The MAGPI survey: effects of spiral arms on different tracers of the interstellar medium and stellar populations at z 0.3

Spiral structures are important drivers of the secular evolution of disc galaxies, however, the origin of spiral arms and their effects on the development of galaxies remain mysterious. In this work, we present two three-armed spiral galaxies at z ~ 0.3 in the Middle Age Galaxy Properties with Integral Field Spectroscopy (MAGPI) survey. Taking advantage of the high spatial resolution (~0.6 arcsec) of the Multi-Unit Spectroscopic Explorer (MUSE), we investigate the two-dimensional (2D) distributions of different spectral parameters: Hα, gas-phase metallicity, and D₄₀₀₀. We notice significant offsets in Hα (~0.2 dex) and gas-phase metallicities (~0.05 dex) among the spiral arms, downstream and upstream of MAGPI 1202197197 (SG 1202). This observational signature suggests the spiral structure in SG 1202 is consistent with arising from density wave theory. No azimuthal variation in Hα or gas-phase metallicities is observed in MAGPI 1204198199 (SG 1204), which can be attributed to the tighter spiral arms in SG 1204 than SG 1202, coming with stronger mixing effects in the disc. The absence of azimuthal D₄₀₀₀ variation in both galaxies suggests the stars at different ages are well mixed between the spiral arms and distributed around the disc regions. The different azimuthal distributions in Hα and D₄₀₀₀ highlight the importance of time-scales traced by various spectral parameters when studying 2D distributions in spiral galaxies. This work demonstrates the feasibility of constraining spiral structures by tracing interstellar medium (ISM) and stellar population at z ~ 0.3, with a plan to expand the study to the full MAGPI survey.