Constraining Spiral Structure Parameters through Galactic Pencil-beam and Large-scale Radial Velocity Surveys

Abstract

We investigate the effect of spiral structure on the Galactic disk as viewed by pencil beams centered on the Sun, relevant to upcoming surveys such as ARGOS, SEGUE, and GAIA. We create synthetic Galactic maps which we call Pencil Beam Maps (PBMs) of the following observables: line-of-sight velocities, the corresponding velocity dispersion, and the stellar number density that are functions of distance from the observer. We show that such maps can be used to infer spiral structure parameters, such as pattern speed, solar phase angle, and number of arms. The mean line-of-sight velocity and velocity dispersion are affected by up to ~35 km/s which is well within the detectable limit for forthcoming radial velocity surveys. One can measure the pattern speed by searching for imprints of resonances. In the case of a two-armed spiral structure it can be inferred from the radius of a high velocity dispersion ring situated at the 2:1 ILR. This information, however, must be combined with information related to the velocities and stellar number density in order to distinguish from a four-armed structure. If the pattern speed is such that the 2:1 ILR is hidden inside the Galactic bulge the 2:1 OLR will be present in the outer Galaxy and thus can equivalently be used to estimate the pattern speed. Once the pattern speed is known the solar angle can be estimated from the line-of-sight velocities and the number density PBMs. Forthcoming radial velocity surveys are likely to provide powerful constraints of the structure of the Milky Way disk.