Spectroscopy of Bright Quest Rr Lyrae Stars: Velocity Substructures Toward Virgo

Abstract

Using a sample of 43 bright (V < 16.1, distance < 13 kpc) RR Lyrae stars (RRLS) from the QUEST survey with spectroscopic radial velocities and metallicities, we find that several separate halo substructures contribute to the Virgo overdensity (VOD). While there is little evidence for halo substructure in the spatial distribution of these stars, their distribution in radial velocity reveals two moving groups. These results are reinforced when the sample is combined with a sample of blue horizontal branch stars that were identified in the SDSS, and the combined sample provides evidence for one additional moving group. These groups correspond to peaks in the radial velocity distribution of a sample of F type main-sequence stars that was recently observed in the same directon by SEGUE, although in one case the RRLS and F star groups may not lie at the same distance. One of the new substructures has a very narrow range in metallicity, which is more consistent with it being the debris from a destroyed globular cluster than from a dwarf galaxy. A small concentration of stars have radial velocities that are similar to the Virgo Stellar Stream (VSS) that was identified previously in a fainter sample of RRLS. Our results suggest that this feature extends to distances as short as ∼ 12 kpc from its previous detection at ∼ 19 kpc. None of the new groups and only one star in the sample have velocities that are consistent with membership in the leading tidal stream from the Sagittarius Dwarf Spheroidal Galaxy, which some authors have suggested is the origin of the VOD.

The Virgo Stellar Stream: Extended sample

“…Several other groups have studied this region and have suggested that the over-density containing the VSS could extend to larger areas of the sky (outside QUEST's observing range). We present the complete spectroscopic follow up of the clump candidates present in QUEST and the composite of the studies we performed along the same l.o.s., including data at brighter magnitudes (Vivas et al 2008). Our study confirmed the nature of the VSS, revealed its likely extent within the QUEST survey and defined a number of its relevant properties.…”

“…On a parallel project by the QUEST collaboration (Vivas et al 2008), another 30 RRLy stars were observed on the same direction but at distances between 4 and 12 kpc. Both samples were combined into one large sample to study the extent of the VSS in distance.…”

A comprehensive view of the Virgo stellar stream

“…We have merged 3 different spectroscopic data sets: 18 RRLS from Duffau et al (2006), 29 RRLS from Vivas et al (2008), and 36 RRLS which are reported here for the first time. In this latest data set most stars come from the QUEST survey (Vivas et al 2004) but we also took spectra of 3 of the RRLS discovered by Ivezić et al (2000) (and followed up by Wu et al 2005), with SDSS data just beyond the northern limit of QUEST (listed in all relevant tables using the prefix W).…”

“…2. Labels correspond to the following references: D06 (internal rectangle), Duffau et al (2006), N07 N and N07 S (large circles), Newberg et al (2007); V08 (polygon), Vivas et al (2008); S09 (small triangles), Starkenburg et al (2009); P09 (small circles), Prior et al (2009); B10 (tiny squares), Brink et al (2010); C12 (medium size circles), Casey et al (2012). The star symbol, arrow and dot-dashed line correspond to the location of an RRLS whose proper motion was determined by Casetti-Dinescu et al (2009, C09), the direction of the proper motion and part of the past orbit, respectively.…”

“…Jurić et al (2008) suggested that the Virgo overdensity (VOD) was produced by the merger of a low A&A 566, A118 (2014) surface brightness galaxy with the Milky Way (see also Carlin et al 2012). Spectroscopic observations have revealed, however, a complex system of substructures in the kinematic distribution of stars (Duffau et al 2006;Newberg et al 2007;Vivas et al 2008;Starkenburg et al 2009;Prior et al 2009;Brink et al 2010;Casey et al 2012). Furthermore, it appears that the strengths of these features depend on the tracer used.…”