Photon-dominated chemistry in the nucleus of M82:: Widespread HOC+ emission in the inner 650 parsec disk

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] similar to 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 (HCO+)-C-13 1 --> 0 line. This implies an [HCO+]/[HOC+] ratio of similar to40. 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 10(4) - 10(5) cm(-3)) clouds immersed in an intense UV field (G(0) ∼ 10(4) in units of the Habing field). The detection of the hydrocarbons c-C3H2 and CH3C2H in the nucleus of M82 suggests that a complex carbon chemistry is developing in this giant PDR.