The Detection of Anomalous Dust Emission in the Nearby Galaxy NGC 6946

TL;DR

This study reports the first likely detection of anomalous dust emission outside the Milky Way in NGC 6946, highlighting its potential impact on interpreting Ka-band data for star formation in external galaxies.

Contribution

It provides observational evidence of anomalous dust emission in a nearby galaxy and demonstrates that spinning dust models can explain this phenomenon.

Findings

Excess Ka-band emission detected in 50% of regions, significant in one.

Anomalous dust emission may account for up to 10% of total Ka-band flux.

Spinning dust models successfully reproduce observed excesses.

Abstract

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.