How galaxies convert their gas reservoirs into stars is a stumbling block in understanding galaxy evolution. The far IR and submm cooling lines arising from photodissociation regions (PDRs) are the key diagnostics of the interplay between star formation and the evolving ISM throughout the course of the history of the universe. While effort has been put into physics of PDRs and star formation in the Galaxy and other metal-rich galaxies, little attention has been put into understanding the effects of the lower metal abundance on the these processes. Here we propose PACS ([CII] 158 um, [OI] 63 um, [OI] 145 um, [OIII] 88 um, [NII] 122 um) and SPIRE FTS spectroscopy towards carefully selected star forming sites in the nearest low metallicity galaxies, the Magellanic Clouds (MCs) as a benchmark study to calibrate the primary diagnostics of low metallicity environments which are crucial for ALMA high redshift science. The proximity of the MCs allows Herschel to resolve molecular clouds at 3 pc scale. Our targeted regions span a wide diversity of environments, including dense molecular gas and ionised regions. These new data, together with existing IRS spectra, Herschel and Spitzer photometry, and a wealth of groundbased data including the MAGMA CO data, will allow us to address important unresolved issues: 1) the amount of molecular gas hidden in a low extinction phase where CO is photodissociated and thus not detected; 2) the dependence of the FIR fine structure lines, CO excitation and gas thermodynamics on environment; and 3) the structure of PDRs at low metallicity. Our results will be key to interpreting FIR fine structure lines and CO observations of more distant systems. We will produce PDR models tested at low metallicity, a library of CO excitation at low metallicity, and a calibration of the amount of COfree molecular gas as a function of environment. In addition, multiline maps covering the main coolants of the a...tomic and molecular gas, represent a data product with lasting legacy value.
Herschel was launched on 14 May 2009! It is the fourth 'cornerstone' mission in the ESA science programme. With a 3.5 m Cassegrain telescope it is the largest space telescope ever launched. It is performing photometry and spectroscopy in approximately the 55-671 µm range, bridging the gap between earlier infrared space missions and groundbased facilities.
