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Volker Ossenkopf-Okada
Universität zu Köln

The need for FIR heterodyne observations to understand the Milky Way interstellar medium

In particular the SOFIA/upGREAT observations of the last years have shown that the traditionally used main cooling lines of the interstellar medium, [CII] at 158µm and [OI] at 63 µm, never just trace the conditions of their emitting region but their profile bears the information on the superposition of all components along the line of sight including large columns of absorbing gas. The nature of this gas is still puzzleing. The columns exceed those of neutral atomic and the expected CO-dark molecular gas. So far we do not have any significant statistics of this material because our observations were biased towards the brightest sources in the Milky Way. Systematic mapping is needed to really understand the transition between the neutral and the molecular phase that may be reflected in these absorbing components. The maps have to include the optically thin [13CII] and [OI] 145µm lines for a reliable analysis. A heterodyne receiver array covering the frequency range 1900-2070GHz and around 4745GHz on a balloon mission would allow us to obtain the required large-scale statistics to really assess the phase composition if the interstellar medium in the Milky Way including the large fraction of this dark absorbing component.