Exploring the central molecular zone of the Galaxy using spectroscopy of H3+ and CO

Abstract

The central 400 parsecs of the Milky Way, a region known as the central molecular zone (CMZ), contains interstellar gas in a wide range of physical environments, from ultra-hot, rarified and highly ionized to warm, dense and molecular. The combination of infrared spectroscopy of H3+ and CO is a powerful way to determine the basic propertiesof molecular interstellar gas, because the abundance ratio of H3+ to CO in 'dense' clouds is quite different from that in 'diffuse' clouds. Moreover, the energy-level structure and the radiative properties of H3+ combined with the unusually warm temperatures of molecular gas in the CMZ make H3+ a unique probe of the physical conditions there. This paper describes how, using infrared absorption spectroscopy of H3+ and CO, it has been discovered that a large fraction of the volume of the CMZ is filled with warm, diffuse and partially molecular gas moving at speeds of up to approximately 200kms-1 and that the mean cosmic ray ionization rate in the CMZ exceeds by roughly an order of magnitude values found in diffuse molecular clouds elsewhere in the Galaxy. © 2012 The Royal Society.