I will present recent results from the IRAM 30m large program “Line Emission as a tool for Galaxy Observations” (LEGO; Kauffmann+17, Barnes+20, Neumann to be subm.). This survey aims at studying star-forming regions in the Milky Way across a wide range of environments to help interpret extragalactic observations of molecular line emission. Here, I will present new results from the three high-mass star-forming regions of the LEGO sample, which are the ideal candidates to connect to active star forming galaxies, such as those probed by the PHANGS/ALMOND (Neumann+23a) and EMPIRE (Jimenez-Donaire+19) surveys. I will show that, in these regions, molecular line emission from N2H+ is the only robust tracer of cold, dense gas. On the contrary, classical extragalactic dense gas tracers, such as HCN, HCO+, HNC or CS can also efficiently emit at moderate gas densities. However, if averaged over the whole extent of the cloud, HCN (HCO+, HNC, CS) and N2H+ have a roughly fixed line ratio, suggesting that HCN is still a decent proxy for dense gas at cloud scales. Moreover, I will demonstrate that the HCN-to-CO line ratio is sensitive to gas density from parsec (LEGO) over 100 parsec (Neumann+24a) to kiloparsec scales (Neumann+24b), showing that HCN traces denser gas than that traced by the low-J CO transitions and hence remains a powerful extragalactic tool to study physical gas conditions in external galaxies.
