Encoding the infrared excess (IRX) in the NUVrK color diagram for star-forming galaxies

TL;DR

This paper introduces an empirical color-based method to estimate the star formation rate of galaxies by relating their UV and optical colors to infrared excess, enabling SFR assessment without far-IR data.

Contribution

The study presents a novel, simple color-color diagram calibration to accurately recover infrared luminosity and SFR in star-forming galaxies using only rest-frame UV and optical colors.

Findings

The IRX can be effectively described by a single vector, NRK, combining two colors.

The method recovers L_IR with ~0.2 dex accuracy.

SFR estimates agree with traditional UV+IR and SED fitting methods for most galaxies.

Abstract

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 24 micron 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 ~0.21 dex for the COSMOS sample and ~0.27 dex 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 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 rest-frame UV and 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.