Structure and kinematical properties of the galaxy at intermediate galactic latitudes

Abstract

We have carried out a sample survey in UBVR photometry and proper motions in different directions in the Galaxy, as part of an investigation of galactic structure and evolution. Three fields in the direction of galactic anticentre (l = 167°, b = 47°), galactic centre (l = 3°, b = 47°) and galactic anti-rotation (l = 278°, b = 47°) have been surveyed. Using our photographic photometry, we determine photometric distances for a sub-sample of stars in the color range 0.3≤(B-V)≤0.9. The stellar space velocities (U, V and W) are derived directly from the measured proper motions and distances. Using our new data together with wide-area surveys in other fields available to date, we discuss the radial and vertical structure of the Galaxy. We have derived the density laws for stars as a function of distance from the galactic plane for each absolute magnitude interval. The density laws for stars with M_V≥3.5 follow a sum of two exponentials with scale heights of 260±50pc (thin disk) and 760±50pc, respectively. This second exponential corresponds to a thick disk with a local density of 7.4^+2.5_-1.5% relative to the thin disk. The scale lengths for these two populations are respectively 2.3±0.6kpc and 3±1kpc. The kinematical distribution of F and G-type stars have been probed to distances up to 3.5kpc above the galactic plane. A new value for the solar motion has been determined from moderately distant stars (1<z<2kpc). It is consistent with local determinations and implies that there is no large motion of the LSR relative to the mean motion of stars at 1-2kpc above the galactic plane. The rotational velocity curve is found flat in the solar neighborhood. The radial gradient in velocity dispersions has been determined for the thin disk population. The thick disk appears as a kinematically distinct population from the thin disk and shows no vertical gradient. A multivariate discriminant analysis is also used to distinguish the thick disk from the thin disk and to estimate its asymmetric drift. It is found to be 53±10km/s, independent of the galactic radius. Of the many models that have been proposed for the origin of the thick disk, the evidence at present seems to favour a model in which thick disk formed through the rapid dynamical heating of an early disk by satellite accretion into the disk.