We present a high-resolution ( \sim 5 ” ) image of the nucleus of M 82 showing the presence of widespread emission of the formyl radical ( HCO ) .
The HCO map , the first obtained in an external galaxy , reveals the existence of a structured disk of \sim 650 pc full diameter .
The HCO distribution in the plane mimics the ring morphology displayed by other molecular/ionized gas tracers in M 82 .
More precisely , rings traced by HCO , CO and HII regions are nested , with the HCO ring lying in the outer edge of the molecular torus .
Observations of HCO in galactic clouds indicate that the abundance of HCO is strongly enhanced in the interfaces between the ionized and molecular gas .
The surprisingly high overall abundance of HCO measured in M 82 ( X ( HCO ) \sim 4 10 ^ { -10 } ) indicates that its nuclear disk can be viewed as a giant Photon Dominated Region ( PDR ) of \sim 650 pc size .
The existence of various nested gas rings , with the highest HCO abundance occurring at the outer ring ( X ( HCO ) \sim 0.8 10 ^ { -9 } ) , suggests that PDR chemistry is propagating in the disk .
We discuss the inferred large abundances of HCO in M 82 in the context of a starburst evolutionary scenario , picturing the M 82 nucleus as an evolved starburst .