Star Formation Rates in Resolved Galaxies: Calibrations with Near- and Far-infrared Data for NGC 5055 and NGC 6946

Creators: Li, Yiming; Crocker, Alison F.; Calzetti, Daniela; Wilson, Christine D.; Kennicutt, Robert C.; Murphy, Eric J.; Brandl, Bernhard R.; Draine, B. T.; Galametz, M.; Johnson, B. D.; Armus, L.; Gordon, K. D.; Croxall, K.; Dale, D. A.; Engelbracht, C. W.; Groves, B.; Hao, C.-N.; Helou, G.; Hinz, J.; Hunt, L. K.; Krause, O.; Roussel, H.; Sauvage, M.; Smith, J. D. T.

Abstract

We use the near-infrared Brγ hydrogen recombination line as a reference star formation rate (SFR) indicator to test the validity and establish the calibration of the Herschel/PACS 70 μm emission as a SFR tracer for sub-galactic regions in external galaxies. Brγ offers the double advantage of directly tracing ionizing photons and of being relatively insensitive to the effects of dust attenuation. For our first experiment, we use archival Canada-France-Hawaii Telescope Brγ and Ks images of two nearby galaxies: NGC 5055 and NGC 6946, which are also part of the Herschel program KINGFISH (Key Insights on Nearby Galaxies: a Far-Infrared Survey with Herschel). We use the extinction corrected Brγ emission to derive the SFR(70) calibration for H II regions in these two galaxies. A comparison of the SFR(70) calibrations at different spatial scales, from 200 pc to the size of the whole galaxy, reveals that about 50% of the total 70 μm emission is due to dust heated by stellar populations that are unrelated to the current star formation. We use a simple model to qualitatively relate the increase of the SFR(70) calibration coefficient with decreasing region size to the star formation timescale. We provide a calibration for an unbiased SFR indicator that combines the observed Hα with the 70 μm emission, also for use in H II regions. We briefly analyze the PACS 100 and 160 μm maps and find that longer wavelengths are not as good SFR indicators as 70 μm, in agreement with previous results. We find that the calibrations show about 50% difference between the two galaxies, possibly due to effects of inclination.

Additional Information

© 2013 American Astronomical Society. Received 2012 November 12; accepted 2013 April 4; published 2013 April 26. Based on observations obtained with WIRCam, a joint project of CFHT, Taiwan, Korea, Canada, France, and the Canada–France–Hawaii Telescope (CFHT) which is operated by the National Research Council (NRC) of Canada, the Institute National des Sciences de l'Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii. This work is based in part on observations made with Herschel, a European Space Agency Cornerstone Mission with significant participation by NASA. Partial support for this work was provided by NASA through the award 1369560 issued by JPL/Caltech. This work has also been partially supported by the NASA–ADAP grant NNX10AD08G. The research of C.D.W. is supported by grants from the Natural Sciences and Engineering Research Council of Canada. The authors would like to thank Daniel Devost for the WIRCam observations with RUNID 07AD86 for NGC 5055 and 07BD91 for NGC 6946. This work has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. The authors thank an anonymous referee for valuable comments that have helped improve this paper.

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