Structure of the velocity distribution of the Galactic disc: a maximum entropy statistical approach - Part I

Abstract

The maximum entropy approach is proposed to describe the local structures of the veloc- ity distribution, which are collected through its sample moments. The method is used with several samples from the HIPPARCOS and Geneva-Copenhagen survey catalogues. For the large-scale distribution, the phase density function may be obtained by fitting moments up to sixth order as a product of two exponential functions, one giving a background ellipsoidal shape of the distribution and the other accounting for the skewness and for the slight shift in the ellipsoidal isocontours in terms of the rotation velocity. The small-scale distribution can be deduced from truncated distributions, such as velocity-bounded samples with |V| ≤ 51 km s−1, which contain a complex mixture of early-type and young disc stars. By fitting up to ten-order moments, the maximum entropy approach gives a realistic portrait of actual asymmetries, showing a clear bimodal pattern: (i) around the Hyades-Pleiades stream, with negative radial mean velocity and (ii) around the Sirius-UMa stream, with slightly positive radial mean velocity. The “U-anomaly” along the radial direction is estimated straightfor- wardly 30 − 35 km s−1 from the contour plots.