The Detection of Anomalous Dust Emission in the Nearby Galaxy NGC 6946
  We report on the Ka-band (26-40 GHz) emission properties for 10 star-forming
regions in the nearby galaxy NGC 6946. From a radio spectral decomposition, we
find that the 33 GHz flux densities are typically dominated by thermal
(free-free) radiation. However, we also detect excess Ka-band emission for an
outer-disk star-forming region relative to what is expected given existing
radio, submillimeter, and infrared data. Among the 10 targeted regions,
measurable excess emission at 33 GHz is detected for half of them, but in only
one region is the excess found to be statistically significant
($\approx7\sigma$). We interpret this as the first likely detection of so
called `anomalous' dust emission outside of the Milky Way. We find that models
explaining this feature as the result of dipole emission from rapidly rotating
ultrasmall grains are able to reproduce the observations for reasonable
interstellar medium conditions. While these results suggest that the use of
Ka-band data as a measure of star formation activity in external galaxies may
be complicated by the presence of anomalous dust, it is unclear how significant
a factor this will be for globally integrated measurements as the excess
emission accounts for $\la$10% of the total Ka-band flux density from all 10
regions.
