New light on faint stars – III. Galactic structure towards the South Pole and the Galactic thick disc

Abstract

We have derived absolute magnitudes from photometric parallaxes for a complete sample of ∼ 12 500 stars brighter than I = 18 in 18.24 square degrees towards the South Galactic Pole. From these data we derive the stellar luminosity function in absolute visual magnitude, bolometric magnitude and mass in the solar neighbourhood for all absolute magnitudes above the thermonuclear burning limit. The total mass in main-sequence stars in the solar neighbourhood is 0.038 m_⊙ pc⁻³, and the total mass-to-light ratio is 1.2 (m_⊙/L_v⊙). We have also derived the density laws with distance from the Galactic plane for each absolute magnitude. The stellar luminosity function shows a systematic change in shape, with stars with $$M_\text v \lesssim +4$$ substantially more concentrated to the plane than fainter stars. The density laws for stars with $$M_\text v \gtrsim +4$$ follow a single exponential with scale height ∼ 300 pc for $$100 \lesssim z \lesssim 1000 \enspace\text {pc}$$, and a second exponential with scale height ∼ 1450 pc for z distances from ∼ 1000 to at least 5000 pc. This second exponential contains ∼ 2 per cent of the stars in the solar neighbourhood. We identify the 300 pc scale height component as old disc, and the 1350 pc scale height component as a Galactic ‘thick disc’. The luminosity function and density law of the ‘thick disc’ are consistent with substantial flattening of the spheroid isodensity contours by the gravitational potential of the Galactic disc, in agreement with recent observations of several external galaxies.