Volume-Limited Dependent Galactic Model Parameters

Abstract

AbstractWe estimate 34 sets of Galactic model parameters for three intermediate-latitude fields with Galactic longitudes l = 60°, l = 90°, and l = 180°, and we discussed their dependence on the volume. Also, we confirm the variation of these parameters with absolute magnitude and Galactic longitude. The star samples in two fields are restricted with bright and unit absolute-magnitude intervals, 4 < Mg ≤ 5, and 5 < Mg ≤ 6, whereas for the third field (l = 60°) a larger absolute-magnitude interval is adopted, 4 < Mg ≤ 10. The limiting apparent magnitudes of the star samples are g0 = 15 and g0 = 22.5 mag, which provide space densities within distances in the line of sight of ∼0.9 and 25 kpc.The Galactic model parameters for the thin disc are not volume dependent. However, the ones for the thick disc and halo do show spectacular trends in their variation with volume, except for the scalelength of the thick disc. The local space density of the thick disc increases, whereas the scaleheight of the same Galactic component decreases monotonically. However, both model parameters approach asymptotic values at large distances.The relative local space density of the halo estimated by fitting the density laws to the space densities evaluated for all volumes is constant, except for the small ones. However it is absolute-magnitude and Galactic longitude dependent. The axial ratio of the halo increases abruptly for the volumes where a thick disc is dominant, whereas it approaches an asymptotic value gradually for larger volumes, indicating a continuous transition from a disc-like structure to a spherical one at the outermost region of the Galaxy. The variation of the Galactic model parameters with absolute magnitude can be explained by their dependence on the stellar luminosity, whereas the variation with volume and Galactic longitude at short distances is a bias in analysis.