Spiral arms across stellar populations in simulations via the local dimension method

Context. The origin and nature of spiral arms remain unclear. Star-forming regions and young stars are generally strongly associated with the spiral structure, but there are few quantitative predictions from simulations about the contribution of stars of different ages. Aims. We aim to quantify the interplay between spiral arms and different populations. Methods. To this end, we used a hydrodynamical simulation of an isolated disc galaxy displaying a dynamic multi-armed spiral structure. Inspired by cosmological structure metrics, we developed a new method, the local dimension, that robustly delineates arms across populations and through space and time. Results. We find that all stars, including those as old as 11 Gyr, support the arms. The spiral strength decreases with stellar age up to 2 Gyr, and it remains nearly constant for older stars. However, the scaling between arm strength and age (or velocity dispersion) depends on the strength of the global spiral structure at each time. Almost all stars formed in arms remain within them for no more than 140–180 Myr, whereas old stars leave arms about three times faster. Even if the youngest populations dominate in the production of the spiral torques at early times, all populations contribute equally at later times. Conclusions. Our results highlight the power of the local dimension for studying complex spiral structures and show that spiral arms are present in all stellar populations. Since we see spiral arms in populations with velocity dispersions up to 90 km/s, which are comparable to those of the Milky Way, we predict that old Galactic populations could also exhibit spiral structure.