The Radcliffe Wave as the gas spine of the Orion Arm
  The Radcliffe Wave is a $\sim3$ kpc long coherent gas structure containing
most of the star-forming complexes near the Sun. In this Letter we aim to find
a Galactic context for the Radcliffe Wave by looking into a possible
relationship between the gas structure and the Orion (Local) Arm. We use
catalogs of massive stars and young open clusters based on \textit{Gaia} EDR3
astrometry, in conjunction with kiloparsec-scale 3D dust maps, to investigate
the Galactic \textit{XY} spatial distributions of gas and young stars. We find
a quasi-parallel offset between the luminous blue stars and the Radcliffe Wave,
in that massive stars and clusters are found essentially inside and downstream
from the Radcliffe Wave. We examine this offset in the context of color
gradients observed in the spiral arms of external galaxies, where the interplay
between density wave theory, spiral shocks, and triggered star formation has
been used to interpret this particular arrangement of gas/dust and OB stars,
and outline other potential explanations as well. We hypothesize that the
Radcliffe Wave constitutes the gas reservoir of the Orion (Local) Arm, and
presents itself as a prime laboratory to study the interface between Galactic
structure, the formation of molecular clouds in the Milky Way, and star
formation.
