Star Formation Rates in Resolved Galaxies: Calibrations with Near and
  Far Infrared Data for NGC5055 and NGC6946
  We use the near--infrared Br\gamma hydrogen recombination line as a reference
star formation rate (SFR) indicator to test the validity and establish the
calibration of the {\it Herschel} PACS 70 \mu m emission as a SFR tracer for
sub--galactic regions in external galaxies. Br\gamma 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 CFHT Br\gamma and Ks images of two nearby galaxies: NGC\,5055 and
NGC\,6946, which are also part of the {\it Herschel} program KINGFISH (Key
Insights on Nearby Galaxies: a Far-Infrared Survey with Herschel). We use the
extinction corrected Br\gamma emission to derive the SFR(70) calibration for
H{\sc 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\mu m 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 H\alpha with the 70 \mu m emission, also for use in H{\sc ii}
regions. We briefly analyze the PACS 100 and 160 \mu m maps and find that
longer wavelengths are not as good SFR indicators as 70\mu m, in agreement with
previous results. We find that the calibrations show about 50% difference
between the two galaxies, possibly due to effects of inclination.
