Constraints on the Galactic bar from the Hercules stream as traced with RAVE across the Galaxy^⋆

¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
e-mail: tantoja@rssd.esa.int
² University of Leicester, University Road, Leicester LE1 7RH, UK
³ Université de Strasbourg, CNRS, Observatoire, 11 rue de l’Université, 67000 Strasbourg, France
⁴ Sydney Institute for Astronomy, School of Physics, University of Sydney, NSW 2006 Sydney, Australia
⁵ RSAA Australian National University, Mount Stromlo Observatory, Cotter Road, Weston Creek, ACT 2611 Canberra, Australia
⁶ Jeremiah Horrocks Institute, University of Central Lancashire, Preston, PR1 2HE, UK
⁷ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
⁸ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr. 12–14, 69120 Heidelberg, Germany
⁹ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
¹⁰ INAF Osservatorio Astronomico di Padova, 36012 Asiago (VI), Italy
¹¹ Senior CIfAR Fellow. University of Victoria, Victoria BC Canada V8P 5C2, Canada
¹² Department of Physics and Astronomy, Macquarie University, NSW 2109 Sydney, Australia
¹³ Research Centre for Astronomy, Astrophysics and Astrophotonics, Macquarie University, NSW 2109 Sydney, Australia
¹⁴ Australian Astronomical Observatory, PO Box 915, North Ryde, NSW 1670, Australia
¹⁵ Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, RH5 6NT, UK
¹⁶ Johns Hopkins University, Homewood Campus, 3400 N Charles Street, Baltimore MD 21218, USA
¹⁷ University of Ljubljana, Faculty of Mathematics and Physics, Jadranska 19, 1000 Ljubljana, Slovenia
¹⁸ Center of excellence Space-SI, Askerceva 19, 1000 Ljubljana, Slovenia

Received: 6 September 2013
Accepted: 16 January 2014

Abstract

Non-axisymmetries in the Galactic potential (spiral arms and bar) induce kinematic groups such as the Hercules stream. Assuming that Hercules is caused by the effects of the outer Lindblad resonance of the Galactic bar, we model analytically its properties as a function of position in the Galaxy and its dependence on the bar’s pattern speed and orientation. Using data from the RAVE survey we find that the azimuthal velocity of the Hercules structure decreases as a function of Galactocentric radius, in a manner consistent with our analytical model. This allows us to obtain new estimates of the parameters of the Milky Way’s bar. The combined likelihood function of the bar’s pattern speed and angle has its maximum for a pattern speed of Ω_b = (1.89 ± 0.08) × Ω₀, where Ω₀ is the local circular frequency. Assuming a solar radius of 8.05 kpc and a local circular velocity of 238 km s^-1, this corresponds to Ω_b = 56 ± 2 km s^-1 kpc^-1. On the other hand, the bar’s orientation φ_b cannot be constrained with the available data. In fact, the likelihood function shows that a tight correlation exists between the pattern speed and the orientation, implying that a better description of our best fit results is given by the linear relation Ω_b/Ω₀ = 1.91 + 0.0044(φ_b(deg) − 48), with standard deviation of 0.02. For example, for an angle of φ_b = 30 deg the pattern speed is 54.0 ± 0.5 km s^-1 kpc^-1. These results are not very sensitive to the other Galactic parameters such as the circular velocity curve or the peculiar motion of the Sun, and are robust to biases in distance.