The Norma arm: star formation in a bar-driven shock front in the Milky Way

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 (Quadrant I). Recent work suggests that these masers may reside along a spiral arm or occupy the dominant $x_1$ stellar orbits that give the bar its shape. We now argue for a very different interpretation using a new generation of N-body, hydrodynamic simulations. We believe the million-year old masers instead trace a feature of the bar's dynamics, specifically, a shock front induced by the bar's torque on the surrounding gas. The bar-driven shock front is also delineated by dust, revealing that this feature is a site of ongoing star formation. This insight brings new light on our understanding of star formation processes in the Milky Way. By associating the dust lane and masers with the simulated shock front, we determine the bar's parameters to be: $a_{\rm bar} \approx 4.3$~kpc, $b_{\rm bar} \approx 2.3$~kpc and $20^{\circ} \lesssim \phi \lesssim 25^{\circ}$, where $a$ and $b$ are the bar’s semi-major and semi-minor half-lengths, and $\phi$ its inclination angle.