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arXiv · 1105.4877

Calibrating Extinction-Free Star Formation Rate Diagnostics with 33GHz Free-Free Emission in NGC6946

E. J. Murphy·J. J. Condon·E. Schinnerer·R. C. Kennicutt Jr.·D. Calzetti·L. Armus·G. Helou·J. L. Turner·G. Aniano·P. Beirão·A. D. Bolatto·B. R. Brandl·K. V. Croxall·D. A. Dale·J. L. Donovan Meyer·B. T. Draine·C. Engelbracht·L. K. Hunt·C. -N. Hao·J. Koda·H. Roussel·R. Skibba·J. -D. T. Smith
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Abstract

Abridged: Using free-free emission measured in the Ka-band (26-40GHz) for 10 star-forming regions in the nearby galaxy NGC6946, including its starbursting nucleus, we compare a number of SFR diagnostics that are typically considered to be unaffected by interstellar extinction: i.e., non-thermal radio (i.e., 1.4GHz), total infrared (IR; 8-1000um), and warm dust (i.e., 24um) emission, along with the hybrid (obscured + unobscured) indicators of H\alpha+24um and UV+IR. The 33GHz free-free emission is assumed to provide the most accurate measure of the current SFR. Among the extranuclear star-forming regions, the 24um, H\alpha+24um and UV+IR SFR calibrations are in good agreement with the 33GHz free-free SFRs. However, each of the SFR calibrations relying on some form of dust emission overestimate the nuclear SFR by a factor of ~2. This is more likely the result of excess dust heating through an accumulation of non-ionizing stars associated with an extended episode of star formation in the nucleus rather than increased competition for ionizing photons by dust. SFR calibrations using the non-thermal radio continuum yield values which only agree with the free-free SFRs for the nucleus, and underestimate the SFRs from the extranuclear star-forming regions by a factor of ~2. This result likely arises from the CR electrons decaying within the starburst region with negligible escape compared to the young extranuclear star-forming regions. Finally, we find that the SFRs estimated using the total 33GHz emission agree well with the free-free SFRs due to the large thermal fractions present at these frequencies even when local diffuse backgrounds are not removed. Thus, rest-frame 33GHz observations may act as a reliable method to measure the SFRs of galaxies at increasingly high redshift without the need of ancillary radio data to account for the non-thermal emission.

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E. J. Murphy, J. J. Condon, E. Schinnerer, R. C. Kennicutt Jr., D. Calzetti, L. Armus, G. Helou, J. L. Turner, G. Aniano, P. Beirão, A. D. Bolatto, B. R. Brandl, K. V. Croxall, D. A. Dale, J. L. Donovan Meyer, B. T. Draine, C. Engelbracht, L. K. Hunt, C. -N. Hao, J. Koda, H. Roussel, R. Skibba, J. -D. T. Smith. 2011-05-24. Calibrating Extinction-Free Star Formation Rate Diagnostics with 33GHz Free-Free Emission in NGC6946. https://doi.org/10.1088/0004-637x/737/2/67

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