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How do the first gas clouds form? What chemical processes occur within
them and how do their characteristics change as the first traces of metals
are injected into them by stellar processing? The capabilities of SAFIR
are ideal for addressing these questions. Once even traces of metals have
formed, the C+ line at 158m becomes very bright. Its luminosity in nearby
spiral galaxies is typically a few tenths of a percent of the entire
bolometric luminosity of the galaxy. Although this line is marginally
accessible in the poor atmospheric windows between 300 and 700um, it can be
routinely observed from the ground only at specific redshifts z>4. The N+
lines at 122 and 205um also play important roles in cloud cooling. Study
of the molecular hydrogen and these light metal fine structure emission
lines in the early Universe and as a function of redshift promises to
reveal many of the processes occurring in the gas clouds that build early
galaxies. Space-borne observations in the far-infrared and submillimeter
with an optimized observatory like SAFIR must be a major component of this
study. The far-infrared fine structure lines also control the cooling of
molecular clouds in the Milky Way. Understanding this process and related
ones revealed by far-infrared spectroscopy is a key to advancing our
knowledge of how these clouds begin their collapse into stars and planets
(see below).
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