The spiral potential of the Milky Way

Context. The location of young sources in the Galaxy suggests a four-armed spiral structure, whereas tangential points of spiral arms observed in the integrated light at infrared and radio wavelengths indicate that only two arms are massive. Aims. Variable extinction in the Galactic plane and high light-to-mass ratios of young sources make it difficult to judge the total mass associated with the arms outlined by such tracers. The current objective is to estimate the mass associated with the Sagittarius arm by means of the kinematics of the stars across it. Methods. Spectra of 1726 candidate B-and A-type stars within 3 degrees of the Galactic center (GC) were obtained with the FLAMES instrument at the VLT with a resolution of approximate to 6000 in the spectral range of 396-457 nm. Radial velocities were derived by least-squares fits of the spectra to synthetic ones. The final sample was limited to 1507 stars with either Gaia DR2 parallaxes or main-sequence B-type stars having reliable spectroscopic distances. Results. The solar peculiar motion in the direction of the GC relative to the local standard of rest (LSR) was estimated to U-circle dot = 10.7 +/- 1.3 km s(-1). The variation in the median radial velocity relative to the LSR as a function of distance from the sun shows a gradual increase from slightly negative values near the sun to almost 5 km s(-1) at a distance of around 4 kpc. A sinusoidal function with an amplitude of 3.4 +/- 1.3 km s(-1) and a maximum at 4.0 +/- 0.6 kpc inside the sun is the best fit to the data. A positive median radial velocity relative to the LSR around 1.8 kpc, the expected distance to the Sagittarius arm, can be excluded at a 99% level of confidence. A marginal peak detected at this distance may be associated with stellar streams in the star-forming regions, but it is too narrow to be associated with a major arm feature. Conclusions. A comparison with test-particle simulations in a fixed galactic potential with an imposed spiral pattern shows the best agreement with a two-armed spiral potential having the Scutum-Crux arm as the next major inner arm. A relative radial forcing dF(r) approximate to 1.5% and a pattern speed in the range of 20-30 km s(-1) kpc(-1) yield the best fit. The lack of a positive velocity perturbation in the region around the Sagittarius arm excludes it from being a major arm. Thus, the main spiral potential of the Galaxy is two-armed, while the Sagittarius arm is an inter-arm feature with only a small mass perturbation associated with it.