Letter to the Editor

100 μm and 160 μm emission as resolved star-formation rate estimators in M 33 (HERM33ES) ^*

¹ University of Massachusetts, Department of Astronomy, LGRT-B 619E, Amherst, MA 01003, USA e-mail: boquien@astro.umass.edu
² Instituto Radioastronomia Milimetrica, Av. Divina Pastora 7, Nucleo Central, 18012 Granada, Spain
³ Institute of Astronomy and Astrophysics, National Observatory of Athens, P. Penteli, 15236 Athens, Greece
⁴ Argelander Institut für Astronomie. Auf dem Hügel 71, 53121 Bonn, Germany
⁵ Laboratoire d'Astrophysique de Bordeaux, Université Bordeaux 1, Observatoire de Bordeaux, OASU, UMR 5804, CNRS/INSU, B.P. 89, Floirac 33270, France
⁶ Observatoire de Paris, LERMA, 61 Av. de l'Observatoire, 75014 Paris, France
⁷ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁸ ATNF, CSIRO, PO Box 76, Epping, NSW 1710, Australia
⁹ IPAC, MS 100-22 California Institute of Technology, Pasadena, CA 91125, USA
¹⁰ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, England
¹¹ KOSMA, I. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
¹² Department of Astronomy, Cornell University, Ithaca, NY 14853, USA
¹³ Max Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹⁴ JAC, 660 North A'ohoku Place, University Park, Hilo, HI 96720, USA
¹⁵ SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands
¹⁶ Dept. Física Teórica y del Cosmos, Universidad de Granada, Spain

Received: 31 March 2010
Accepted: 21 April 2010

Abstract

Context. Over the past few years several studies have provided estimates of the SFR (star-formation rate) or the total infrared luminosity from just one infrared band. However these relations are generally derived for entire galaxies, which are known to contain a large scale diffuse emission that is not necessarily related to the latest star-formation episode.

Aims. We provide new relations to estimate the SFR from resolved star-forming regions at 100 μm and 160 μm.

Methods. We select individual star-forming regions in the nearby (840 kpc) galaxy M 33. We estimate the SFR combining the emission in Hα and at 24 μm to calibrate the emission at 100 μm and 160 μm as SFR estimators, as mapped with PACS/Herschel. The data are obtained in the framework of the HERM33ES open time key program.

Results. There is less emission in the HII regions at 160 μm than at 100 μm. Over a dynamic range of almost 2 dex in Σ_SFR we find that the 100 μm emission is a nearly linear estimator of the SFR, whereas that at 160 μm is slightly superlinear.

Conclusions. The behaviour of individual star-forming regions is surprisingly similar to that of entire galaxies. At high Σ_SFR, star formation drives the dust temperature, whereas uncertainties and variations in radiation-transfer and dust-heated processes dominate at low Σ_SFR. Detailed modelling of both galaxies and individual star forming regions will be needed to interpret similarities and differences between the two and assess the fraction of diffuse emission in galaxies.