2013A&A...558A..67A

We present an empirical method of assessing the star formation rate (SFR) of star-forming galaxies based on their locations in the rest-frame color-color diagram (NUV-r) vs. (r-K). By using the Spitzer 24µm sample in the COSMOS field (∼16400 galaxies with 0.2≤z≤1.3) and a local GALEX-SDSS-SWIRE sample (∼700 galaxies with z≤0.2), we show that the mean infrared excess <IRX≥<L_IR/L_UV> can be described by a single vector, NRK, that combines the two colors. The calibration between <IRX> and NRK allows us to recover the IR luminosity, L_IR_, with an accuracy of σ∼0.21 for the COSMOS sample and 0.27dex for the local one. The SFRs derived with this method agree with the ones based on the observed (UV+IR) luminosities and on the spectral energy distribution (SED) fitting for the vast majority (∼85%) of the star-forming population. Thanks to a library of model galaxy SEDs with realistic prescriptions for the star formation history, we show that we need to include a two-component dust model (i.e., birth clouds and diffuse ISM) and a full distribution of galaxy inclinations in order to reproduce the behavior of the <IRX> stripes in the NUVrK diagram. In conclusion, the NRK method, based only on the rest-frame UV/optical colors available in most of the extragalactic fields, offers a simple alternative of assessing the SFR of star-forming galaxies in the absence of far-IR or spectral diagnostic observations.