Sub-kpc star-formation law in the local luminous infrared galaxy IC 4687
  as seen by ALMA
  We analyze the spatially resolved (250 pc scales) and integrated
star-formation (SF) law in the local luminous infrared galaxy (LIRG) IC4687.
This is one of the first studies of the SF law on a starburst LIRG at these
small spatial scales. We combined new interferometric ALMA CO(2-1) data with
existing HST/NICMOS Pa$\alpha$ narrow-band imaging and VLT/SINFONI near-IR
integral field spectroscopy to obtain accurate extinction corrected SF rate
(SFR) and cold molecular gas surface densities ($\Sigma_{gas}$ and
$\Sigma_{SFR}$). We find that IC4687 forms stars very efficiently with an
average depletion time ($t_{dep}$) of 160 Myr for the individual 250 pc
regions. This is approximately one order of magnitude shorter than the
$t_{dep}$ of local normal spirals and also shorter than that of main-sequence
high-z objects, even when we use a Galactic $\alpha_{CO}$ conversion factor.
This result suggests a bimodal SF law in the $\Sigma_{SFR} \propto
\Sigma_{gas}^{N}$ representation. A universal SF law is recovered if we
normalize the $\Sigma_{gas}$ by the global dynamical time. However, at the
spatial scales studied here, we find that the SF efficiency (or $t_{dep}$) does
not depend on the local dynamical time for this object. Therefore, an
alternative normalization (e.g., free-fall time) should be found if a universal
SF law exists at these scales.
