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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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Laboratory for Astronomy and Solar Physics (LASP) at NASA Goddard Space Flight Center
Responsible NASA Official: Dominic Benford,