The Spiral Pattern Speed in the Milky Way Galaxy

Abstract

Based on a sample of masers, we have solved the basic kinematic equations with the inclusion of the Galactic rotation parameters and the peculiar solar velocity as the sought-for unknowns. Based on a spectral analysis, we have obtained the following estimates: \(|f|_{R,\theta}=(7.0,5.1)\pm(1.2,1.4)\) km s\({}^{-1}\), the corresponding wavelengths \(\lambda_{R,\theta}=(1.9,1.7)\pm(0.4,0.7)\) kpc, and \(\chi_{\odot}={-}140^{\circ}\pm 15^{\circ}\). We have confirmed the presence of periodic perturbations in the vertical maser velocities with an amplitude \(|f|_{W}=3.1\pm 1.4\) km s\({}^{-1}\) and a wavelength \(\lambda=1.9\pm 0.8\) kpc. We show that the velocity perturbations \(f_{R}\) and \(f_{\theta}\) can have both the same and opposite signs. Therefore, we have obtained a large spread of estimates. For example, if \(f_{R}\) and \(f_{\theta}\) have the same signs, then \(\Omega_{p}=25.8\pm 2.0\) km s\({}^{-1}\) kpc\({}^{-1}\) and \(R_{\textrm{cor}}=9.1\pm 0.8\) kpc, while if \(f_{R}\) and \(f_{\theta}\) have opposite signs, then \(\Omega_{p}=35.4\pm 2.0\) km s\({}^{-1}\) kpc\({}^{-1}\) and \(R_{\textrm{cor}}=6.8\pm 0.8\) kpc.