2005ApJ...619L.155F

The nucleus of M82 has been mapped in several 3 and 1 mm lines of CN, HCN, C₂H, c-C₃H₂, CH₃C₂H, HC₃N, and HOC⁺ using the IRAM 30 m telescope. These species have been purposely selected as good tracers of photon-dominated chemistry. We have measured [CN]/[HCN]∼5 in the inner 650 pc galaxy disk. Furthermore, we have detected the HOC⁺ 1⟶0 line with an intensity similar to that of the H¹³CO⁺ 1⟶0 line. This implies an [HCO⁺]/[HOC⁺] ratio of ∼40. These results corroborate the existence of a giant photodissociation region (PDR) in the nucleus of M82. In fact, the low [HCO⁺]/[HOC⁺] ratio can only be explained if the nucleus of M82 is formed by small (r<0.02-0.2 pc) and dense (n∼a few times 10⁴-10⁵/cm3) clouds immersed in an intense UV field (G₀∼10⁴ in units of the Habing field). The detection of the hydrocarbons c-C₃H₂and CH₃C₂H in the nucleus of M82 suggests that a complex carbon chemistry is developing in this giant PDR.