Towards a fully consistent Milky Way disc model - II. The local disc model and SDSS data of the NGP region

We have used the self-consistent vertical disc models of the solar neighbourhood presented in Paper I, which are based on different star formation histories (SFRs) and fit the local kinematics of main-sequence stars equally well, to predict star counts towards the North Galactic Pole (NGP). We combined these four different models with the local main sequence in the filter system of the Sloan Digital Sky Survey (SDSS) and predicted the star counts in the NGP field with b > 80 degrees. All models fit the Hess diagrams in the F-K dwarf regime better than +/- 20 per cent and the star number densities in the solar neighbourhood are consistent with the observed values. The chi(2) analysis shows that model A is clearly preferred with systematic deviations of a few per cent only. The SFR of model A is characterized by a maximum at an age of 10 Gyr and a decline by a factor of 4 to the present-day value of 1.4 M-circle dot pc(-2) Gyr(-1). The thick disc can be modelled very well by an old isothermal simple stellar population. The density profile can be approximated by a sech(alpha) function. We found a power-law index alpha(t) = 1.16 and a scaleheight h(t) = 800 pc corresponding to a vertical velocity dispersion of sigma(t) = 45.3 km s-1. About 6 per cent of the stars in the solar neighbourhood are thick-disc stars.