A. Spagna et al., GALACTIC STRUCTURE ALONG THE MAIN MERIDIONAL SECTION OF THE GALAXY .1. THE NORTH GALACTIC POLE (N321) FIELD, Astronomy and astrophysics, 311(3), 1996, pp. 758-777
This work presents the results of a new magnitude limited survey of co
lors (BVR(c)) and proper motions in a new field at the North Galactic
Pole (NGP), with extent 4 degrees x 5 degrees and centered at (l simil
ar or equal to 162 degrees, b similar or equal to 87 degrees). The pla
te material, digitized at the Space Telescope Science Institute, is fr
om sky surveys taken with the Oschin Schmidt telescope on Palomar Moun
tain: 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 a
n absolute reference frame based on external galaxies, and their mean
(total) internal error is about 3 mas/yr. The procedures for the astro
metric reduction and for the photometric calibration are described in
detail, with emphasis on the techniques for the detection and the remo
val of systematic errors. External checks have been performed with ind
ependent astrometric and photometric catalogs. A comparison with the L
ick NPM1 catalog shows an offset of about 4 mas/yr in mu(alpha), which
does not seem to be due to the different procedures adopted in establ
ishing the absolute frame. Photometric parallaxes and galactic U,V vel
ocities have been derived adopting a statistical model for the chemica
l effect based on a vertical metallicity gradient of partial derivativ
e[Fe/H]/partial derivative z = -0.3 kpc(-1). The subsample of stars wi
th 4.5 less than or equal to M(V) less than or equal to 5.0 has been a
nalyzed in order to investigate the vertical density distribution up t
o z approximate to 5000 pc; a least squares decomposition of the verti
cal volume density with exponential density laws has converged to a th
in disk with scale height of z(0) similar or equal to 259 +/- 12 pc an
d a thick disk with z(0) similar or equal to 1137 +/- 61 pc and a dens
ity rho(0,thick) 23 times smaller than rho(0,thin). Mean V velocities
(along with their dispersion have been tabulated as a function of dist
ance, and the asymmetric drift is seen with outstanding clarity. A dir
ect deconvolution of the histogram, assuming Gaussian-like distributio
ns for each component, requires the presence of a halo with a rotation
lag of -225 km/s with respect to the Sun, and an intrinsic velocity d
ispersion of 140 km/s.Finally, a set of 28 stars with similar kinemati
cs at an average distance of 4400 pc has been isolated, which, if conf
irmed, would support the existence of kinematically homogeneous non-th
ermalized groups within the population of the extended galactic disk.