FIRESS for PRIMA: Sensitive Wideband Spectroscopy Across the Far-Infrared

Recognizing the promise of the far-infrared waveband for studying the pathways to habitable worlds and the drivers of galaxy growth, the Astro 2020 Decadal Survey recommended that a far-IR Probe (or its X-ray counterpart) be built for science operations starting early in the 2030 decade. Revolutionary advances are possible at probe scale in the far-IR, because we have not yet fielded a platform that operates at the fundamental limits in the band between JWST and ALMA. The cryogenic PRIMA Probe and its workhorse far-IR enhanced survey spectrometer (FIRESS) will create this opportunity. FIRESS uses four long-slit grating spectrometer modules to cover the full 24 to 235 micron waveband at a resolving power greater than 100. Kinetic inductance detector (KID) arrays provide sensitivity close approaching the natural astrophysical backgrounds, translating to mapping speed improvements of a factor of 10,000 or more over the previous facilities. The long slits combined with a steering mirror support pointed observations and a range of mapping modes suitable for programs ranging from deep extragalactic fields reaching across cosmic time, to sensitive mapping of low-surface brightness cosmic ecosystems in nearby galaxies and the Milky Way. For detailed study of targets of interest, FIRESS also employs a Fourier-transform module (FTM) which is engaged to process the incident light as it prior to its detection in the grating modules. The FTM mode also covers the full FIRESS band, and boosts the resolving power to more than 4,000 at 112 microns. The FTS enables detection of the 112-micron HD rotational fundamental, as well as the full spectrum of warm and cool water vapor in protoplanetary disks.