The prevalence of star formation as a function of Galactocentric radius
  We present large-scale trends in the distribution of star-forming objects
revealed by the Hi-GAL survey. As a simple metric probing the prevalence of
star formation in Hi-GAL sources, we define the fraction of the total number of
Hi-GAL sources with a 70-micron counterpart as the "star-forming fraction" or
SFF. The mean SFF in the inner galactic disc (3.1 kpc < R_GC < 8.6 kpc) is 25%.
Despite an apparent pile-up of source numbers at radii associated with spiral
arms, the SFF shows no significant deviations at these radii, indicating that
the arms do not affect the star-forming productivity of dense clumps either via
physical triggering processes or through the statistical effects of larger
source samples associated with the arms. Within this range of Galactocentric
radii, we find that the SFF declines with R_GC at a rate of -0.026 +/- 0.002
per kiloparsec, despite the dense gas mass fraction having been observed to be
constant in the inner Galaxy. This suggests that the SFF may be weakly
dependent on one or more large-scale physical properties of the Galaxy, such as
metallicity, radiation field, pressure or shear, such that the dense
sub-structures of molecular clouds acquire some internal properties inherited
from their environment.
