Multi-line modelling in nearby galaxies: The link between dense gas and star formation in different environments

The J=1–0 lines of HCN and HCO+ have become the default tracers of dense molecular gas in external galaxies. To study the relation between HCN and HCO+ emission and density, we have mapped their J=3–2 lines with the ALMA Compact Array (ACA) in the nearby star-forming galaxies NGC3351, NGC3627, and NGC4321. Combined with archival line maps of HCN(1–0), HCO+(1–0), CO(1–0), and CO(2–1), we use this data set to: (1) compare the excitation of dense gas tracers in normal and starburst/AGN galaxies; (2) explore how line ratios vary across galaxy disks; (3) infer the density distribution and other cloud properties on a pixel-by-pixel basis through a physically-motivated radiative transfer model.