parameters we probe, variation in any of them can generate significant scatter in the TF relation, but the effects of the spin parameter and halo concentration can be reduced significantly by introducing R-d While the scatter caused by varying m(d) (the ratio between disc mass and halo mass) is most effectively reduced by introducing the parameters which describe the rotation-curve shape. The TF and FP relations combined should therefore provide useful constraints on models of galaxy formation

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