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

TL;DR

This study uses self-consistent vertical disc models combined with SDSS data to accurately predict star counts towards the North Galactic Pole, favoring a specific star formation history model and characterizing the thick disc structure.

Contribution

It introduces a refined local disc model that fits SDSS star count data and provides detailed parameters for the thick disc component.

Findings

Model A best fits the SDSS star count data.

Thick disc constitutes about 6% of local stars.

Derived thick disc scale height is 800pc with a velocity dispersion of 45.3 km/s.

Abstract

We have used the self-consistent vertical disc models of the solar neighbourhood presented in Just & Jahreiss (2010), which are based on different star formation histories (SFR) 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 SDSS and predicted the star counts in the NGP field with b>80deg. All models fit the Hess diagrams in the F-K dwarf regime better than 20 percent 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 percent only. The SFR of model A is characterised by a maximum at an age of 10Gyr and a decline by a factor of four to the present day value of 1.4Msun/pc^2/Gyr. 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_t) function. We found a power law index alpha_t=1.16 and a scale height of 800pc corresponding to a vertical velocity dispersion of 45.3km/s. About 6 percent of the stars in the solar neighbourhood are thick disc stars.