Star formation takes place in giant molecular clouds (GMCs), with most stars forming in clusters or associations. How clusters/associations form is still an open problem, as is the cause of differences in their properties. For instance, Young Massive Clusters (YMCs; masses > 10^4 Msun; ages ~1 Myr) are not as common in the Milky Way (MW) as they are in starburst galaxies. The YMCs that are found in our Galaxy are typically found near the Galactic Centre or bar.
We investigate how the formation of such clusters depends on galactic environment using smoothed particle hydrodynamics simulations. We extract 10^6 Msun GMC complexes from a MW-like galaxy model, enhance the resolution, and re-run the region with improved methods to track star formation and feedback. This includes ray-traced photoionization which is not possible on galactic scales. We retain galactic potentials and cloud-cloud interactions, which are usually neglected in standard, isolated cloud models.
We model GMC complexes from the bar, inner spiral arm, outer spiral arm, and inter-arm region. We find that the relation between star formation rate surface density and gas surface density follows the Kennicutt-Schmidt relation with power law 1.4, with the bar having higher densities than the arms. However, the inter-arm is 2-3x less star-forming for the same gas surface density as the arm regions. We also find that the bar and inner arm preferentially produce smaller, denser clusters whereas the outer arm and inter-arm regions produce larger, lower density clusters more similar to associations.