Spiral Structure Boosts Star Formation in Disk Galaxies
  We investigate the impact of spiral structure on global star formation using
a sample of 2226 nearby bright disk galaxies. Examining the relationship
between spiral arms, star formation rate (SFR), and stellar mass, we find that
arm strength correlates well with the variation of SFR as a function of stellar
mass. Arms are stronger above the star-forming galaxy main sequence (MS) and
weaker below it: arm strength increases with higher $\log\,({\rm SFR}/{\rm
SFR}_{\rm MS})$, where ${\rm SFR}_{\rm MS}$ is the SFR along the MS. Likewise,
stronger arms are associated with higher specific SFR. We confirm this trend
using the optical colors of a larger sample of 4378 disk galaxies, whose
position on the blue cloud also depends systematically on spiral arm strength.
This link is independent of other galaxy structural parameters. For the subset
of galaxies with cold gas measurements, arm strength positively correlates with
HI and H$_2$ mass fraction, even after removing the mutual dependence on
$\log\,({\rm SFR}/{\rm SFR}_{\rm MS})$, consistent with the notion that spiral
arms are maintained by dynamical cooling provided by gas damping. For a given
gas fraction, stronger arms lead to higher $\log\,({\rm SFR}/{\rm SFR}_{\rm
MS})$, resulting in a trend of increasing arm strength with shorter gas
depletion time. We suggest a physical picture in which the dissipation process
provided by gas damping maintains spiral structure, which, in turn, boosts the
star formation efficiency of the gas reservoir.
