Because of the great sensitivity advance provided by SAFIR, the scientific potential is tremendous. It may not be possible to accurately forecast the mission's most important contributions some two decades before its launch. But we know that SAFIR will provide unique capabilities to address key questions about the formation of stars, galaxies, and planets that will likely remain unanswered until its launch.

1. When And How Did The First Stars Form?
   
   Stars form when immense clumps of interstellar gas (mostly hydrogen) and dust contract under their own gravitational pull until they become so hot that they ignite in nuclear fusion. Though it may seem counterintuitive, these clouds have to maintain very cold temperatures as they collapse.
   
   As gravity pulls the cloud's atoms and molecules in toward each other, they collide more and more frequently, heating the cloud and increasing its outward pressure. If this energy could not be radiated away, the outward pressure would eventually counter the inward gravitational pull, and the cloud would stop collapsing before it ingites as a star.
   
   Gas comprised purely of hydrogen and helium is poor at radiating energy at the temperatures of collapsing interstellar clouds (50-500 K), but a small fraction of certain heavier elements such as carbon, nitrogen, oxygen, and silicon mixed in with the gas do a much better job. When a collision causes one of these "cooling agents" to rotate, vibrate, or jump to an excited electronic state, it quickly emits a photon and returns to a less energetic state. By this process, energy is removed from the gas, and the cloud is able to continue its collapse into a star. Even though they represent a small percentage of a cloud's makeup, these cooling agents are critical to star formation.
   
   But the heavier elements that effectively cool the gas are manufactured in stars, and released to the interstellar medium when the stars die. So when the very first stars formed, there were no heavy elements. The question of how stars formed without cooling agents is the subject of much study, to which SAFIR is expected to contribute a great deal.
   
   While the entire process is not well understood, theoretical models suggest that hydrogen molecules were able to radiate enough energy to allow the original gas clouds to collapse into stars. The most important spectral lines at which they emitted energy are believed to be the pure rotational transitions of hydrogen (that is, the wavelengths at which energy is radiated away when hydrogen molecules that are set spinning by collisions with other molecules give up their rotational energy) at 28, 17, 12, 9.6, 8.1, and 6.9 microns.
   
   As more material accreted onto the growing cores that would become stars, the temperatures rose and the infrared continuum (a broad band of infrared wavelengths) and ro-vibrational transitions (energy given off by molecules that were both rotating and vibrating) of H₂ at wavelengths around 2 µm are thought to have become important coolants.
   
   This emitted light has been redshifted with the expansion of the universe, and would be observed between 40 and 1000 µm today. Spectroscopy with SAFIR will provide a unique platform to observe the formation of the universe's first protostellar condensations from primordial gas.



2. When Did Galaxies Begin Forming, And What Is The History Of Galaxy Evolution And Energy/Element Production In The universe?
   
   The detailed history of galaxy formation is only beginning to be addressed. But according to current theories, after the first stars enriched the ISM with metals, smaller objects coalesced into larger ones, eventually forming the progenitors of present-day galaxies. Once they began to be generated in stars, heavy elements (C, N, O, Si, S, Fe) and the associated dust dominated the energetics of the ISM, and were crucial players in subsequent evolution.
   
   As with present-day luminous galaxies, the presence of dust means that much of the radiation emerges in the far-infrared, even though the primary energy sources emit light in the optical and UV.
   
   Taken together, all the dusty star-forming galaxies at all redshifts have generated a diffuse far-IR background light. This far-IR background is comparable in intensity to the optical / near-IR background, showing that the relatively unexplored far-IR part of the spectrum is important for understanding galaxy evolution.
   
   Recently, the first steps have been taken to resolve this background light into individual sources -- apparently star-forming galaxies at redshifts of 1-4 (when the universe was between 1.5 billion and 6 billion years old). The figure to the right shows an example of these observations. The measurements were taken at a wavelength of nearly 850 microns, or almost 1 µm, and the bright blobs are the distant galaxies. Follow-up work on these and other similar detections has enabled scientists to determine the distance of a few of these galaxies. Taken together with studies at optical wavelengths, the results suggest that energy production in the universe was most vigorous when the universe was from two to three billion years old (compared to its current age of nearly 14 billion years).
   
   In this decade, the Spitzer and the Herschel Space Observatory missions will reveal thousands more sources as they map the continuum emission to the limits of their resolving powers. These continuum surveys alone, however, cannot measure the sources' redshifts (their distances from earth and consequently their distances back in time as seen from earth) or physical conditions. That requires spectroscopy. Spectroscopy can reveal how luminosity varied over time, and whether starburst or black hole accretion is responsible for the energy production of a representative sample of galaxies. The figure shows a handful of far-IR spectra of a few nearby galaxies.
   
   
   The overall shape of each spectrum is determined by the temperature of the dust. These galaxies typically have dust temperatures in the range of 30-50 K, which generates a broad peak in the spectrum at around 50-100 microns wavelength. The sharp features, called spectral lines, occur in both emission (positive peaks) and absorption (negative peaks) and are due to atomic and molecular species in the gas. The dust continuum and the lines together measure the redshift (or distance) of a galaxy, its total luminosity (energy output) and probe the source of the luminosity, either stars or black holes.
   
   At present, the handful of nearby galaxies shown are just about the only ones for which we have spectra. The upcoming Spitzer and Herschel mission will improve this situation somewhat, but as is always the case for any given platform, continuum detection is much easier than spectroscopy. The spectrometers on Herschel and Spitzer will only be sensitive to the brightest of the galaxies detected in their continuum surveys.
   
   With the tremendous gains afforded by its large, cold mirror, SAFIR will be capable of spectroscopy of distant galaxies that Herschel and Spitzer will have detected in their continuum surveys, but will have been unable to resolve with their own spectrometers.
   
