A low density wave's spiral pattern speed, from the tracer separations (age gradient) across a spiral arm in the Milky Way

Abstract

ABSTRACT We observe the density wave's angular pattern speed Ωp to be near 12–17 km s−1 kpc−1, by the separation between a typical optical H ii region (from the spiral arm's dust lane) and using a H ii-evolution time model TH ii to yield its relative speed, and independently by the separation between a typical radio maser (from the spiral arm's dust lane) with a maser model. The assumption of a fixed circular rotational speed of the gas and stars with galactic radius is employed (neglecting spiral perturbation at mid-radii, nuclear bar influence at small radii, and tidal effects at large radii).