Modelling the stellar halo with RR-Lyrae stars

Abstract

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 DFs 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 $\rho\sim r^{-4.5}$ 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 $\sim20\mbox{kpc}$, possibly signalling that the halo is not axisymmetric. The DF is used to predict the velocity distribution of BHB stars for which space velocities are available. The $z$ components are predicted successfully but too much anisotropy in the $v_Rv_\phi$ plane is expected.