1994A&A...284L..23W

The first map of an external galaxy in the ³P₁-³P₀ fine-structure line of atomic carbon (CI) is presented towards M82, and compared to the distribution of the CO J=4-3 molecular line. The CI traces features that are seen in lower transition CO maps, and shows that CI and the CO are well mixed and have similar spatial distributions. There are small differences between the CO J=4-3 line and lower transition data towards the NE part of the molecular ring, where the emission is less prominent. The abundance ratio [CI]/[CO] across M82 is very high, having an average value ∼0.5 across most of the molecular core, a factor at least 5 times that which is typical of dense molecular cloud cores seen in our own Galaxy. This means that on average, CI is overabundant towards M82. This result can be explained using models which provide enhancements to the CI abundance above normal Interstellar Medium values, possibly as a result of a greater cosmic ray flux in M82, or where there is substantial mixing of the gas.