Spiral Structure Properties, Dynamics, and Evolution in Milky Way–mass Galaxy Simulations

Abstract The structure of spiral galaxies is essential to understanding the dynamics and evolution of disk galaxies; however, the precise nature of spiral arms remains uncertain. Two challenges in understanding the mechanisms driving spirals are how galactic environment impacts spiral morphology and how they evolve over time. We present a catalog characterizing the properties, dynamics, and evolution of m = 2 spiral structure in 10 Milky Way–mass galaxies from the FIRE-2 cosmological zoom-in simulations. Consistent with previous literature, we find that FIRE-2 spirals are transient, recurring features simultaneously present in the disk at varying pattern speeds (Ω p ) that broadly decrease with radius. These spirals persist on gigayear timescales (mean duration of 1.90 Gyr), but fluctuate in amplitude on timescales of hundreds of millions of years. Tidal interactions and bar episodes impact the resulting m = 2 spiral structure: strong satellite interactions generally produce shorter-lived, stronger spirals with larger radial extent, and bars can increase Ω p . Galactic environment influences spiral structure, in which kinematically colder disks can support longer-lived, stronger spirals. The properties of identified spirals in FIRE-2 vary widely in radial extent (0.3–10.8 kpc), duration (1.00–6.00 Gyr), and amplitude ( a 2,max = 0.018–0.192). We find the presence of spirals in all age populations, suggesting these are density-wave-driven features. This work represents the first time that spiral structure has been cataloged in this manner in cosmological simulations; the catalog can be leveraged with current and forthcoming observational surveys, enabling systematic comparisons to further our understanding of galaxy evolution.