PRIMA: Far-Infrared Mission for Astrophysics

• PRIMA is a proposed far-infrared mission featuring a cryogenically cooled 1.8m telescope and advanced KID arrays for ultra-sensitive imaging and spectroscopy.
• Its two primary instruments, PRIMAger and FIRESS, enable hyperspectral imaging, polarimetric mapping, and tunable high-resolution spectroscopy across 24–235 μm.
• The mission’s rapid mapping speed and open General Observer program establish a new benchmark for exploring galaxy evolution, ISM conditions, and cosmic dust properties.

The PRobe far-Infrared Mission for Astrophysics (PRIMA) is a proposed space-based observatory dedicated to ultra-high sensitivity imaging and spectroscopy in the far-infrared regime (24–235 μm), employing a cryogenically cooled 1.8 m telescope and two primary instruments. PRIMA is architected as a next-generation survey and discovery machine, featuring mapping speeds 2–4 orders of magnitude higher than preceding far-IR missions, with 75% of mission time offered to open competition under a broad General Observer (GO) program, fostering community-driven science across extragalactic, Galactic, and planetary astrophysics (Moullet et al., 2023).

1. Mission Architecture and Instrumentation

PRIMA’s technical foundation is a 1.8 m primary mirror cooled to ~4.5 K, enabling astrophysical background-limited sensitivity over a factor-of-ten improvement in noise equivalent power (NEP) compared to previous facilities. The mission employs two main science instruments:

• PRIMAger: A hybrid far-IR imager offering (a) hyperspectral imaging at R~8 resolution from 24–84 μm and (b) polarimetric imaging in broader bands spanning 80–264 μm. The imager deploys ultra-sensitive kinetic inductance detector (KID) arrays, with dark NEP ~3×10⁻²⁰ W Hz^–0.5.
• FIRESS (Far-Infrared Enhanced Survey Spectrometer): A modular far-IR spectrometer comprising four R~100 grating modules fed by 24×84 KID arrays, capable of spatial multiplexing and “plug-in” Fourier Transform Module (FTM) for tunable high-resolution spectroscopy (R up to 20,500 at 24 μm), allowing full spectral coverage in two settings across the 24–235 μm range.

This configuration yields a survey instrument with a field of view and angular resolution optimized for mapping faint structures, capturing both broad-band photometry and key diagnostic spectral lines inaccessible to JWST or ALMA (Ciesla et al., 1 Sep 2025, Pontoppidan et al., 1 Sep 2025, M. et al., 2 Sep 2025).

2. Detector Advances and Background-Limited Sensitivity

PRIMA leverages the latest developments in KID technology for the far-IR regime, featuring kilo-pixel arrays with lens-coupled aluminum inductor-absorbers and niobium interdigitated capacitors. Laboratory validation demonstrates that ≥73% of 1,008-pixel prototype arrays attain NEP < 1×10⁻¹⁹ W Hz^–0.5 per pixel at 210 μm, corresponding to background-limited performance under mission conditions (Foote et al., 2023, Kane et al., 7 Aug 2024).

Key detector characteristics:

• Uniform frequency-multiplexed architecture
• Absorption efficiency up to 90% with microlens coupling
• Responsivity modeled and fitted using Mattis–Bardeen theory and

$$\mathrm{NEP}^2_{\mathrm{tot}} = \mathrm{NEP}^2_{\mathrm{photon}} + \mathrm{NEP}^2_{\mathrm{recomb.}} + \mathrm{NEP}_0^2$$

• Multitone RF readout (RFSoC) for efficient, high-yield array characterization and operation

Detectors exhibit a wide dynamic range (photon noise limited up to incident powers ~20 fW/pixel), supporting both deep integrations on faint sources and observations of bright targets without saturation or degradation (Hailey-Dunsheath et al., 2023).

3. Science Drivers: Extragalactic, ISM, and Galaxy Evolution

The PRIMA General Observer Science Book (Moullet et al., 2023) documents community-driven science themes, highlighting PRIMA’s essential role in:

• ISM and Metallicity Evolution: Joint continuum and fine-structure line measurements (e.g., PAHs, [Ne II], [O III], [Si II]) across 24–250 μm for diagnosing dust/metal content and probing the physical state (electron density, ionization, temperature) throughout cosmic history.
• Dust Mass Function and Evolution: Deep blind surveys out to z~3 and beyond (via ALMA synergy) targeting the dust mass function (DMF) to constrain dust production mechanisms (AGB stars, SNe, ISM growth) and to determine the obscured fraction of star formation over time.
• Mid-IR Diagnostics: Hyperspectral imaging and FIRESS spectroscopy of PAH bands, mid-IR crystalline silicate features (e.g., 69 μm forsterite), and the quantification of dust processing and composition in starbursts and early galaxies.
• Epoch of Reionization and High-z Quasars: Coverage of rest-frame mid-IR for z>5 quasars, enabling discrimination of dust production channels and SED diagnostics inaccessible to JWST/ALMA (Moullet et al., 2023).
• Optically Dark and HST-Dark Galaxies: Blind surveys (e.g., PRIDES; A. Faisst et al.) in well-characterized fields (COSMOS), for detection and characterization of massive, dusty galaxies not seen in UV/optical, providing constraints on their star formation, AGN content, and evolutionary links to local ellipticals (Moullet et al., 2023).
• Galaxy Quenching and Feedback: Observations of post-starburst systems probing feedback and quenching mechanisms using mid-IR forbidden line ratios ([Ne III], [O IV], [S III], [O I]) where optical tracers do not suffice.

