Early galaxy evolution from deep wide field star counts. II. First estimate of the thick disc mass function

Abstract

Star counts at high and intermediate galactic latitudes, in the visible and the near infrared, are used to determine the density law and the initial mass function of the thick disc population. The combination of shallow fields dominated by stars at the turnoff with deep fields allows the determination of the thick disc mass function in the mass range 0.2-0.8 Msun. Star counts are compared with simulations of a synthesis population model. The fit is based on a maximum likelihood criterion. The best fit model gives a scale height of 800 pc, a scale length of 2500 pc and a local density of 10e-3 stars/pc3 or 7.1e-4 Msun/pc3 for Mv<8. The IMF is found to follow a power law with a slope alpha=-0.5. This is the first determination of the thick disc mass function.