Galactic structure along the main meridional section of the Galaxy. I. The North Galactic Pole (N321) field.

Details Galactic structure along the main meridional section of the Galaxy. I. The North Galactic Pole (N321) field. Galactic structure along the main meridional section of the Galaxy. I. The North Galactic Pole (N321) field.

Jul 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26a...311..758s&link_type=abstract

Astronomy and Astrophysics, v.311, p.758-777

Astronomy and Astrophysics

Astrophysics

38

Astrometry, Photometry, Galaxy: Structure, Galaxy: Kinematics And Dynamics, Galaxy: Stellar Content

Scientific paper

This work presents the results of a new magnitude limited survey of colors (BVR_c_) and proper motions in a new field at the North Galactic Pole (NGP), with extent 4degx5deg and centered at (l=~162deg, b=~+87deg). The plate material, digitized at the Space Telescope Science Institute, is from sky surveys taken with the Oschin Schmidt telescope on Palomar Mountain: POSS I, Quick-V and, for the first time, POSS II. We provide V, (B-V), (V-R_c_) photographic photometry for a stellar sample down to V=18.5 with an accuracy of about 0.1 mag. Proper motions are on an absolute reference frame based on external galaxies, and their mean (total) internal error is about 3mas/yr. The procedures for the astrometric reduction and for the photometric calibration are described in detail, with emphasis on the techniques for the detection and the removal of systematic errors. External checks have been performed with independent astrometric and photometric catalogs. A comparison with the Lick NPM1 catalog shows an offset of about 4mas/yr in μalpha_, which does not seem to be due to the different procedures adopted in establishing the absolute frame. Photometric parallaxes and galactic U,V velocities have been derived adopting a statistical model for the chemical effect based on a vertical metallicity gradient of d[Fe/H]/dz=-0.3kpc^-1^. The subsample of stars with 4.5<=M_V_<=5.0 has been analyzed in order to investigate the vertical density distribution up to z=~5000pc; a least squares decomposition of the vertical volume density with exponential density laws has converged to a thin disk with scale height of z_0_=~259+/-12pc and a thick disk with z_0_=~1137+/-61pc and a density ρ_0,thick_ 23 times smaller than ρ_0,thin_. Mean V velocities (along with their dispersion) have been tabulated as a function of distance, and the asymmetric drift is seen with outstanding clarity. A direct deconvolution of the histogram, assuming Gaussian-like distributions for each component, requires the presence of a halo with a rotation lag of -225km/s with respect to the Sun, and an intrinsic velocity dispersion of 140km/s. Finally, a set of 28 stars with similar kinematics at an average distance of 4400pc has been isolated, which, if confirmed, would support the existence of kinematically homogeneous non-thermalized groups within the population of the extended galactic disk.

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