Modelling the stellar halo with RR-Lyrae stars

A seven-parameter distribution function (DF) is fitted to 20 000 RR-Lyrae stars for which only astrometric data are available. The observational data are predicted by the DF in conjunction with the gravitational potential of a self-consistent model Galaxy defined by DEs for the dark halo, the bulge, and a four-component disc. 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 RR-Lyrae population confirms that the population is flattened and has a strongly radially biased velocity distribution. At large radii, its density profile tends to rho similar to 1(-43) but no power law provides a good fit inside the solar sphere. 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 similar to 20 kpc, possibly signalling that the halo is not axisymmetric. The DE is used to predict the velocity distribution of BNB stars for which space velocities are available. The z components are predicted successfully but too much anisotropy in the v(R)v(phi). plane is expected.