A high pitch angle structure in the Sagittarius Arm

Context: In spiral galaxies, star formation tends to trace features of the pattern, including arms, spurs, feathers, and branches. However, in our own Milky Way, it has been challenging connect individual star-forming regions their larger Galactic environment owing perspective from within disk. One feature nearly all modern models Way is Sagittarius Arm, located inward Sun with a pitch angle ~12 deg. Aims: We map 3D locations velocities segment Arm using young stellar objects (YSOs) Spitzer/IRAC Candidate YSO (SPICY) catalog compare distribution arm. Methods: Distances for these are derived Gaia EDR3 astrometry molecular line surveys. infer parallaxes proper motions spatially clustered groups YSOs estimate radial associated clouds. Results: identify 25 longitude range l~4.0-18.5 deg arranged narrow, ~1 kpc long linear structure high $\psi = 56$ aspect ratio ~7:1. This includes massive such as M8, M16, M17, M20. The remarkably coherent, direction rotation $240\pm3$ km/s (slightly higher than average) slight drifts toward center (-4.3 km/s) negative Z (-2.9 km/s). rotational shear experienced by 4.6 km/s/kpc. Conclusions: observed 56 Arm. discuss possible interpretations this substructure lower spur, or an isolated structure.