Bibcode
Vásquez, S.; Zoccali, M.; Hill, V.; Renzini, A.; González, O. A.; Gardner, E.; Debattista, V. P.; Robin, A. C.; Rejkuba, M.; Baffico, M.; Monelli, M.; Motta, V.; Minniti, D.
Referencia bibliográfica
Astronomy and Astrophysics, Volume 555, id.A91, 11 pp.
Fecha de publicación:
7
2013
Revista
Número de citas
55
Número de citas referidas
49
Descripción
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.
Based on observations taken with ESO telescopes at the La Silla Paranal
Observatory under program IDs 163.O-0741(A), 085.D-0143(A) and
385.B-0735(B) and on observations taken with the Magellan telescope at
the Las Campanas Observatory.Kinematics for IMACS and FLAMES spectra are
only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr
(ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/555/A91
Proyectos relacionados
Evolución Galáctica en el Grupo Local
La formación y evolución de galaxias es un problema fundamental en Astrofísica. Su estudio requiere “viajar atrás en el tiempo”, para lo cual hay dos enfoques complementarios. El mas extendido consiste en analizar las propiedades de las galaxias a diferentes distancias cosmológicas. Nuestro equipo se concentra en el otro enfoque, denominado
Matteo
Monelli