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Dense gas is important for galaxy evolution and star formation.Optically-thin dense-gas tracers, such as isotopologues of HCN, HCO+, etc., arevery helpful to diagnose excitation conditions of dense molecular gas. However,previous studies of optically-thin dense-gas tracers were mostly focusing onaverage properties of galaxies as a whole, due to limited sensitivity andangular resolution. M82, a nearby prototype starburst galaxy, offers a uniquecase for spatially-resolved studies with single-dish telescopes. With the IRAM30-m telescope, we observed the J = 1 - 0 transition of H13CN, HC15N, H13CO+,HN13C, H15NC, and SiO J = 2 - 1, HC3N J= 10 - 9, H2CO J = 2 - 1 toward fivepositions 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 majoraxis, with lower values in the central region than those on the disk,indicating higher optical depths in the central region. The optical depths ofHCO+ lines are found to be systematically higher than those of HCN lines at allpositions. Futhermore, we find that the 14N/15N ratios have an increasinggradient from the center to the outer disk.

Properties of dense molecular gas along the major axis of M 82