The pattern speed of the OH/IR stars in the Milky Way |
| |
| Authors: | Victor P Debattista Ortwin Gerhard Maartje N Sevenster |
| |
| Institution: | Astronomisches Institut, Universität Basel, Venusstrasse 7, CH-4102 Binningen, Switzerland; RSAA/MAAAO, RSAA/MSSSO, Cotter Road, Weston ACT 2611, Australia |
| |
| Abstract: | We show how the continuity equation can be used to determine pattern speeds in the Milky Way Galaxy (MWG). This method, first discussed by Tremaine & Weinberg in the context of external galaxies, requires projected positions, ( l , b ), and line-of-sight velocities for a spatially complete sample of relaxed tracers. If the local standard of rest (LSR) has a zero velocity in the radial direction ( u LSR), then the quantity that is measured is Δ V ≡Ωp R 0- V LSR , where Ωp is the pattern speed of the non-axisymmetric feature, R 0 is the distance of the Sun from the Galactic centre and V LSR is the tangential motion of the LSR, including the circular velocity. We use simple models to assess the reliability of the method for measuring a single, constant pattern speed of either a bar or spiral in the inner MWG. We then apply the method to the OH/IR stars in the ATCA/VLA OH 1612-MHz survey of Sevenster et al., finding Δ V =252±41 km s-1, if u LSR=0 . Assuming further that R 0=8 kpc and V LSR=220 km s-1, this gives Ωp=59±5 km s-1 kpc-1 with a possible systematic error of perhaps 10 km s?1 kpc?1. The non-axisymmetric feature for which we measure this pattern speed must be in the disc of the MWG. |
| |
| Keywords: | Galaxy: bulge Galaxy: disc Galaxy: kinematics and dynamics Galaxy: structure galaxies: spiral |
|
|
