Towards universal hybrid star formation rate estimators

¹ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
e-mail: mederic.boquien@uantof.cl
² Unidad de Astronomía, Fac. Cs. Básicas, Universidad de Antofagasta, Avda. U. de Antofagasta 02800, Antofagasta, Chile
³ Department of Astronomy, University of Massachusetts-Amherst, Amherst, MA 01003, USA
⁴ Department of Physics and Astronomy, University of Wyoming, Laramie, WY 82071, USA
⁵ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching-bei-München, Germany
⁶ Laboratoire AIM, CEA/IRFU/Service d’Astrophysique, CNRS, Université Paris Diderot, Bât. 709, 91191 Gif-sur-Yvette, France
⁷ Department of Astronomy, The Ohio State University, 140 W 18th Ave., Columbus, OH 43210, USA
⁸ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
⁹ INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
¹⁰ Department of Physics & Astronomy, University College London, Gower Place, London WC1E 6BT, UK
¹¹ Department of Physics and Astronomy, University of Toledo, 2801 West Bancroft Street, Toledo, OH 43606, USA
¹² Department Física Teórica y del Cosmos, Universidad de Granada 18071 Granada, Spain
¹³ Ritter Astrophysical Observatory, University of Toledo, Toledo, OH 43606, USA
¹⁴ Instituto de Astrofísica de Canarias, vía Láctea s/n, 38205 La Laguna, Spain

Received: 16 November 2015
Accepted: 27 March 2016

Abstract

Context. To compute the star formation rate (SFR) of galaxies from the rest-frame ultraviolet (UV), it is essential to take the obscuration by dust into account. To do so, one of the most popular methods consists in combining the UV with the emission from the dust itself in the infrared (IR). Yet, different studies have derived different estimators, showing that no such hybrid estimator is truly universal.

Aims. In this paper we aim at understanding and quantifying what physical processes fundamentally drive the variations between different hybrid estimators. In so doing, we aim at deriving new universal UV+IR hybrid estimators to correct the UV for dust attenuation at local and global scales, taking the intrinsic physical properties of galaxies into account.

Methods. We use the CIGALE code to model the spatially resolved far-UV to far-IR spectral energy distributions of eight nearby star-forming galaxies drawn from the KINGFISH sample. This allows us to determine their local physical properties, and in particular their UV attenuation, average SFR, average specific SFR (sSFR), and their stellar mass. We then examine how hybrid estimators depend on said properties.

Results. We find that hybrid UV+IR estimators strongly depend on the stellar mass surface density (in particular at 70 μm and 100 μm) and on the sSFR (in particular at 24 μm and the total infrared). Consequently, the IR scaling coefficients for UV obscuration can vary by almost an order of magnitude: from 1.55 to 13.45 at 24 μm for instance. This result contrasts with other groups who found relatively constant coefficients with small deviations. We exploit these variations to construct a new class of adaptative hybrid estimators based on observed UV to near-IR colours and near-IR luminosity densities per unit area. We find that they can reliably be extended to entire galaxies.

Conclusions. The new estimators provide better estimates of attenuation-corrected UV emission than classical hybrid estimators published in the literature. Taking naturally variable impact of dust heated by old stellar populations into account, they constitute an important step towards universal estimators.