Dense gas under the microscope: a high-resolution view on dense gas and star formation with ALMA

Dense gas is the fundamental fuel for star formation, yet its conversion efficiency and timescales across and within galaxies remain poorly understood. In this talk, I will present recent ALMA spectral line observations of the nearby star-forming galaxy NGC 4321 (Neumann et al. 2024). These new 260-parsec resolution data provide the most sensitive, high-resolution, full-galaxy perspective on dense gas tracers (HCN, HCO+) in nearby star-forming galaxies. By combining these observations with cloud-scale data on bulk molecular gas (ALMA), hot dust (JWST), and ionised gas (VLT-MUSE) from the PHANGS collaboration, we gain an unprecedented, detailed view of molecular cloud properties and their star-forming potential across diverse galactic environments, including the central molecular zone, the galactic bar, bar ends, spiral arms, and interarm regions. Our findings on the dense gas fraction (traced by HCN/CO) and star formation efficiency (traced by SFR/HCN) reveal that while molecular clouds experience strong external pressure toward the inner galaxy, they tend to decouple from their surroundings in the outer disk, where stars form in a more universal way. Furthermore, we identify the galactic bar as a distinct environment where star formation proceeds in an unusually inefficient manner. These findings underscore the necessity of high-resolution, multi-wavelength studies to fully understand the physical conditions governing star formation across diverse galactic environments.