2025ApJ...993..214L

High-density molecular gas plays a vital role in supporting star formation within galaxies. However, traditional tracers of dense gas, such as the low-J transitions of HCN and HCO⁺, are predominantly optically thick. This characteristic presents a significant challenge in accurately estimating the column density of dense gas and mapping its spatial distribution. Optically thin tracers, including HC₃N (10-9) and isotopologues of HCN, HCO⁺, and HNC (1-0), among others, emerge as better tracers by enabling more precise measurements and analyses of the dense gas in galaxies. Here, we present the first high-resolution (∼3$\mathop{.}\limits^{\unicode{x02033}}$8, corresponding to ∼65 pc) molecular line observations of the nearby starburst galaxy M82 with the IRAM Northern Extended Millimetre Array. Notably, HC₃N (10-9) and H¹³CN (1-0) emission lines are brighter in the northeast part (NE lobe) than those in the southwest part (SW lobe) of M82, suggesting a higher accumulation of dense gas at the NE lobe. The spatial distributions of H41α and 3 mm continuum emission indicate that starburst activity varies across the central starburst disk, with more intense star formation occurring at the SW lobe compared to the NE lobe. The average optical depths of HCN and HCO⁺ (1-0) across the triple-peaked regions exhibit significant variation, with the highest values observed at the NE lobe. Our results suggest a potential evolutionary sequence in M82, where the distributions of star formation and dense gas appear to be partially decoupled-a phenomenon that classical dense gas tracers cannot adequately probe.