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3D kinematics through the X-shaped Milky Way bulge

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Autor Vasquez S.
Autor Zoccali M.
Autor Hill V.
Autor Renzini A.
Autor Gonzalez O.A.
Autor Gardner E.
Autor Debattista V.P.
Autor Robin A.C.
Autor Rejkuba M.
Autor Baffico M.
Autor Monelli M.
Autor Motta V.
Autor Minniti D.
Fecha Ingreso 2014-04-05T00:22:12Z
Fecha Disponible 2014-04-05T00:22:12Z
Fecha en Repositorio 2014-04-04
dc.identifier 10.1051/0004-6361/201220222
dc.description.abstract Context. It has recently been discovered that the Galactic bulge is X-shaped, with the two southern arms of the X both crossing the lines of sight at l = 0 and | b| > 4, hence producing a double red clump in the bulge color magnitude diagram. Dynamical models predict the formation of X-shaped bulges as extreme cases of boxy-peanut bulges. However, since X-shaped bulges were known to be present only in external galaxies, models have never been compared to 3D kinematical data for individual stars. Aims. We study the orbital motion of Galactic bulge stars in the two arms (overdensities) of the X in the southern hemisphere. The goal is to provide observational constraints to bulge formation models that predict the formation of X-shapes through bar dynamical instabilities. Methods. Radial velocities have been obtained for a sample of 454 bulge giants, roughly equally distributed between the bright and the faint red clump, in a field at (l,b) = (0, -6). Proper motions were derived for all red clump stars in the same field by combining images from two epochs, which were obtained 11 years apart, with WFI at the 2.2 m at La Silla. The observed field contains the globular cluster NGC 6558, whose member stars were used to assess the accuracy of the proper motion measurement. At the same time, as a by-product, we provide the first proper motion measurement of NGC 6558. The proper motions for the spectroscopic subsample are analyzed for a subsample of 352 stars, taking into account the radial velocities and metallicities measured from near-infrared calcium triplet lines. Results. The radial velocity distribution of stars in the bright red clump, which traces the closer overdensity of bulge stars, shows an excess of stars moving towards the Sun. Similarly, an excess of stars receding from the Sun is seen in the far overdensity, which is traced by faint red clump stars. This is explained by the presence of stars on elongated orbits, which are most likely streaming along the arms of the X-shaped bulge. Proper motions for these stars are consistent with qualitative predictions of dynamical models of peanut-shaped bulges. Surprisingly, stars on elongated orbits have preferentially metal-poor (subsolar) metallicities, while the metal rich ones, in both overdensities, are preferentially found in more axisymmetric orbits. The observed proper motion of NGC 6558 has been measured as (μlcos (b),μb) = (0.30 ± 0.14, -0.43 ± 0.13), with a velocity dispersion of (σlcos(b),σb) = (1.8,1.7) mas/yr. This is the first proper motion measurement for this cluster. © ESO, 2013. en_US
dc.source Astronomy and Astrophysics
Link Descarga dc.source.uri
Title dc.title 3D kinematics through the X-shaped Milky Way bulge en_US
Tipo dc.type Article
dc.description.keywords Color magnitude diagrams; Dynamical instabilities; Galaxies: Kinematics and dynamics; Galaxy: bulge; Galaxy: kinematics and dynamics; Galaxy: structure; Qualitative predictions; Southern Hemisphere; Astrophysics; Galaxies; Kinematics; Measurements; Oilseeds; Velocity distribution; Stars en_US

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