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

Antoja, T.; Helmi, A.; Dehnen, W.; Bienaymé, O.; Bland-Hawthorn, J.; Famaey, B.; Freeman, K.; Gibson, B. K.; Gilmore, G.; Grebel, E. K.; Kordopatis, G.; Kunder, A.; Minchev, I.; Munari, U.; Navarro, J.; Parker, Q.; Reid, W. A.; Seabroke, G.; Siebert, A.; Steinmetz, M.; Watson, F.; Wyse, R. F. G.; Zwitter, T.

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 Ωb = (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 Ωb = 56 ± 2 km s-1 kpc-1. On the other hand, the bar’s orientation φb 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 Ωb/Ω0 = 1.91 + 0.0044(φb(deg) − 48), with standard deviation of 0.02. For example, for an angle of φb = 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.

Citation

Antoja, T., Helmi, A., Dehnen, W., Bienaymé, O., Bland-Hawthorn, J., Famaey, B., Freeman, K., Gibson, B. K., Gilmore, G., Grebel, E. K., Kordopatis, G., Kunder, A., Minchev, I., Munari, U., Navarro, J., Parker, Q., Reid, W. A., Seabroke, G., Siebert, A., Steinmetz, M., …Zwitter, T. (2014). Constraints on the Galactic bar from the Hercules stream as traced with RAVE across the Galaxy. Astronomy and Astrophysics, 563, A60-1-A60-17. https://doi.org/10.1051/0004-6361/201322623

• Journal Article Type: Article
• Acceptance Date: Jan 16, 2014
• Online Publication Date: Mar 7, 2014
• Publication Date: Mar 1, 2014
• Deposit Date: Apr 14, 2016
• Publicly Available Date: Apr 14, 2016
• Journal: Astronomy and astrophysics
• Print ISSN: 0004-6361
• Publisher: EDP Sciences
• Peer Reviewed: Peer Reviewed
• Volume: 563
• Article Number: ARTN A60
• Pages: A60-1-A60-17
• DOI: https://doi.org/10.1051/0004-6361/201322623
• Keywords: Galaxy : kinematics and dynamics; Galaxy : structure; Galaxy : disc; Galaxy : evolution
• Public URL: https://hull-repository.worktribe.com/output/436062
• Publisher URL: http://www.aanda.org/articles/aa/abs/2014/03/aa22623-13/aa22623-13.html
• Additional Information: This is an authors accepted version of an article published in Astronomy and astrophysics, 2014, v.563. With kind permission of The European Physical Journal (EPJ).
• Contract Date: Apr 14, 2016