SPICE

ABOUT SPICE

SPICE is designed to address the age-old question, How did we get here? How did galaxies like our own Milky Way form and evolve, how do planetary systems like our Solar System develop, and how do planets like Earth acquire their life-enabling water? SPICE is a spatio-spectral interferometer designed to image protoplanetary and debris disks and measure the spectra of many individual distant galaxies to help answer these questions. SPICE will apply century-old interferometry techniques for the first time in space to achieve unprecedented resolution matching that of the Webb telescope but at ten-times longer far-infrared wavelengths. With cryo-cooled telescopes and state-of-the-art detectors, SPICE offers sensitivity about 10 times that of the Herschel Space Observatory, the most powerful far-infrared telescope to date. SPICE provides a moderate-resolution spectrum in every spatial pixel, and the spectrum is full of information about physical and chemical conditions in the objects studied, as well as the redshifts and distances of galaxies.

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• Science Case Survey

GALAXY EVOLUTION

The key goal of galaxy formation studies is to understand the physical processes that drive the evolution of galaxies and their central massive blackholes throughout cosmic time. A crucial component of the baryonic content of galaxies – responsible for almost all the energy emitted by galaxies – is the gas and dust in between stars, i.e., the interstellar medium (ISM). Evidence accumulated over the past decade convinces us that infrared observations of the gas and dust are critical to our understanding of galaxy-building processes. Indeed, the infrared emission of galaxies makes up about half of the cosmic radiation background from galaxy formation and evolution processes, and the star formation rate density of the Universe is dominated by the dust-obscured star formation emitting in infrared, at least out to z~4. The effect of dust attenuation severely biases the UV and optical observations, and hence to gain a complete and unbiased view of the physical and chemical processes inside galaxies, we need to probe their stellar, gaseous, metal and blackhole content in infrared wavelengths. The high spatial resolution of SPICE (~0.3 arcsec) compared to its predecessors, Spitzer (~6-18 arcsec) and Herschel (~5-40 arcsec), and its >10x larger field of view compared to that of ALMA, will enable us, for the first time, to disambiguate the infrared emission of individual (unlensed) galaxies beyond the local Universe in statistically large samples. In addition, the high spatial resolution of SPICE will allow for studies of far-IR cooling lines in individual star forming regions and the diffuse ISM of galaxies other than the Milky Way, providing context for the ALMA detections of these lines at high redshifts and connecting the local Universe to the epoch of reionization and first galaxies.

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PLANET FORMATION

Planets, comets and asteroids are born in the dusty, gas-rich discs that surround young stars: protoplanetary discs. These sites are rich in their chemical and physical diversity, both of which set the initial conditions from which life must emerge. However, observations at far-infrared wavelengths that dominate the emission of protoplanetary discs remain unresolved from previous generation space telescopes (e.g., the Spitzer Space Telescope, and the Herschel Space Observatory). Indeed, detecting and resolving the location of gas species at high-resolution in protoplanetary discs is the essential next step to constrain models of planet formation. Of those species, water is perhaps the most important molecule driving the origins of life, and much of the water is thought to be frozen. With sub-arcsecond spatial resolution, and spectral resolving powers R>10³ from space, SPICE will be the first instrument to detect and resolve the location of water-ice in protoplanetary discs in nearby star forming regions, and define how this is delivered to young planets. Moreover, hunting for carbon and oxygen bearing species necessary for the complex chemical interactions that occurred in our Solar System, yet only accessible at far-infrared wavelengths, SPICE will constrain the chemical evolution with which planets, comets and asteroids form.

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PLANETARY SYSTEM ARCHITECTURE

Debris disks are analogues of the Solar system’s small-body populations seen to orbit other stars. Most commonly these disks resemble analogues of our Kuiper belt, with semi-major axes of tens of astronomical units. While observations detect the dust emission from these disks, this dust is replenished by collisions between larger bodies, which provide a mass reservoir that can supply dust for the entire main-sequence stellar lifetime. Current sensitivity is at best able to detect disks about ten times dustier than Solar system levels, with disks detected around approximately 20% of stars. That is, we know that the Solar system’s debris disk lies somewhere among the faintest 80%, but do not know whether it is unusually faint or not. Sometimes these disks have prominent rings, as seen in the image of Fomalhaut on the right, but narrow rings are easier to image, and evidence is emerging from ALMA that most debris disks are actually broader. A few disks show radial substructure (show HD 107146), but conclusions about whether such structure is generic will require greater sensitivity than is currently available. The origin of a debris disk’s radial profile, whether it be structured or not, remains unclear; possible scenarios invoke primordial origins in the protoplanetary disk, or subsequent dynamical sculpting by planets. A key hypothesis to test is whether debris disks show similar radial structure to the brightest and largest protoplanetary disks…

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INSTRUMENT PERFORMANCE MODELING

With the cutting edge of space-based, far-infrared astronomy at somewhat of a halt, with the ESA Herschel Space Observatory and NASA Spitzer Space Telescope finished their missions, we must now consider the next big leap in far-infrared astronomical observations. Although both Herschel and Spitzer operated successful missions with high spectral resolution, they were both limited in spatial resolution due to the fundamental diffraction limit of their 3.5 m and 0.85 m primary mirrors, respectively. Clearly then, high spatial resolution (sub-arc second) should be our next step in observing at these science-rich, but relatively long wavelengths. We aim to identify the scientific questions that can be answered with high spatial resolution far-infrared observations, and to translate these questions into a technological definition of a far-infrared space-based mission. The Instrument Performance Modeling working group are using the python Far-Infrared Interferometer Simulator (pyFIInS) to more accurately understand and predict the abilities and limitations of a SPICE-like system. The three astronomy & astrophysics working groups will feed realistic sky model data cubes to our group, for input to the pyFIInS tool.

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SPICE

DESIGN OVERVIEW