DISENTANGLING THE VIRGO OVERDENSITY WITH RR LYRAE STARS

Abstract

ABSTRACT We use a combination of spatial distribution and radial velocity to search for halo substructures in a sample of 412 RR Lyrae stars (RRLSs) that covers a region of ∼525 square degrees of the Virgo overdensity (VOD) and spans distances from the Sun from 4 to 75 kpc. With a friends-of-friends algorithm we identified six high-significance groups of RRLSs in phase space, which we associate mainly with the VOD and with the Sagittarius stream. Four other groups were also flagged as less significant overdensities. Three high-significance and three lower-significance groups have distances between ∼10 and 20 kpc, which places them in the distance range attributed by others to the VOD. The largest of these is the Virgo stellar stream at 19 kpc, which has 18 RRLSs, a factor of two increase over the number known previously. While these VOD groups are distinct according to our selection criteria, their overlap in position and distance and, in a few cases, similarity in radial velocity are suggestive that they may not all stem from separate accretion events. Even so, the VOD appears to be caused by more than one overdensity. The Sagittarius (Sgr) stream is a very obvious feature in the background of the VOD at a mean distance of 44 kpc. Two additional high-significance groups were detected at distances . Their radial velocities and locations differ from the expected path of the Sgr debris in this part of the sky, and they are likely to be remnants of other accretion events.