HCO+ in the Starburst Galaxy M82

Abstract

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 mm continuum, the distribution of radio SNR's 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 an LVG analysis, reflect mean gas densities in the range 10^4 cm^{-3} - 10^5 cm^{-3} for kinetic temperatures in the range 20K - 60K. The comparative uniformity of these conditions and the low filling factor suggest that each sampled point comprises alarge 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.