Star Formation On Sub-kpc Scale Triggered By Non-linear Processes In
  Nearby Spiral Galaxies
  We report a super-linear correlation for the star formation law based on new
CO($J$=1-0) data from the CARMA and NOBEYAMA Nearby-galaxies (CANON) CO survey.
The sample includes 10 nearby spiral galaxies, in which structures at sub-kpc
scales are spatially resolved. Combined with the star formation rate surface
density traced by H$\alpha$ and 24 $\mu$m images, CO($J$=1-0) data provide a
super-linear slope of $N$ = 1.3. The slope becomes even steeper ($N$ = 1.8)
when the diffuse stellar and dust background emission is subtracted from the
H$\alpha$ and 24 $\mu$m images. In contrast to the recent results with
CO($J$=2-1) that found a constant star formation efficiency (SFE) in many
spiral galaxies, these results suggest that the SFE is not independent of
environment, but increases with molecular gas surface density. We suggest that
the excitation of CO($J$=2-1) is likely enhanced in the regions with higher
star formation and does not linearly trace the molecular gas mass. In addition,
the diffuse emission contaminates the SFE measurement most in regions where
star formation rate is law. These two effects can flatten the power law
correlation and produce the apparent linear slope. The super linear slope from
the CO($J$=1-0) analysis indicates that star formation is enhanced by
non-linear processes in regions of high gas density, e.g., gravitational
collapse and cloud-cloud collisions.
