M. Pereira-Santaella, L. Colina, S. García-Burillo, P. Planesas, A. Usero, A. Alonso-Herrero, S. Arribas, S. Cazzoli, B. Emonts, J. Piqueras López, M. Villar-Martín. 2016. Sub-kpc star formation law in the local luminous infrared galaxy IC 4687 as seen by ALMA. Astronomy and Astrophysics 587, DOI: 10.1051/0004-6361/201527693

We analyze the spatially resolved (250 pc scales) and integrated star formation (SF) law in the local luminous infrared galaxy (LIRG) IC 4687. 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 narrowband 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 IC 4687 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) proportional to Sigma(N)(gas) 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.