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The history of star formation determines the evolution of galaxies and the
generation rate of production of for heavy elements. It has been traced by
deep Hubble Space Telescope (HST) imaging followed up with large
ground-based telescopes. Even at modest redshifts, however, these
techniques only probe the rest-frame ultraviolet. Here, SAFIR will make a
critical contribution. Interstellar dust can absorb nearly all the UV in
star-forming galaxies. In the best-studied starburst galaxies such as M82,
a debate raged for more than a decade on how to correct even the
near-infrared emission for interstellar extinction. Such corrections are
poorly determined for galaxies at high redshift, so there are large
uncertainties in the star-forming rate for z>1 when most of the heavy
elements were created. These uncertainties can be removed only by
measuring the far-infrared emission from dust heated by young stars in
these galaxies. The importance of this approach is underlined by the large
cosmic far-infrared and submillimeter energy density discovered by COBE.
This background has been partially resolved by ISO in the very far-infrared
and is thought to arise from starburst galaxies at redshifts of up to z~3.
SAFIR will resolve most of this highredshift background into individual
galaxies. This will allow us to image the dominant phases of dust embedded
star formation and nuclear activity throughout the Universe. Since
ultradeep optical images (e.g., Hubble Deep Field) reveal many galaxies too
faint to contribute significantly to the submillimeter diffuse background,
an entirely new population of optically faint, young galaxies must be
responsible for it. A full understanding of star formation in the early
Universe requires that we extend far-infrared and submillimeter
measurements to these small systems. In this respect, SAFIR is crucial for
understanding the processes by which primordial structure in the Universe
leads to the first galaxies in it. In this luminosity range and over
1<z<5, ALMA and other ground-based submillimeter telescopes are mostly
sensitive to widespread and diffuse infrared cirrus emission which is
thought to be the output of cold dust grains that are not necessarily
heated by recent star formation. The rate of star formation in modest
galaxies for 1<z<5 can best be determined through high sensitivity imaging
from 20 to 500um. In addition to reliable detection of a galaxyŐs infrared
output, quantitative measurements of star formation require a reliable
distance to it. Combining SAFIR and ALMA measurements of SEDs will give
photometric redshift estimates, and SAFIR spectroscopy can measure
redshifts directly using the strong PAH features in galaxy spectra near
8m. In this way the sensitivity of SAFIR will allow us to measure galaxy
luminosities to below L*, even out to z ~ 5. Equally important is the
resolution afforded by a 10m aperture; precursor farinfrared missions like
SIRTF will be limited in this science objective by confusion noise.
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