Characterizing the stellar halo with distribution function based models
Abstract
A new class of models is used to build a self-consistent model of our Galaxy. The model is defined by the parameters that specify the distribution functions (DFs) f(J) of four stellar discs (three thin-disc age cohorts and a thick disc), a rotating, axisymmetric bulge and oblate stellar and dark haloes. Then a seven-parameter distribution function (DF) is fitted to ~20 000 RR-Lyrae stars for which only astrometric data are available. Tests of the technique developed to deal with missing line-of-sight velocities show that adding such velocities tightens constraints on the DF only slightly. The recovered model of the RRLyrae population confirms that the population is flattened and has a strongly radially biased velocity distribution. The model is shown to provide an excellent fit to the data for stars brighter than r = 16.5 but at certain longitudes it predicts too few faint stars at Galactocentric radii ~ 20 kpc, possibly signalling that the halo is not axisymmetric.