The LEGO Large Program: Constraining the Physics of Line Emission in Galaxy Observations

The IRAM Large Program LEGO studies molecular clouds in the Milky Way to constrain the physics controlling molecular line emission in galaxies. This is done by imaging two dozen clouds with setups that provide continuous spectral coverage of about 85–115 GHz. To give one example, research in this area permits to “calibrate” extragalactic observations of molecules like HCN and N 2 H + to explore how the star formation activity in galaxies depends on their dense gas contents (i.e., Gao & Solomon relation). Interestingly, LEGO and other studies now reveal a substantial cloud-to-cloud variation in line ratios. Once understood properly, this diversity can be used to constrain the properties of extragalactic molecular clouds at great detail. Here we outline the LEGO sample and describe the project status.