Contributed Talk - Splinter ISM

Thursday, 24 September 2020, 09:00   (virtual room F)

Interpreting line emission in unresolved observations

V. Ossenkopf-Okada
I. Physics Institute, University of Cologne

The derivation of the physical properties of the ISM in external galaxies is usually based on the fit of line and continuum intensities and their ratios measured within a telescope beam that covers whole molecular clouds or even galaxies. The derived parameters are then interpreted as some average properties of the ISM within the beam. This assumption now can be verified based on existing large data sets that cover the dynamic range from a hundredth of a parsec up to the scales accessible in the extragalactic observations. Here we used the combination of several data sets covering the Orion-A molecular cloud complex at a resolution of about 10arcsec in an area of about 2 square degrees to provide a template for a region with active star-formation. Similar to Barnes et al. (2020) we find that the integrated emission of different tracers is dominated by different origin, not simply described in terms of a critical density. In particular for the low-J lines of CO isotopologues suprathermal excitation has to be taken into account. This allows to assess the density of the material. The very bright emission of regions like Orion KL is statistically insignificant when diluted in a large beam. On global scales, low density gas dominates the total intensity. For low-J lines of CO isotopologues, this is material with densities somewhat above 10^3 cm-3, for [CII] material at densities below 10^3 cm-3. The CO lines have a very weak temperature - and therfore UV field - dependence, while [CII] shows a strong dependence. [CI] measures material at higher temperatures and higher densities, NH3 lower temperatures at high densities, and so on. At the end we find that most of the emission of the different tracers stems from regions with different properties so that any interpretation of their ratios in terms of global properties will fail.