UNIVERSITY OF COLOGNE

Ahmad Ali

Star formation takes place in giant molecular clouds, with most stars forming in clusters or associations. How these clusters/associations form is still an open problem, as is the cause of differences in their properties. Importantly, the role of galactic environment is also uncertain. Young massive clusters (YMCs; masses >10⁴ M_☉, radii ~ 1 pc, ages ~ 1 Myr) are of particular interest as they may be the progenitors of globular clusters. These questions are difficult to investigate in full galaxy-scale simulations because of limited resolution and less accurate stellar feedback methods compared to cloud-scale models; meanwhile, cloud-scale models usually start with isolated spherical clouds, which are not realistic initial conditions.

We have carried out zoom-in simulations to bridge this gap. Using smoothed particle hydrodynamics, we extract 100-300 pc regions of a Milky Way-like galaxy (from Pettitt et al. 2020) and re-simulate them at higher resolution (0.4 M_☉ vs. originally 600 M_☉ per particle). This allows us to use more accurate methods for star formation and stellar feedback compared to the original galaxy model, including cluster-sink particles, ray-traced photoionization from O stars, and ISM heating/cooling with H₂/CO chemistry. The original galactic potentials are still present in the zoom-ins.

We have chosen 10⁶ M_☉ cloud complexes from the bar, inner spiral arm, outer arm, and inter-arm region (in increasing order of galactocentric radius; see the Figure). Denser regions form stars at a higher rate, following the Kennicutt-Schmidt relation Σ_SFR ∝ Σ_gas^1.3 (where Σ_SFR is the star formation rate per unit area and Σ_gas is the gas mass per unit area). The bar is always the most star-forming model. The inter-arm region forms stars less efficiently than the spiral arm regions, as Σ_SFR is a factor of 2-3 below the arms for the same Σ_gas.  Almost all the clusters in the bar and inner arm are smaller than 5 pc. Half the clusters in the outer arm and a third in the inter-arm are larger than 5 pc, with radii more similar to associations. The bar and inner arm regions are able to form faster rotating clusters, while the outer arm and inter-arm regions tend to produce slower rotators on average. These results highlight the importance of galactic environment for star/cluster formation.

Ali et al. 2023, MNRAS, 524, 555. Also available on arXiv at https://arxiv.org/abs/2306.12945