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