HCO+ in the Starburst Galaxy M82

We report observations and an analysis of the distribution of HCO+ (1-0) and HCO+ (4-3) emission in the central 1 kpc star-forming region of M82. Comparisons are made with other star formation indicators such as the millimeter continuum, the distribution of radio supernova remnants, and the molecules CO and OH. In a broad sense, the HCO+ is distributed in a way similar to the CO, although there are noticeable differences in detail, including an inward displacement of spiral arm emission relative to CO. A comparison of the position-velocity plots for CO, HCO+ (1-0), HCO+ (4-3), and ionized gas, with orbits expected in the presence of the nuclear bar, suggest an inward transfer of gas associated with star formation toward the nucleus. The HCO+ (4-3)/(1-0) line ratios are comparatively uniform in the observed region, and according to a large velocity gradient analysis, reflect mean gas densities in the range 104-105 cm-3 for kinetic temperatures in the range 20-60 K. The comparative uniformity of these conditions and the low filling factor suggest that each sampled point comprises a large number of clouds occupying a broad range of density, and possibly temperature. We briefly examine fractal-type models in the context of the HCO+ data as an alternative way to analyze molecular line emission in M82.