2018AJ....156..248T

In this work, we derive a spatially resolved map of the line-of-sight velocity of the interstellar medium and use it, along with a second map of line-of-sight velocity from Paper I of this series, to determine the nature of the gaseous spiral structure in the Milky Way (MW). This map is derived from measurements of the 1.527 µm diffuse interstellar band (DIB) in stellar spectra from the Apache Point Observatory Galactic Evolution Experiment survey and covers the nearest 4-5 kpc of the Northern Galactic plane. We cross-check this new DIB-based line-of-sight velocity map with the map derived in Paper I and find that they agree. We then compare these maps with line-of-sight velocity maps derived from simulations of quasi-stationary density wave spiral structure and dynamic, or material, spiral structure in a MW-like galaxy. While none of the maps derived from these simulations is an exact match to the measured velocity field of the MW, the measurements are more consistent with simulations of dynamic spiral structure. In the dynamic spiral structure simulation that best matches the measurements, the Perseus spiral arm is being disrupted.