LEGO: Wide–Field Mapping of Milky Way Clouds to understand Extragalactic Line Emission

Jens Kauffman

Thursday December 4th, 17:20

Modern instruments like NOEMA, ALMA, and the IRAM 30m–telescope now allow us to probe molecular emission from other galaxies on a regular basis. Past work focussed on bright lines of, e.g., CO and HCN, but investigations are now increasingly moving on to much fainter species. Line intensities and their ratios provide provide us with unique tools to assess, for example, the gas temperatures and densities in other galaxies. No other means exist to extract many of these critical parameters. It is therefore essential that we truly understand the physics driving line emission from molecular clouds. Our actual knowledge about the drivers of molecular line emission is, however, extremely limited. Consider HCN, a molecule that is used as a workhorse tracer of dense gas in galaxies. Current extragalactic research usually assumes that emission this molecular traces gas at densities of order 10^5 cm^-3. First wide–field imaging surveys in the Milky Way, however, demonstrate that this molecule is actually associated with gas of typical densities of order 10^3 cm^-3. This difference of two orders of magnitude has immediate implications for our understanding of star formation efficiencies in galaxies. LEGO, the first wide–field molecular line imaging survey systematically covering all environments in the Milky Way, currently generates the first comprehensive data set available to study how molecular line emission couples to the conditions in a molecular cloud. I present first results from the IRAM Large Program (about 600h allocated). The data can inform our future analysis of extragalactic molecular clouds, and they tell us important lessons about the formation and dispersion of molecular clouds in the Milky Way.