Photon-dominated Chemistry in the Nucleus of M82: Widespread HOC ⁺ Emission in the Inner 650 Parsec Disk

Abstract

The nucleus of M82 has been mapped in several 3 and 1 mm lines of CN, HCN, C2H, c-C3H2, CH3C2H, HC3N,
\nand HOC using the IRAM 30 m telescope. These species have been purposely selected as good tracers of
\nphoton-dominated chemistry. We have measured [CN]/[HCN] ∼ 5 in the inner 650 pc galaxy disk. Furthermore,
\nwe have detected the HOC 1 r 0 line with an intensity similar to that of the H13CO 1 r 0 line. This implies
\nan [HCO ]/[HOC ] ratio of ∼40. These results corroborate the existence of a giant photodissociation region
\n(PDR) in the nucleus of M82. In fact, the low [HCO ]/[HOC ] ratio can only be explained if the nucleus of
\nM82 is formed by small (r ! 0.02–0.2 pc) and dense (n ∼ a few times 104–105 cm 3) clouds immersed in an
\nintense UV field ( in units of the Habing field). The detection of the hydrocarbons c-C3H2 and CH3C2G ∼ 104 H 0
\nin the nucleus of M82 suggests that a complex carbon chemistry is developing in this giant PDR.