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

TL;DR

This study maps star formation and metallicity in nine spiral galaxies to test theories of spiral arm formation, finding evidence supporting both density wave and dynamic spiral theories depending on the galaxy.

Contribution

It provides the first detailed azimuthal analysis of SFR and metallicity across entire galaxy disks, testing predictions of spiral arm theories with new observational data.

Findings

Higher SFR and metallicity on the trailing edge support density wave theory.

No significant azimuthal offsets in most galaxies support dynamic spiral theory.

Metallicity fluctuations near co-rotation suggest gravitational perturbations.

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 disk, 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~dex and 0.068~dex, respectively) and NGC~1566 (by 0.119~dex 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 co-rotation 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.