PRIMA’s sensitivity and mapping speed facilitate statistically robust, multi-epoch surveys and enable detailed follow-up spectroscopy across diverse environments, supporting direct tests of galaxy evolution models.

4. Confusion, Polarimetry, and Survey Depth Limitations

Far-IR confusion noise, arising from the blending of numerous faint sources in each beam, sets a practical sensitivity floor for both photometric and polarimetric observations. Simulations reveal:

• The classical confusion limit grows rapidly with observing wavelength and is ~100× lower for polarization compared to intensity, due to vectorial cancellation of Stokes Q and U from uncorrelated sources (Béthermin et al., 5 Apr 2024).
• Even basic source extraction in intensity maps will reach L* galaxy completeness up to z~3 and main-sequence 10¹¹ M_⊙ galaxies up to z~5.
• PRIMAger’s polarization mode is expected to provide ∼8,000 galaxy detections in polarization up to z=2.5 in a conservative survey scenario, enabling the first population studies of high-z dust polarization.
• Confusion in photometry can be mitigated using prior-based deblending with higher-resolution short-wavelength data and advanced algorithms (e.g., XID+), while polarization surveys are less affected by these issues.
• Fractional uncertainties at the confusion limit are 8–15% (PHI bands, intensity) and a few degrees in polarization angle at the bright end (Béthermin et al., 5 Apr 2024).

These constraints inform survey design and highlight the synergy between PRIMA’s hyperspectral and polarimetric modes for deblending and robust extragalactic science.

5. Polarimetric Instrumentation and Methods

The PRIMA Polarimetric Imager (PPI) employs arrays of single-polarization KIDs, with each array element oriented in one of three angles (0°, 60°, 120°), enabling full Stokes I, Q, and U measurement per scan (Dowell et al., 25 Apr 2024). Key features and methodologies:

• No polarization modulator: polarimetry is achieved directly via detector orientation, facilitating simultaneous multi-angle sampling.
• Observations rely on an agile beam-steering mirror, enabling fast cross-linked scans and “destriping” for baseline removal, as commonly performed with Herschel/SPIRE and Planck.
• Simulations demonstrate recovery of input I, Q, U maps at or near fundamental noise limits, with only a 30–40% noise penalty under pessimistic 1/f detector noise assumptions.
• Map reconstruction employs least-squares inversion and calibration sequences using sky rotation, ensuring robust gain calibration and mitigation of systematic errors such as pointing and beam differences (Dowell et al., 25 Apr 2024).

PRIMA’s approach to FIR polarimetry is thus poised to deliver transformational advances in mapping magnetic fields and dust alignment across Galactic and extragalactic environments.

6. Community Science: General Observer Program

The mission designates 75% of telescope time within a five-year prime mission as competitive, enabling community-defined science through the General Observer program. This open-access model is intended to maximize the scientific return and exploit PRIMA’s extraordinary sensitivity and broad spectral coverage. Submitted proposals within the Science Book span topics from ISM and galaxy evolution to feedback, quenching, AGN, and cosmic infrared background mapping (Moullet et al., 2023), illustrating the mission’s breadth and flexibility.

7. Synergies and Legacy

PRIMA is designed to operate in concert with JWST, ALMA, Euclid, Roman, and other forthcoming facilities, covering diagnostic wavelengths and phenomena inaccessible to other infrared/sub-mm telescopes (e.g., bridging the JWST cutoff at ≲28 μm and the long-wavelength capabilities of ALMA). This unique spectral access, coupled with ultra-high sensitivity, enables PRIMA to fill critical gaps in the global multi-wavelength paper of cosmic star formation, dust, metals, and the evolution of baryons in galaxies across cosmic time (Moullet et al., 2023).

In summary, PRIMA combines background-limited detector technology, broad-band imaging and spectroscopy, and open survey strategies to enable a transformative legacy in far-infrared astrophysics—quantifying dust, gas, and metals; tracing feedback, magnetic fields, and obscured AGN; and providing the first population studies of polarized dust at high redshift, thereby advancing the understanding of key processes in galaxy formation and evolution.