The Sweet Spot: Spectroscopy from 12 to 230 μm
The power of infrared diagnostics
The mid- to far-IR spectral range hosts a suite of atomic, molecular and dust features covering a wide range of excitation, density and metallicity, directly tracing the physical conditions both in the nuclei of galaxies as well as in the regions where stars and planets form. Ionic fine structure lines (e.g. [NeII], [SIII], [OIII]) probe HII regions around hot young stars, providing a measure of the star formation rate, stellar type, and the density of the gas. Lines from highly ionized species (e.g. [OIV], [NeV], [SIV]) trace the presence of extremely energetic photons emitted from AGN, providing a measure of the accretion rate. Photo-dissociation regions (PDR), the transition between young stars and their parent molecular clouds, can be studied via the strong [CII] and [OI] lines and the emission from small dust grains and Poly-cyclic Aromatic Hydrocarbons (PAHs). The major coolants of the diffuse warm gas (e.g. [NII]) in galaxies are also in the far-infrared giving us a complete picture of the Interstellar Material (ISM).
The rest-frame mid-infrared is also home to pure rotational H2, HD and OH lines (including OH ground state 79 and 119 μm) and mid- to high-J CO and H2O lines. Finally, the strong PAH emission features (carrying 1- 10% of the total IR emission in star- forming galaxies) with their unique spectral signature, can be used to determine the redshift of galaxies too red to be measured at shorter wavelengths. The IR also hosts numerous unique dust features from minerals such as olivine, calcite and dolomite that probe evolution from pristine to processed dust (e.g. aqueous alteration), CO2 ice and molecules like C2H2 and fullerenes.
The SPICA mission is a joint European-Japanese project to realize an infrared space observatory with a cryogenically cooled telescope aimed to achieve background limited performance. The launch and operations of SPICA are anticipated for the end of the next decade.
SPICA is designed to explore the Universe at mid- to far-IR wavelengths between 12-230µm, with unprecedented sensitivity and unique spectroscopic capabilities. With its 2.5m primary mirror actively cooled to < 8K, SPICA will achieve a gain in sensitivity of more than two orders of magnitude over both Spitzer and Herschel in the mid and far-IR – a gigantic leap in capabilities for exploring the universe.
The SPICA observatory will offer the community a unique astronomical facility, able to cover a broad spectral range to unprecedented depths (a few 10-20 W/m2 for spectroscopy, 5σ, 1hour). Such an exceptional leap in performance is destined to produce many exceptional scientific advances.
The mission builds on heritage from the earlier infrared missions both in Europe and Japan, and profits fully from the advances in modern detector technologies. Combined with an extremely low background cold telescope this new generation of detectors will yield a world class observatory with unprecedented sensitivity in the infrared.