How mergers and flybys shape azimuthal age patterns in spiral galaxies

ABSTRACT Spiral structures are one of the most common features in galaxies, yet their origins and evolution remain debated. Stellar age distributions offer crucial insights into galaxy evolution and star formation, though environmental effects can obscure the intrinsic age patterns. Using the Auriga cosmological gravo-magnetohydrodynamical zoom-in simulations, we investigate the azimuthal age distribution of young stars (${<} 2$ Gyr) in a sample of five Milky Way-mass spiral galaxies over the past 5 Gyr. We quantify the age gradients across spiral arms using the mean age offset ($\mathrm{\Delta \tau }$) and the non-overlap fraction ($f_{\mathrm{non{\text{-}}overlap}}$). We further analyse the impact of mergers and fly-by events on the age gradients. Our results show that Auriga spiral galaxies generally feature younger stars in their leading edges compared to the trailing edges, with a typical $\mathrm{\Delta \tau }$ between 30 and 80 Myr. However, gas-rich interactions can disrupt this age offset, resulting in similar age distributions on each side of the spiral arms. In three snapshots, we observe similar mean ages on both sides of spiral arms but differing age distribution broadness, coinciding with satellite interactions crossing the host galaxy’s disc plane. Our simulation data suggest that the typical azimuthal age variation recovers within $\sim$600 Myr after galaxy interactions. This work highlights the transient role of environmental interactions in shaping spiral arm age patterns.