Small-scale structure of the disc velocity distribution. A maximum entropy statistical approach - Part II

Abstract

Among metallicity, colour, and other star properties, the eccentricity of the star’s orbit behaves as a very good sampling parameter to find a more detailed structure for the disc velocity distribution, allowing distinctions between different eccentricity layers. For subsam- ples with eccentricities e < 0.15, star velocities are approximately symmetrically distributed around the LSR in the radial direction, with a dearth of stars at the LSR. For e = 0.15, the core distribution of the thin disc is supported by two major stellar groups with opposite radial velocities. Several simulations confirm that such a double-peaked distribution comes from the lognormal distribution of the velocity amplitudes. For maximum eccentricity 0.3 and maximum distance to the Galactic plane 0.5 kpc a representative thin disc sample is obtained. An explanation of the apparent vertex deviation of the disc from the swinging of those major kinematic groups around the LSR is possible, which predicts a continuously changing orientation of the disc’s pseudo ellipsoid.