   The result of a systematic spectroscopy program will be an unambiguous history of energy and element production since the first galaxies were formed, and a detailed understanding of the origin of present-day galaxies.



3. What Is The Nature Of The Interaction Between Black Holes And The Material In The Galaxies That Host Them?
   
   Nearly all galaxies are thought to harbor black holes at their centers. And surprisingly, many of these black holes are responsible for enormous outputs of energy. While it's true that nothing -- not even light -- can escape from black holes, huge amounts of energy are released when matter spirals furiously into them. The greater the rate at which matter from the host galaxy falls into a black hole, the more energy is released and the brighter it appears.
   
   For most nearby galaxies, the central black hole is quiescent, that is, very little matter is falling onto the black hole. The galaxy's luminosity is dominated by star formation at locations well outside the central few parsecs (one parsec equals about 3.26 light years). In galaxies called Seyfert or AGN galaxies however, the luminosity is greatest within the central few parsecs of the very center (hence the term, "AGN," which stands for Active Galactic Nucleus). The energy source for AGN galaxies has traditionally been believed to be accretion onto the central black hole, though it has also been suggested that very compact areas of intense star formation known as "starbursts" could contribute substantially to the luminosity.
   
   In general, the mechanism by which some galaxies feed their nuclear black holes while others do not is not well understood. A dominant model for Seyfert galaxies incorporates a dense torus of material around the central black hole, which obscures the very central object at optical wavelengths. If true, this circumnuclear torus may be an important intermediate step in the delivery of gas from the host galaxy to its nuclear black hole.
   
   Validating the torus model in nearby Seyfert galaxies requires SAFIR's arcsecond spatial resolution (what it would take to see a dime at a distance of 2.3 miles) to distinguish the torus from the gas of the host galaxy. At this spatial resolution, spectroscopy of H2, CO and a host of other species at wavelengths between 10 and 300 µm can confirm or deny the presence of a dense torus.
   
   Closer to home, the center of our own Galaxy offers, at very high spatial resolution, an excellent probe of the general processes at work in galactic nuclei. Because of our position in the plane of the Galaxy, intervening dust obscures our view of the Galactic Center at optical and near-infrared wavelengths. So far-infrared and submillimeter wavelengths are best for mapping structures and measuring conditions at the Galactic Center. SAFIR's arcsecond spatial resolution at these wavelengths corresponds to 0.04 parsec (pc). This is sufficient to distinguish the central source, SGR A*, presumably a black hole with a modest accretion disk, from the surrounding circumnuclear disk and infalling streamers.



4. How Do Solar Systems Form?
   
   Detailed study of the formation of planetary systems has been challenging. Forming planetary systems tend to be about 100 AU across (an AU, or Astronomical Unit, is the distance between the earth and the sun) and the typical distance to a nearby protoplanetary system is about 100 pc. To study something that size at that distance requires spatial resolution of around one arc second. SAFIR will, for the first time, provide this capability at mid- and far-infrared wavelengths, which are the most important for dusty protoplanetary disks.
   
   To determine how planetary systems form, we need to measure the temperature and size of dust grains and the molecular state of gas at various distances (from 1-1000 AU) from their central protostar. This requires multiwavelength imaging and spectroscopy at high spatial resolution in the mid- and far-infrared.
   
   While moderate-resolution spectroscopy is sufficient for measuring the gas and dust composition of a collapsing interstellar cloud, very high resolution spectroscopy is essential for measuring the dynamics -- that is, how the collapse is physically taking place.
   
   Spectroscopy determines the velocity of an object by measuring the Doppler shift of the light it emits. Just as a train whistle sounds higher as it approaches and lower as it recedes, the wavelength of light from a cosmic object is shorter if the object is approaching us and longer if it's moving away. A spectrum of the object shows its characteristic emission and absorption lines shifted to shorter or longer wavelengths due to its motion toward or away from earth.
   
   In a star-forming cloud, these shifts are very subtle because the gas and dust are moving slowly by cosmic standards, as little as one kilometer per second or less (or about one millionth the speed of light). Extremely high spectral resolution is needed to measure these motions (R ~ 1,000,000). Only heterodyne spectrometers have the resolving power required for detailed observations of the motions within collapsing protostars.
   
   In our own solar system, Kuiper Belt Objects (KBOs) make excellent probes of the early solar nebula. The Kuiper Belt is a region beyond Neptune's orbit that is the source of short-period comets like Halley. (There is some argument about whether Pluto should be classified as a full-fledged planet or merely a Kuiper Belt Object.) Unlike the planets and asteroids in the inner solar system, KBOs formed slowly with little or no processing. SAFIR will have the sensitivity to measure albedos (reflectivities) and surface temperatures of KBOs at far-IR wavelengths, key parameters in determining their composition and surface chemistry.



5. Which Pre-Biotic Molecules Are Present In Planet-Forming Regions, And What Are Their Abundances?
   
   Sensitive spectroscopy of protostellar disks in the far-infrared and submillimeter spectral regime can reveal the presence of large, organic molecules, which are uniquely identifiable through their torsional and floppy vibrational modes (the wavelengths at which they emit photons to give up the energy they acquire when collisions with other molecules set them twisting or bending).
   
   

   This figure is reproduced with permission of its author, R. Ruiterkamp of Lieden Observatory of The Netherlands.

   These building blocks of life are thought to exist in the material from which planetary systems form, and it is hoped that the Herschel Space Observatory may detect some of the most abundant of these molecules (glycine is a favored candidate) in the most nearby disks.
   
   SAFIR's improved sensitivity, however, will allow the first systematic approach to the measurement of several key molecules in dozens of protoplanetary systems throughout our region of the Galaxy. With these capabilities, SAFIR can perhaps be deemed the first true astrobiology observatory.