| 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. |
