The nature of the Norma arm as a bar-driven star-forming shock front

Abstract

The Milky Way is commonly depicted as having four major spiral arms, including the Norma-Outer arm, and a central stellar bar whose properties remain debated. Masers associated with young high-mass stars have been used to trace a segment of the "Norma arm" extending from one end of the Galaxy's central stellar bar. Recent work suggests that these masers may reside along a spiral arm or occupy the dominant x1 stellar orbits that give the bar its shape. We now use a new generation of N-body, hydrodynamic simulations to argue that these million-year old masers instead trace the Galactic bar's Quadrant I shock front or, equivalently, its dust lane. This implies that the bar dust-lane is star-forming, shedding new light on our understanding of star formation processes in the Milky Way. By associating the masers with the simulated shock front, we determine the bar's parameters to be: a ~ 4.3 +/- 0.2 kpc, b ~ 2.3 +/- 0.2 kpc, phi = 20 (+5/-2) deg, Omega_p <~ 40 km s^-1 kpc^-1, representing the bar's semi-major and semi-minor half-lengths, inclination angle and pattern speed, respectively. This raises questions about the Milky Way's spiral structure and suggests that the Galactic bar is long, slow and star-forming.