Astronomical imaging spectroscopy at far-infrared wavelengths

宇宙发射的辐射能量大约有一半落在远红外(FIR)光谱范围(301000微米)，原因有两个：第一个原因是，遥远宇宙中的源、本地宇宙中的星系或我们银河系中的原恒星经常被尘埃和气体所笼罩。尘埃有效地散射和吸收较短波长的辐射，然后以连续(尘埃)和线(离子、原子、分子)发射的形式在较长波长重新辐射。第二个原因是，遥远星系的红外亮度不会随着距离的增加而降低，因为它们的发射是红移到红外的。*由于大气吸收，大部分FIR无法从地面进入。此外，星载仪器必须在~4K下工作，以最大限度地减少其自身发射，否则将主导微弱的天文信号。在以前的FIR任务中，冷却是通过将整个望远镜放在低温恒温器中实现的，将主镜的直径限制在~60厘米，导致空间分辨率相对较低。赫歇尔空间天文台打破了这一趋势，使用了直径3.5米的被动冷却主镜，该主镜位于仪器有效载荷之外，仪器套件冷却到约4K。这种设计在空间分辨率和灵敏度方面有了很大的进步，然而，后者仍然受到来自相对温暖(~80K)望远镜的光子噪声的限制。按照欧空局/日本宇宙航空研究开发机构Spica任务的建议，通过主动冷却大口径望远镜，可能会使灵敏度提高2-3个数量级。然而，为了实现这些收益，必须对仪器本身的噪声性能进行相应的更高程度的了解。*这项提议有两个组成部分，第一个是继续使用赫歇尔/SPIRE的数据来研究星际介质和恒星形成的最早阶段，即分子云分裂成冷而致密的引力束缚核心的时间。没有恒星的核心可以通过研究它们的热(连续)发射和线发射(或吸收)来探测，热(连续)发射提供了关于尘埃性质的信息，线发射(或吸收)提供了这些区域的化学信息。它们共同为了解原恒星崩塌的初始条件提供了一个独特的窗口。SPIRE具有广泛的光谱覆盖范围和中等分辨率，非常适合这项任务。*第二个组成部分是开发和评估为Spica/Safari仪器提议的光谱仪原型的性能。最近投入使用的大体积、低本底的低温恒温器将用于研究光谱仪关键部件的光学和热性质，如标准具、机制和计量。评估集成系统在低温下的性能的能力将使加拿大成为未来FIR空间天文学任务的首选伙伴。

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.********