Calibrating mid-infrared emission as a tracer of obscured star formation on H II-region scales in the era of JWST

¹ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy
e-mail: francesco.belfiore@inaf.it
² Department of Astronomy, The Ohio State University, 140 West 18th Avenue, Columbus OH 43210, USA
³ Sub-department of Astrophysics, Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
⁴ European Southern Observatory, Karl-Schwarzschild Straße 2, 85748 Garching bei München, Germany
⁵ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁶ Instituto de Alta Investigación, Universidad de Tarapacá, Casilla 7D, Arica, Chile
⁷ Max-Planck-Institut für Extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching, Germany
⁸ Sterrenkundig Observatorium, Ghent University, Krijgslaan 281-S9, 9000 Gent, Belgium
⁹ European Southern Observatory (ESO), Alonso de Córdova 3107, Casilla 19, Santiago 19001, Chile
¹⁰ Department of Physics and Astronomy, University of Wyoming, Laramie, WY 82071, USA
¹¹ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstraße 12-14, 69120 Heidelberg, Germany
¹² Univ. Lyon, Univ. Lyon1, ENS de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, 69230 Saint-Genis-Laval, France
¹³ Universität Heidelberg, Zentrum für Astronomie, Institut für theoretische Astrophysik, Albert-Ueberle-Straße 2, 69120 Heidelberg, Germany
¹⁴ International Centre for Radio Astronomy Research, University of Western Australia, 7 Fairway, Crawley, 6009 WA, Australia
¹⁵ Universität Heidelberg, Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, Im Neuenheimer Feld 205, 69120 Heidelberg, Germany
¹⁶ Max-Planck-Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
¹⁷ Observatorio Astronómico Nacional (IGN), C/Alfonso XII, 3, 28014 Madrid, Spain
¹⁸ Department of Physics, University of Alberta, Edmonton, AB T6G 2E1, Canada
¹⁹ Departamento de Física de la Tierra y Astrofísica, Universidad Complutense de Madrid, 28040 Madrid, Spain
²⁰ Instituto de Física de Partículas y del Cosmos IPARCOS, Facultad de CC Físicas, UCM, 28040 Madrid, Spain
²¹ Department of Astronomy & Astrophysics, University of California, San Diego, 9500 Gilman Drive, San Diego, CA 92093, USA
²² Department of Physics and Astronomy, McMaster University, 1280 Main St. West, Hamilton, ON L8S 4M1, Canada
²³ Canadian Institute for Theoretical Astrophysics (CITA), University of Toronto, 60 St George Street, Toronto, ON M5S 3H8, Canada

Received: 13 June 2023
Accepted: 1 September 2023

Abstract

Measurements of the star formation activity on cloud scales are fundamental to uncovering the physics of the molecular cloud, star formation, and stellar feedback cycle in galaxies. Infrared (IR) emission from small dust grains and polycyclic aromatic hydrocarbons (PAHs) is widely used to trace the obscured component of star formation. However, the relation between these emission features and dust attenuation is complicated by the combined effects of dust heating from old stellar populations and an uncertain dust geometry with respect to heating sources. We used images obtained with NIRCam and MIRI as part of the PHANGS–JWST survey to calibrate the IR emission at 21 μm, and the emission in the PAH-tracing bands at 3.3, 7.7, 10, and 11.3 μm as tracers of obscured star formation. We analysed ∼20 000 optically selected H II regions across 19 nearby star-forming galaxies, and benchmarked their IR emission against dust attenuation measured from the Balmer decrement. We modelled the extinction-corrected Hα flux as the sum of the observed Hα emission and a term proportional to the IR emission, with a_IR as the proportionality coefficient. A constant a_IR leads to an extinction-corrected Hα estimate that agrees with those obtained with the Balmer decrement with a scatter of ∼0.1 dex for all bands considered. Among these bands, 21 μm emission is demonstrated to be the best tracer of dust attenuation. The PAH-tracing bands underestimate the correction for bright H II regions, since in these environments the ratio of PAH-tracing bands to 21 μm decreases, signalling destruction of the PAH molecules. For fainter H II regions, all bands suffer from an increasing contamination from the diffuse IR background. We present calibrations that take this effect into account by adding an explicit dependence on 2 μm emission or stellar mass surface density.