Encoding of the infrared excess in the NUVrK color diagram for star-forming galaxies^⋆

¹ Canada-France-Hawaii Telescope Corporation, 65-1238 Mamalahoa Hwy, Kamuela, HI 96743, USA
e-mail: arnouts@cfht.hawaii.edu
² Laboratoire AIM, CEA/DSM-CNRS-Université Paris Diderot, IRFU/Service d’Astrophysique, Bât. 709, CEA-Saclay, 91191 Gif- sur-Yvette Cedex, France
³ UPMC-CNRS, UMR7095, Institut d’Astrophysique de Paris, 75014 Paris, France
⁴ Aix-Marseille Université, CNRS, Laboratoire d’Astrophysique de Marseille, UMR 7326, 13388 Marseille, France
⁵ Spitzer Science Center, MS 314-6, California Institute of Technology, Pasadena, CA 91125, USA
⁶ Institute for Astronomy, 2680 Woodlawn Dr., University of Hawaii, Honolulu, HI 96822, USA
⁷ California Institute of Technology, MC 249-17, 1200 East California Boulevard, Pasadena, CA 91125, USA
⁸ INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁹ MPE, Giessenbachstrasse 1, 85748 Garching, Germany
¹⁰ Excellence Cluster, Boltzmann Strasse 2, 85748 Garching, Germany

Received: 25 April 2013
Accepted: 25 July 2013

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  μm sample in the COSMOS field (~16 400 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 0.27 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 (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.