The distribution of HCO+ J=4-3 and HCN J=4-3 in the nuclear region of M82

We present the first detection of HCN J = 4-3 in the starburst galaxy M82 a nd a fully sampled grid of the distribution of its emission in the nuclear region. The angular resolution is 14" and the velocity resolution 6.3 km s( -1). The distribution of HCN emission is compared with that of HCO+ J = 4-3 made with the same resolution, and the conclusion is that the HCO+/HCN bri ghtness ratio is higher at J = 4-3 than at lower transitions, with a median ratio of 3.5. There is evidence for a variation in this ratio across the n uclear region, with higher values toward the edge of the nuclear disk. This variation is consistent with a variation in gas density, with higher densi ties associated with regions closer to the nucleus of M82. However, some of this variation could also be produced by a variation in the HCO+/HCN abund ance ratio. A large-scale velocity gradient (LVG) analysis of three transit ions in each molecule centered at the nucleus shows that the line ratios wi thin each species and between the two species are consistent with a common set of physical conditions for the emitting regions, namely, a kinetic temp erature of 50 K, gas density 10(5) cm(-3), and gas column density 10(22) cm (-2). Although it has been suggested that HCO+ may be subject to mechanisms of excitation other than collisions with H-2, such as electron collisions, our analysis shows that there is no reason to invoke any such mechanisms t o account for the line ratios present. The HCO+/HCN intensity ratio is know n to vary significantly from one galaxy to another, with M82 at the high en d of the spectrum of variation. This effect is briefly discussed in the con text of a bistable chemical equilibrium model in which the HCO+ abundance i s a sensitive function of the cosmic-ray ionization rate and gas density in the cloud cores. Within the framework of this picture, a high cosmic-ray f lux could be inversely related to HCO+ abundance, contrary to what is gener ally assumed.