Sub-kpc star formation law in the local luminous infrared galaxy IC 4687 as seen by ALMA^⋆

¹ Centro de Astrobiología (CSIC/INTA), Ctra de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid Spain
e-mail: mpereira@cab.inta-csic.es
² ASTRO-UAM, UAM, Unidad Asociada CSIC, Spain
³ Observatorio Astronómico Nacional (OAN-IGN)-Observatorio de Madrid, Alfonso XII, 3, 28014 Madrid, Spain
⁴ Instituto de Física de Cantabria, CSIC-Universidad de Cantabria, 39005 Santander, Spain

Received: 3 November 2015
Accepted: 30 December 2015

Abstract

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 Paα narrowband imaging and VLT/SINFONI near-IR integral field spectroscopy to obtain accurate extinction-corrected SF rate (SFR) and cold molecular gas surface densities (Σ_gas and Σ_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 α_CO conversion factor. This result suggests a bimodal SF law in the Σ_SFR∝Σ_gas^N representation. A universal SF law is recovered if we normalize the Σ_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.