QUEST RR Lyrae survey. II. Halo overdensities

These RRLSs are contained in a band 23 wide in declination that spans 165{deg} in right ascension and lie 4 to 60kpc from the Sun. Away from the major overdensities, the distribution of these stars is adequately fitted by a smooth halo model, in which the flattening of the halo decreases with increasing galactocentric distance (as reported by Preston et al., 1991ApJ...375..121P).

This model was used to estimate the background of RRLSs on which the halo overdensities are overlaid. A procedure was developed for recognizing groups of stars that constitute significant overdensities with respect to this background. To test this procedure, a Monte Carlo routine was used to make artificial RRLS surveys that follow the smooth halo model but with Poissondistributed noise in the numbers of RRLSs and, within limits, random variations in the positions and magnitudes of the artificial stars.