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

TL;DR

This study investigates the star-formation law at 250 pc scales in the luminous infrared galaxy IC 4687, revealing rapid star formation efficiency and suggesting a bimodal star-formation law that varies with local conditions.

Contribution

First spatially resolved analysis of the star-formation law at 250 pc scales in a starburst LIRG, combining ALMA, HST, and VLT data to explore star formation efficiency.

Findings

IC 4687 forms stars very efficiently with a depletion time of 160 Myr.

A bimodal star-formation law is suggested, differing from normal spirals.

Star formation efficiency does not depend on local dynamical time at these scales.

Abstract

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.