Investigation of the ELAIS field by Vega photometry: absolute magnitude-dependent Galactic model parameters

Abstract

We estimate the density laws of the Galactic stellar populations as a function of absolute magnitude in a near-polar Galactic field. The density laws are determined by direct fitting to photometric parallaxes from Vega photometry in the ELAIS field (alpha = 16(h)10(m)00(s), delta = +54 degrees 30'00''; l = 84 degrees.27, b = +44 degrees.90; 6.571 deg(2); epoch 2000), both independently for each population and simultaneously for all stellar populations. Stars have been separated into different populations based on their spatial location. The thick disc and halo are fitted best by an exponential. However, the thin disc is fitted best by using a sech(2) law for stars at faint absolute magnitudes [10 < M(g') <= 11, 11 < M(g') <= 12 and 12 < M(g') <= 13], and an exponential law for stars at relatively bright absolute magnitudes [5 < M(g') <= 6, 6 < M(g') <= 7, 7 < M(g') <= 8, 8 < M(g') <= 9 and 9 < M(g') <= 10] The scaleheights for the sech(2) density laws are the equivalent exponential scaleheights. Galactic model parameters are dependent on absolute magnitude. The scaleheight for the thin disc decreases monotonically from stars at bright absolute magnitudes [M(g') = 5] to stars at faint absolute magnitudes [M(g') = 13] in the range 363-163 pc, except for the minimum H= 211 pc at < M(g') <= 10 where the sech density law fits better. Its local density is flat at bright absolute magnitudes but increases at faint absolute magnitudes. For the thick disc, the scaleheight is flat within the uncertainties. The local space density of the thick disc relative to the local space density for the thin disc is almost flat in the absolute magnitude intervals 5 < M(g') <= 6 and 6 < M(g') <= 7, 7.59 and 7.41 per cent respectively, whereas it decreases down to 3.31 per cent in the absolute magnitude interval 7 < M(g') <= 8. The axial ratio for the halo is kappa= 0.60, 0.73 and 0.78 for the absolute magnitude intervals 4 < M(g') <= 5, 5 < M(g') <= 6 and 6 < M(g') <= 7 respectively, and its local space density relative to the local space density for the thin disc is 0.06 and 0.04 per cent for the intervals 5 < M(g') <= 6 and 6 < M(g') <= 7, respectively [the local space density relative to the thin disc could not be derived for the absolute magnitude interval 4 < M(g') <= 5 owing to a lack of the local space density for the thin disc for this interval]. The simultaneous fit of all three stellar populations agrees within uncertainties with the most recent values in the literature. Also, each parameter is close to one of the corresponding parameters estimated for different absolute magnitude intervals in this work, but with one exception: i.e. the scaleheight for the thick disc is relatively small and its error is rather large (H= 760(-55)(+62)pc).