Astronomical imaging spectroscopy at far-infrared wavelengths

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Approximately half of the radiant energy emitted by the universe falls in the far-infrared (FIR) spectral range (301000 µm) for two reasons: the first is that sources in the distant universe, galaxies in the local universe, or protostars in our own galaxy are often shrouded in dust and gas. The dust efficiently scatters and absorbs shorter wavelength radiation, which is subsequently re-radiated at longer wavelengths, both as continuum (dust) and line (ions, atoms, molecules) emission. The second reason is that distant galaxies do not decrease in IR brightness with increasing distance, because their emission is redshifted into the IR. Most of the FIR is inaccessible from the ground due to atmospheric absorption. Moreover, space borne instruments must operate at ~ 4 K to minimize their self-emission, which would otherwise dominate the weak astronomical signal. In previous FIR missions cooling was achieved by placing the entire telescope in a cryostat, limiting the diameters of primary mirrors to ~60 cm and resulting in relatively low spatial resolution. The Herschel Space Observatory broke this trend by employing a 3.5 m diameter passively cooled primary mirror located outside of the instrument payload, the instrument suite being cooled to ~4K. This design provided a major advance in spatial resolution and sensitivity, however, the latter remained limited by photon noise from the relatively warm (~80K) telescope. Potential gains in sensitivity of 2 -3 orders of magnitude exist by actively cooling large aperture telescopes as proposed for the ESA/JAXA SPICA mission. However, to realize these gains, the noise performance of the instruments themselves must be understood to a corresponding higher degree. This proposal has two components, the first is to continue to use data from Herschel/SPIRE in the study of the interstellar medium and the earliest stages of star formation, the time when molecular clouds fragment into cold and dense gravitationally bound cores. Starless cores can be probed by studying their thermal (continuum) emission, which provides information about the dust properties, and their line emission (or absorption), which provides information on the chemistry in these regions. Together they provide a unique window into the initial conditions of protostellar collapse. With its broad spectral coverage and intermediate resolution SPIRE is well suited to this task. The second component is to develop and evaluate the performance of a prototype of the spectrometer proposed for the SPICA/SAFARI instrument. A recently commissioned, large volume, low background, test facility cryostat will be used to study optical and thermal properties of key components of the spectrometer such as etalons, mechanisms and metrology. The ability to evaluate the performance of an integrated system at cryogenic temperatures will position Canada to be a partner of choice in future FIR space astronomy missions.