The nucleus of M82 has been mapped in several 3mm and 1mm lines of CN , HCN , C _ { 2 } H , c-C _ { 3 } H _ { 2 } , CH _ { 3 } C _ { 2 } H , HC _ { 3 } N and HOC ^ { + } using the IRAM 30m telescope .
These species have been purposely selected as good tracers of photon-dominated chemistry .
We have measured [ CN ] / [ HCN ] \sim 5 in the inner 650 pc galaxy disk .
Furthermore , we have detected the HOC ^ { + } 1 \rightarrow 0 line with an intensity similar to that of the H ^ { 13 } CO ^ { + } 1 \rightarrow 0 line .
This implies a [ HCO ^ { + } ] / [ HOC ^ { + } ] ratio of \sim 40 . These results corroborate the existence of a giant photo-dissociation 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 \sim a few 10 ^ { 4 } –10 ^ { 5 } cm ^ { -3 } ) clouds immersed in an intense UV field ( G _ { 0 } \sim 10 ^ { 4 } in units of the Habing field ) .
The detection of the hydrocarbons c-C _ { 3 } H _ { 2 } and CH _ { 3 } C _ { 2 } H in the nucleus of M82 suggests that a complex carbon chemistry is developing in this giant PDR .