Spectroscopy of QUEST RR Lyrae Variables: The New Virgo Stellar Stream

Abstract

Eighteen RR Lyrae variables (RRLs) that lie in the ``12.4h clump'' identified by the Quasar Equatorial Survey Team (QUEST) have been observed spectroscopically to measure their radial velocities and metal abundances. Ten blue horizontal branch (BHB) stars identified by the Sloan Digital Sky Survey (SDSS) were added to this sample. Six of the nine stars in the densest region of the clump have a mean radial velocity in the Galactic rest frame (Vgsr) of 99.8 and σ=17.3 km s-1, which is slightly smaller than the average error of the measurements. The whole sample contains eight RRLs and five BHB stars that have values of Vgsr suggesting membership in this stream. For seven of these RRLs, the measurements of [Fe/H], which have an internal precision of 0.08 dex, yield <[Fe/H]>=-1.86 and σ=0.40. These values suggest that the stream is a tidally disrupted dwarf spheroidal galaxy of low luminosity. Photometry from the database of the SDSS indicates that this stream covers at least 106 deg2 of the sky in the constellation Virgo. The name Virgo stellar stream is suggested

The Virgo Stellar Overdensity: Mapping the Infall of the Sagittarius Tidal Stream onto the Milky Way Disk

“…An alternative explanation for the presence of the unexpected main-sequence (MS) feature in the CMD reported in several studies (the so-called VOD; Newberg et al 2002;Duffau et al 2006;Jurić et al 2005; see x 1) is to postulate the existence of a nonaxisymmetric Galactic model component, such as a triaxial halo (Newberg & Yanny 2005;Xu et al 2006). We have explored this possibility by inspecting a sample of deep CMDs (with limiting magnitude R lim $ 24:5) of Galactic fields taken with the Wide Field Camera (WFC) at the prime focus of the Isaac Newton 2.5 m telescope (La Palma, Spain) during different observing runs devoted to the search of tidal debris associated with known Galactic satellites or globular clusters (Martínez-Delgado et al 2004a).…”

“…The region in which Jurić et al (2005) detect the Virgo overdensity is marked by the white wire-frame box. The blue wire-frame box marks the location of the Virgo stellar stream as reported by Duffau et al (2006). [The left and right panels of this figure are available as mpeg animations showing the oblate and prolate halos, respectively, in the electronic edition of the Journal.…”

“…The coordinate system origin is located at the Galactic center, with the red axis (X ) pointing toward the Sun ( yellow circle) and the blue axis (Z ) pointing toward the north Galactic pole. The region in which Jurić et al (2005) detect the VOD is marked by the white wire-frame box, whereas the location of the VSS as reported by Duffau et al (2006;see x 1) is shown by the blue wire-frame box. This coincidence points to a plausible identification of the VOD with the leading arm of the Sagittarius tidal stream or, at the very least, to likely significant pollution of the VOD as seen in the SDSS density maps by the Sagittarius tidal debris.…”

“…Martínez-Delgado et al (2004b) suggested that the VSS might be related to the complex debris structure of the Sagittarius stream predicted by theoretical models for this region of the sky. However, spectroscopic follow-up of a sample of these RR Lyrae stars by Duffau et al (2006) showed a fairly narrow velocity distribution that pointed to a progenitor galaxy less massive than the Sagittarius dwarf and, therefore, to a hitherto unknown tidally disrupted dwarf galaxy. Using SDSS photometric data, these authors estimated that the VSS covers roughly an area of 106 deg 2 in the sky.…”

The Future of Stellar Populations Studies in the Milky Way and the Local Group

“…Not only will these surveys allow deep searches for streams and satellites through color-magnitude filtering for specific stellar tracers of these structures, but their inclusion of a time series dimension will enable the search for pulsational variables, which are high quality standard candles that have already proven to be quite powerful in the search for halo substructure -e.g., with u, v , g , r, i, z 22.9, 22.7, 22.9, 22.6, 22.0, 21.5 RR Lyrae stars from SDSS (e.g., Ivezić et al 2004, Watkins et al 2009 or the QUEST project (e.g., Duffau et al 2006. The discovery and exploration of increasing numbers of tidal streams not only tells us about the assemblage of the luminous MW, but also the structure and size of the MW's dark halo because the shapes and orbits of tidal streams are extremely sensitive probes of the overall mass distribution of the host galaxy.…”