| Description |
Stars form in dense, cold clouds of molecular hydrogen (H2), best observed in lines of the tracer carbon monoxide (CO). As stars turn on, the dense gas is heated, ionisation occurs, and a photon-dominated region (PDR) is formed. Here, CO dissociates to neutral carbon, much of which is subsequently ionized. Both neutral and ionized carbon cool the interstellar gas by strong line emission at far-infrared-sub-millimeter wavelengths. Thus, CO emission is a good tracer for cold and dense, [CI] for warm dense, and [CII] for warm tenuouis gas. Cold tenuous gas is traced by neutral atomic hydrogen emission. Galaxies have different metallicities and gradients, strongly affecting the physical processes ruling the heating and cooling of the interstellar medium (ISM). In order to understand the ISM in distant galaxies, we need to understand how metallicities and radiation fields affect ISM physics. By observing the ISM in the low-metallicity Large and Small Magellanic Clouds, the response of the ISM to various radiation fields is gauged as a function of the very different metallicities of the SMC, the LMC, and the Milky Way. We have selected in the LMC and the SMC interstellar clouds exposed to a range of radiation field intensities. Ideally, we would fully map these clouds, but time constraints limit us to strips in lines requiring long integration times: the 1.9 THz [CII] and 809 GHz [CI] lines, which are in effect unobservable from the ground, requiring Herschel space-borne observations, to complement a series of CO and [CI] (492 GHz) observations from the ground. Together with HI maps, they will provide us with the carbon and hydrogen budgets, and determine, in detail, the distribution of the various carbon phases over a wide range of phyical conditions. Analysis of the HI-[CI]-[CII]-CO-H2 interfaces is well-feasible thanks to the advanced ISM-PDR codes developed by members of our team. |
