Photon dominated chemistry in the nucleus of M82: Detection of HOC + in a starburst galaxy

Abstract

The nucleus of M82 has been mapped in 3mm and 1mm lines of CN, HCN, C 2H, c-C3H2, CH3C2H, HC3N and HOC+ using the IRAM 30m telescope. These species have been selected for constituting a good diagnostic of photon-dominated chemistry. We have measured [CN]/[HCN] ratios ≥ 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 a [HCO+]/[HOC+] ratio of ∼ 40. This low HCO+/HOC+ abundance ratio unambiguously shows that the chemistry of the molecular gas reservoir in the nucleus of M82 is determined by the intense interstellar UV flux (G0 ∼ 104 in units of Habing field). Finally, the detection of the hydrocarbons c-C3H 2 and CH3C2H in the nucleus of M82 suggests that a complex carbon chemistry is developing in this giant photo-dissociation region (PDR).