2022ApJ...933..139L

Dense gas is important for galaxy evolution and star formation. Optically thin dense-gas tracers, such as isotopologues of HCN, HCO⁺, etc., are very helpful in diagnosing the excitation conditions of dense molecular gas. However, previous studies of optically thin dense-gas tracers mostly focused on the average properties of galaxies as a whole, due to limited sensitivity and angular resolution. M82, a nearby prototype starburst galaxy, offers a unique case for spatially resolved studies with single-dish telescopes. With the IRAM 30 m telescope, we observed the J = 1 - 0 transition of H¹³CN, HC¹⁵N, H¹³CO⁺, HN¹³C, H¹⁵NC, and SiO J = 2 - 1, HC₃N J = 10 - 9, and H₂CO J = 2 - 1 toward five positions along the major axis of M82. The intensity ratios of I(HCN)/I(H¹³CN) and I(HCO⁺)/I(H¹³CO⁺) show a significant spatial variation along the major axis, with lower values in the central region than those on the disk, indicating higher optical depths in the central region. The optical depths of HCO⁺ lines are found to be systematically higher than those of HCN lines at all positions. Furthermore, we find that the ¹⁴N/¹⁵N ratios have an increasing gradient from the center to the outer disk.