Quantifying azimuthal variations within the interstellar medium of z ~ 0 spiral galaxies with the TYPHOON survey

Abstract

Most star formation in the local Universe occurs in spiral galaxies, but their origin remains an unanswered question. Various theories have been proposed to explain the development of spiral arms, each predicting different spatial distributions of the interstellar medium. This study maps the star formation rate (SFR) and gas-phase metallicity of nine spiral galaxies with the TYPHOON survey to test two dominating theories: density wave theory and dynamic spiral theory. We discuss the environmental effects on our galaxies, considering reported environments and merging events. Taking advantage of the large field of view covering the entire optical disc, we quantify the fluctuation of SFR and metallicity relative to the azimuthal distance from the spiral arms. We find higher SFR and metallicity in the trailing edge of NGC 1365 (by 0.117 and 0.068 dex, respectively) and NGC 1566 (by 0.119 and 0.037 dex, respectively), which is in line with density wave theory. NGC 2442 shows a different result with higher metallicity (0.093 dex) in the leading edge, possibly attributed to an ongoing merging. The other six spiral galaxies show no statistically significant offset in SFR or metallicity, consistent with dynamic spiral theory. We also compare the behaviour of metallicity inside and outside the corotation radius (CR) of NGC 1365 and NGC 1566. We find comparable metallicity fluctuations near and beyond the CR of NGC 1365, indicating gravitational perturbation. NGC 1566 shows the greatest fluctuation near the CR, in line with the analytic spiral arms. Our work highlights that a combination of mechanisms explains the origin of spiral features in the local Universe.