SIGNALS on the mixing of oxygen and nitrogen in the spiral galaxy NGC 6946

Abstract

As part of the SIGNALS (Star formation, Ionized Gas, and Nebular Abundances Legacy Survey) survey, which comprises a sample of approximately 40 nearby galaxies observed with the Fourier transform spectrometer SITELLE, we present a study of metal mixing in the spiral galaxy NGC 6946. Taking advantage of the blue sensitivity of our set-up, we measure the oxygen and nitrogen abundances of 638 H ii regions, and focus our analysis on the abundance fluctuations about the radial gradients. We detect an azimuthal variation of about 0.1 dex in these abundances across the north-east spiral arm, with the leading edge being more metal-poor than the trailing edge. This result aligns with galaxy simulations, where radial gas flows along the spiral arms lead to dilution on the leading edge and enrichment on the trailing edge, due to the presence of radial metallicity gradients. Our 2D analysis reveals that oxygen and nitrogen exhibit comparable spatial correlation scales, despite the different injection energies and distinct nucleosynthetic origins – core-collapse supernovae in the case of oxygen and primarily asymptotic giant branch stars for nitrogen. The observed similarity suggests that stellar processes drive these two elements into the interstellar medium over equivalent spatial scales.