Measuring star formation with resolved observations: the test case of M 33⋆
1 Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
2 Aix-Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
3 Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA
4 Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Observatory, 43994 Onsala, Sweden
5 Laboratoire d’Astrophysique de Bordeaux, Université de Bordeaux, CNRS UMR 5804, 33271 Floirac, France
6 Observatoire de Paris, LERMA, CNRS, 61 Av. de l’Observatoire, 75014 Paris, France
7 Sterrewacht Leiden, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
8 Instituto Radioastronomia Milimetrica, 18012 Granada, Spain
9 Infrared Processing and Analysis Center, California Institute of Technology, MS 100–22, Pasadena, CA 91125, USA
10 Department Física Teórica y del Cosmos, Universidad de Granada 18071, Granada, Spain
11 Department of Physics, University of Alberta, 2–115 Centennial Centre for Interdisciplinary Science, Edmonton T6G2E1, Canada
12 Department of Astronomy, Cornell University, Ithaca, NY 14853, USA
13 Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
14 SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands
15 Kapteyn Astronomical Institute, University of Groningen, 9712 Groningen, The Netherlands
16 Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing, National Observatory of Athens, 15236 Athens, Greece
Received: 28 January 2014
Accepted: 4 February 2015
Context. Measuring star formation on a local scale is important to constrain star formation laws. It is not clear yet, however, whether and how the measure of star formation is affected by the spatial scale at which a galaxy is observed.
Aims. We wish to understand the impact of the resolution on the determination of the spatially resolved star formation rate (SFR) and other directly associated physical parameters such as the attenuation.
Methods. We carried out a multi-scale, pixel-by-pixel study of the nearby galaxy M 33. Assembling FUV, Hα, 8 μm, 24 μm, 70 μm, and 100 μm maps, we have systematically compared the emission in individual bands with various SFR estimators from a resolution of 33 pc to 2084 pc.
Results. There are strong, scale-dependent, discrepancies of up to a factor 3 between monochromatic SFR estimators and Hα+24 μm. The scaling factors between individual IR bands and the SFR show a strong dependence on the spatial scale and on the intensity of star formation. Finally, strong variations of the differential reddening between the nebular emission and the stellar continuum are seen, depending on the specific SFR (sSFR) and on the resolution. At the finest spatial scales, there is little differential reddening at high sSFR. The differential reddening increases with decreasing sSFR. At the coarsest spatial scales the differential reddening is compatible with the canonical value found for starburst galaxies.
Conclusions. Our results confirm that monochromatic estimators of the SFR are unreliable at scales smaller than 1 kpc. Furthermore, the extension of local calibrations to high-redshift galaxies presents non-trivial challenges because the properties of these systems may be poorly known.
Key words: galaxies: individual: M 33 / galaxies: ISM / galaxies: star formation
The maps (FITS files) and the data cube used in this article are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (126.96.36.199) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/578/A8
© ESO, 2015