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SCIENTIFIC ABSTRACT One the most exciting discoveries of the IRAS mission was that a significant number of galaxies emit a large fraction (up to 90%) of their energy in the far-IR. With luminosities exceeding 10^12 L(solar) these IR bright (IRB) galaxies may represent the initial dust enshrouded stages of quasar formation. IRBs are an important component of the extragalactic sky at all luminosities, and are the single most numerous class of objects in the local universe with luminosities in excess of 10^11 L(solar). What is the source of this prodigious far-IR energy? For some galaxies, it is clear the energy source is a tremendous burst of star formation. For others, the far-IR luminosity may be the outward ramification of an obscured active galactic nucleus (AGN). Many of these galaxies appear to be in collision or merger, suggesting that the far-IR emission is the cooling radiation of galaxy-galaxy collisions. We propose LWS spectroscopic measurements of a representative set of these IRBs to shed light on the nature of these objects, with special attention to the origins of the far-IR luminosity. Detailed studies of the line intensity ratios will (1) determine the physical conditions of the interstellar medium (ISM) in IRBs including abundances, hence the degree of processing of the ISM (2) reveal the spatial extent and age of any starburst (3) determine the hardness of the UV radiation field in these galaxies and (together with SWS GT Proposal observations) distinguish the origins of the field, be it AGN or starburst, and (4) determine the contribution from galaxy-galaxy collisions to the far-IR luminosity. By striving for the highest signal-to-noise thought possible, we will also be sensitive to the presence of low level emission of molecular species such as H20, CO, and OH. We will also use the SWS to observe [FeII] 26.0 um and [FeIII] 22.9 um line emission in a selected set of interacting galaxies with prominent circumnuclear starbursts. These observations are designed to constrain the ionization equilibrium of gas phase iron in the starbursts, and to explore the effect of supernova activity and star formation on [Fe II] and [Fe III] line emission in interaction induced starbursts. OBSERVATIONAL SUMMARY. Our scientific program requires detection of more than half a dozen bright fine-structure lines. Therefore, for observational efficiency, we will take full LWS01 scans on most of our objects. Our LWS01 program is separated into two primary sections: detailed mapping of a few nearby representative IRB galaxies, and LWS01 scans of the nuclei of a sample of ultraluminous, IR bright galaxies. Our first priority is full LWS grating scans of several representative IRB galaxies including objects known to be powered by starbursts (e.g. M82), objects thought to contain embedded AGNs (e.g. Arp 220) and colliding galaxy pairs (e.g. IC694/NGC 3690). Our Priority 1 also includes the ultraluminous "protogalaxy" IRAS1021+4724. These full scans and the full scan maps outlined below will provide a template by which we may compare the more distant or enigmatic sources. Our primary diagnostic lines include [CII] 158 um, [OI] 63 and 146 um, [NII] 122 um, [OIII] 52 and 88 um, and [NIII] 57 um (Table 1). The first three lines trace the physical conditions of the atomic and photodissociated molecular gas and together with photodissociation region models constrain the strength of the local UV radiation field. The [NII] line traces the diffuse ionized medium, and is used to discern the fraction of [CII] emission which arises from this low density ionized gas. The [OIII] pair are a somewhat higher density tracer for the ionized gas, and together with the [NIII] and [NII] lines, they trace the O/N abundance ratio, hence, age of the interstellar medium. The [NIII]/[NII] line intensity ratio is a sensitive indicator of the hardness of the UV radiation field when T(eff) 33,000 K. This pair thus indicates the upper mass limit (or, age) of any purported starburst. Our [OIII] data, when combined with SWS GT observations of the [OIV] 26 um line will be a sensitive indicator of the very hard UV radiation fields associated with AGNs. Finally, clear signatures of the shocks associated with galaxy-galaxy collisions include a high [OI]/[CII] ratio, and detectable [SI] 56 um and/or CO (14-13) 186 um radiation. Naturally, our LWS01 spectra may also reveal unexpectedly strong lines from other important species. Of special interest would be strong H20 line emission - an indicator of warm dense molecular gas (shocked or UV exposed). For the Priority 1 objects, we set the integration time so that we would detect the weakest of our seven priority lines (likely [NIII]) with SNR > 7 (except in the case of the "protogalaxy" IRAS 1021+4724, where the signal-to-noise ratio achieved on the continuum is 2-10). Line flux estimates were based on typical observed KAO [CII] line-to-continuum ratios and standard models of the ISM. Our Priority 2 observations consist of LWS01 full scans of several ultraluminous galaxies. Priority 3 includes