Abstract
We use the near--infrared Brhydrogen recombination line as a reference
star formation rate (SFR) indicator to test the validity and establish the
calibration of the Herschel PACS 70 m emission as a SFR tracer for
sub--galactic regions in external galaxies. Broffers 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 CFHT Brand 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 Bremission to derive the SFR(70) calibration for
Hii 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 70m 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 Hwith the 70 m emission, also for use in Hii
regions. We briefly analyze the PACS 100 and 160 m maps and find that
longer wavelengths are not as good SFR indicators as 70m, in agreement with
previous results. We find that the calibrations show about 50% difference
between the two galaxies, possibly due to effects of inclination.
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