The QUEST RR Lyrae Survey. II. The Halo Overdensities in the First Catalog

The first catalog of the RR Lyrae stars (RRLS) in the Galactic halo by theQUEST Survey has been searched for significant overdensities that may be debrisfrom disrupted dwarf galaxies or globular clusters. Away from the majoroverdensities, the distribution of these stars is adequately fit by a smoothhalo model, in which the flattening of the halo decreases with increasinggalactocentric distance (Preston et al 1991). This model was used to estimatethe ``background'' of RRLS on which the halo overdensities are overlaid. Aprocedure was developed for recognizing groups of stars that constitutesignificant overdensities with respect to this background. To test thisprocedure, a Monte Carlo routine was used to make artificial RRLS surveys thatfollow the smooth halo model, but with Poisson distributed noise in the numbersof RRLS and, within limits, random variations in the positions and magnitudesof the artificial stars. The artificial surveys created by this routine wereexamined for significant groups in exactly the same way as the QUEST survey.These calculations provided estimates of the frequencies with which randomfluctuations produce significant groups. In the QUEST survey, there are sixsignificant overdensities that contain six or more stars and several smallerones. The small ones and possibly one or two of the larger ones may beartifacts of statistical fluctuations, and they need to be confirmed bymeasurements of radial velocity and/or proper motion. The most prominent groupsare the northern stream from the Sagittarius dwarf spheroidal galaxy and alarge group in Virgo. Two other groups lie in the direction of the Monocerosstream and at approximately the right distance for membership. Another group isrelated to the globular cluster Palomar 5.Comment: Accepted for publication in the Astronomical Journa