Volume 569, September 2014
|Number of page(s)||16|
|Section||Galactic structure, stellar clusters and populations|
|Published online||30 September 2014|
The Gaia-ESO Survey: metallicity and kinematic trends in the Milky Way bulge⋆
1 Laboratoire Lagrange (UMR 7293), Université Nice Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, CS 34229, 06304 Nice, cedex 04, France
2 GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
3 Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Santiago, Chile
4 The Milky Way Millennium Nucleus, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
5 Vatican Observatory, V 00120 Vatican City State, Italy
6 Departamento de Ciencias Fisicas, Universidad Andres Bello, Republica 220, Santiago, Chile
7 European Southern Observatory, A. de Cordova 3107, Casilla 19001, Santiago 19, Chile
8 Lund Observatory, Department of Astronomy and Theoretical Physics, Box 43, 221 00 Lund, Sweden
9 Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
10 INAF − Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125, Florence, Italy
11 INAF − Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
12 Instituto de Astrofísica de Andalucía-CSIC, Apdo. 3004, 18080 Granada, Spain
13 INAF − Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
14 INAF − Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
15 Dipartimento di Fisica e Astronomia, Sezione Astrofisica, Universitá di Catania, via S. Sofia 78, 95123 Catania, Italy
16 Department for Astrophysics, Nicolaus Copernicus Astronomical Center, ul. Rabiańska 8, 87-100 Toruń, Poland
17 European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
18 ZAH − Landessternwarte Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
19 Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
Received: 2 May 2014
Accepted: 11 August 2014
Aims. Observational studies of the Milky Way bulge are providing increasing evidence of its complex chemo-dynamical patterns and morphology. Our intent is to use the iDR1 Gaia-ESO Survey (GES) data set to provide new constraints on the metallicity and kinematic trends of the Galactic bulge, exploring the viability of the currently proposed formation scenarios.
Methods. We analyzed the stellar parameters and radial velocities of ~1200 stars in five bulge fields wich are located in the region −10°<l< 7° and −10°<b< −4°. We use VISTA Variables in the Via Lactea (VVV) photometry to verify the internal consistency of the atmospheric parameters recommended by the consortium. As a by-product, we obtained reddening values using a semi-empirical Teff-color calibration. We constructed the metallicity distribution functions and combined them with photometric and radial velocity data to analyze the properties of the stellar populations in the observed fields.
Results. From a Gaussian decomposition of the metallicity distribution functions, we unveil a clear bimodality in all fields, with the relative size of components depending of the specific position on the sky. In agreement with some previous studies, we find a mild gradient along the minor axis (−0.05 dex/deg between b = −6° and b = −10°) that arises from the varying proportion of metal-rich and metal-poor components. The number of metal-rich stars fades in favor of the metal-poor stars with increasing b. The K-magnitude distribution of the metal-rich population splits into two peaks for two of the analyzed fields that intersects the near and far branches of the X-shaped bulge structure. In addition, two lateral fields at (l,b) = (7, −9) and (l,b) = (−10, −8) present contrasting characteristics. In the former, the metallicity distribution is dominated by metal-rich stars, while in the latter it presents a mix of a metal-poor population and and a metal-intermediate one, of nearly equal sizes. Finally, we find systematic differences in the velocity dispersion between the metal-rich and the metal-poor components of each field.
Conclusions. The iDR1 bulge data show chemo-dynamical distributions that are consistent with varying proportions of stars belonging to (i) a metal-rich boxy/peanut X-shaped component, with bar-like kinematics; and (ii) a metal-poor more extended rotating structure with a higher velocity dispersion that dominates far from the Galactic plane. These first GES data already allow studying the detailed spatial dependence of the Galactic bulge populations, thanks to the analysis of individual fields with relatively high statistics.
Key words: Galaxy: formation / Galaxy: abundances / Galaxy: bulge / Galaxy: stellar content / stars: abundances
© ESO, 2014
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