Estimation of Galactic model parameters in high latitudes with 2MASS

Abstract

Context. In general, studies focused on the Milky Way's structure show a range of values when deriving different Galactic parameters, such as radial scalelengths, vertical scaleheights, or local space densities. Those values are also dependent on the Galactic coordinates under consideration for the corresponding analysis, as a direct consequence of observing a structure (our Galaxy) that is far from being as smooth and well-behaved as models usually treat. Aims. In this paper, we try to find any dependence of the Galactic structural parameters on the Galactic longitude for either the thin disc or the thick disc of the Milky Way that would indicate possible inhomogeneities or asymmetries in those Galactic components. Methods. Galactic model parameters have been estimated for a set of 36 high-latitude fields with Two Micron All Sky Survey (2MASS) photometry. Possible variations with the Galactic longitude in either the scaleheight and the local space density of these components are explored. Results. Galactic model parameters for the different fields show that they are Galactic longitude-dependent. The thick disc scaleheight changes from ∼800 pc at 150° < / < 230° to ∼1050 pc at |l| < 30°. A plausible explanation for this finding might be the effect of the flare on this Galactic component, which changes the scaleheight (hz) with Galactocentric distance (R) following the approximate law: hz(R) = (940 ± 20) × [1 - (0.12 ± 0.02)(R - R⊙)]. The effect of the flare is more prominent in some lines of sight than in others, producing the observed changes in the parameters with the Galactic coordinates used to derive them. © ESO 2007.