The role of spiral arms in Milky Way star formation
  What role does Galactic structure play in star formation? We have used the
Herschel Hi-GAL compact-clump catalogue to examine trends in evolutionary stage
over large spatial scales in the inner Galaxy. We examine the relationship
between the fraction of clumps with embedded star formation (the star-forming
fraction, or SFF) and other measures of star-formation activity. Based on a
positive correlation between SFF and evolutionary indicators such as the
luminosity-to-mass ratio, we assert that the SFF principally traces the average
evolutionary state of a sample and must depend on the local fraction of
rapidly-evolving, high-mass young stellar objects. The spiral-arm tangent point
longitudes show small excesses in the SFF, though these can be accounted for by
a small number of the most massive clusters, just 7.6% of the total number of
clumps in the catalogue. This suggests that while the arms tend to be home to
the Galaxy's massive clusters, the remaining 92.4% of Hi-GAL clumps in our
catalogue do not show an enhancement of star formation within arms. Globally,
the SFF is highest at the Galactic midplane and inner longitudes. We find no
significant trend in evolutionary stage as a function of position across spiral
arms at the tangent-point longitudes. This indicates that the angular offset
observed between gas and stars, if coordinated by a density wave, is not
evident at the clump phase; alternatively, the onset of star formation is not
triggered by the spiral density wave.
