Metallicity and kinematics of the bar in situ^⋆,⋆⋆

¹ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France
e-mail: Carine.Babusiaux@obspm.fr
² Laboratoire Lagrange, University of Nice Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, BP 4229, 06304 Nice Cedex 4, France
³ LERMA, Observatoire de Paris, CNRS, Université Pierre et Marie Curie, 61 Av. de l’Observatoire, 75014 Paris, France
⁴ Institute of Astronomy, University of Cambridge, Cambridge, CB30 HA, UK
⁵ Institut Utinam, CNRS UMR6213, OSU Theta de Franche-Comté Bourgogne, Université de Franche-Comté, BP1615, 25010 Besançon, France
⁶ CESBIO (UMR 5126), Observatoire de Midi-Pyrénées (CNES, CNRS, Université Paul Sabatier, IRD), 18 Av Ed. Belin, BP 2801, 31401 Toulouse Cedex 9, France

Received: 13 November 2013
Accepted: 28 December 2013

Abstract

Context. Constraints on the Galactic bulge and bar structures and on their formation history from stellar kinematics and metallicities mainly come from relatively high-latitude fields (|b|  > 4°) where a complex mix of stellar population is seen.

Aims. We aim here to constrain the formation history of the Galactic bar by studying the radial velocity and metallicity distributions of stars in situ (|b|  ≤ 1°).

Methods. We observed red clump stars in four fields along the bar’s major axis (l = 10°, −6°, 6° and b = 0° plus a field at l = 0°, b = 1°) with low-resolution spectroscopy from FLAMES/GIRAFFE at the VLT, observing around the Ca ii triplet. We developed robust methods for extracting radial velocity and metallicity estimates from these low signal-to-noise spectra. We derived distance probability distributions using Bayesian methods rigorously handling the extinction law.

Results. We present radial velocities and metallicity distributions, as well as radial velocity trends with distance. We observe an increase in the radial velocity dispersion near the Galactic plane. We detect the streaming motion of the stars induced by the bar in fields at l =  ±6°, the highest velocity components of this bar stream being metal-rich ([Fe/H]  ~ 0.2 dex). Our data is consistent with a bar that is inclined at 26 ± 3° from the Sun-Galactic centre line. We observe a significant fraction of metal-poor stars, in particular in the field at l = 0°, b = 1°. We confirm the flattening of the metallicity gradient along the minor axis when getting closer to the plane, with a hint that it could actually be inverted.

Conclusions. Our stellar kinematics corresponds to the expected behaviour of a bar issued from the secular evolution of the Galactic disc. The mix of several populations, seen further away from the plane, is also seen in the bar in situ since our metallicity distributions highlight a different spatial distribution between metal-poor and metal-rich stars, the more metal-poor stars being more centrally concentrated.