PASSAGE: Slitless Spectroscopy for Galaxy Evolution

Updated 5 September 2025

The paper demonstrates the use of JWST NIRISS pure-parallel slitless spectroscopy to perform an unbiased, volume-efficient survey of emission-line galaxies over a wide redshift range.
It employs advanced techniques such as orthogonal grism orientations and contamination modeling to mitigate spectral overlap and reduce cosmic variance, yielding thousands of high-quality spectra.
The survey provides critical constraints on star formation, metallicity, and reionization by capturing detailed emission-line data from low-mass galaxies, setting a legacy standard for future studies.

The Parallel Application of Slitless Spectroscopy to Analyze Galaxy Evolution (PASSAGE) survey is a large-scale extragalactic program utilizing the James Webb Space Telescope (JWST) Near Infrared Imager and Slitless Spectrograph (NIRISS) in pure-parallel slitless grism mode. PASSAGE is designed as an unbiased, volume-efficient search for emission-line galaxies over a wide range of masses and redshifts, leveraging JWST’s spatial resolution, spectral coverage, and the multiplexing intrinsic to slitless spectroscopy. The program aims to provide definitive constraints on the physical properties and demographics of galaxies, particularly low-mass, emission-line dominated systems, across cosmic time.

1. Survey Architecture and Observational Strategy

PASSAGE was awarded the largest JWST Cycle 1 program time, comprising 591 hours of NIRISS observations, ultimately executing about 400 hours spanning 63 independent, high-latitude extragalactic fields in pure-parallel mode (Malkan et al., 30 Aug 2025). The program uses NIRISS wide field slitless spectroscopy (WFSS), which covers an instantaneous field of view of 2.2' × 2.2' and simultaneously acquires both direct imaging and dispersive grism data. The survey is organized around several key principles:

Unbiased Spectroscopic Sampling: All sources detected in the field contribute dispersed spectra, without preselection from broadband imaging or photometric criteria.
Cosmic Variance Mitigation: The use of dozens of independent, high-latitude fields sampled in parallel with primary JWST observations provides a large number of uncorrelated sightlines.
Spectral Baseline and Resolution: NIRISS grisms (GR150C and GR150R) are employed, typically covering 1.0–2.3 μm with R ≈ 150. Multiple orientations (orthogonal dispersions) are sometimes acquired, enabling better deblending of overlapping spectra.
Data Release: Data products, including direct images, extracted spectra, and catalogs, are released to the public with minimal proprietary period, supporting community-wide analysis.

2. Methodological Innovations in Pure-Parallel Slitless Spectroscopy

Slitless spectroscopy in the pure-parallel context maximizes the multiplex advantage by capturing spectra for all sources in the field, including faint, low-mass, and emission-line dominated galaxies that are often missed by targeted spectroscopy (Atek et al., 2010, Weiner, 2012, Willott et al., 2022). Critical features include:

Emission-Line Driven Detection: The most numerous sources at faint fluxes are compact, low-mass, and exhibit strong optical nebular emission lines (e.g., Hα, [O III], Hβ), making direct, line-driven redshift and physical property measurements accessible for the statistical galaxy population at z > 1.
Mitigation of Spectral Overlap: Orthogonal grism orientations and advanced contamination modeling (using multiple images and roll angles) minimize the impact of spectral overlap and confusion, a longstanding challenge in crowded fields and at faint flux limits (Runnholm et al., 26 Feb 2025, Pirzkal et al., 2023).
Spectral Multiplexing and Volume Efficiency: Each field yields thousands of spectra, collectively providing tens of thousands of independent measurements with minimized cosmic variance, enabling population studies of rare objects and faint galaxies.

3. Galaxy Evolution Science Enabled by PASSAGE

PASSAGE delivers key advances in several domains of galaxy evolution research by enabling:

Star Formation in Low-Mass Galaxies: The survey reaches deep emission-line flux limits, detecting star formation rates (SFRs) down to a few tenths of solar masses per year in galaxies at intermediate redshift, including systems several magnitudes below L* (Atek et al., 2010, Runnholm et al., 26 Feb 2025).
Physical Parameter Inference from Emission Lines: The ability to detect multiple diagnostic lines (Hα, Hβ, [O III], [O II], [N II]) across 1.0–2.3 μm facilitates SFR, metallicity, and ionization parameter measurements using established relations (e.g., Kennicutt SFR calibration, strong-line metallicity diagnostics).
Demography of Extreme Emission-Line Galaxies: PASSAGE is uniquely sensitive to the population of extreme emission line galaxies (EELGs), which are found to be significantly more common at high redshift (the comoving number density increasing by an order of magnitude from z ~ 0.5 to z > 1.5) (Maseda et al., 2016). The unbiased selection captures these systems, which often drive global SFR density and reionization.
Probing the Epoch of Reionization: The program operates without photometric preselection, enabling a direct census of rare Lyman-α emitters (LAEs) at 7.5 ≤ z ≤ 9.5 (Runnholm et al., 26 Feb 2025), as well as early galaxies forming in over-dense large-scale structures (Fudamoto et al., 19 Mar 2025).
Quantifying Field-to-Field Variance: The distribution of LAEs and emission-line sources across the independent pointings allows for statistical tests of reionization morphology (patchiness, bubble size) and the impact of cosmic variance.

4. Data Quality, Limitations, and Technical Challenges

The pure-parallel JWST/NIRISS approach introduces both strengths and technical challenges:

Spatial Resolution: JWST’s diffraction-limited PSF and fine pixel scale allow morphology-resolved measurements and help deblend overlapping sources, enhancing sensitivity to compact, faint galaxies (Willott et al., 2022).
Spectral Coverage and Line Diagnostics: Continuous coverage from 1.0–2.3 μm at R ~ 150 enables rest-frame optical line measurements out to z ~ 3.5. The use of orthogonal grisms increases both completeness and reliability of line identification (Pirzkal et al., 2023).
Spectral Confusion and Overlap: Even with JWST’s resolution, overlapping spectra remain a challenge at deep flux limits. Techniques such as cross-orientation validation, forward modeling, and SED-informed contamination subtraction (as in NGDEEP and SAPPHIRES) are essential (Pirzkal et al., 2023, Sun et al., 19 Mar 2025).
SNR and Sensitivity Limits: The survey achieves deep flux limits (3σ ~ 2 × 10⁻¹⁸ erg s⁻¹ cm⁻² in NGDEEP, similar in PASSAGE), adequate for detecting faint emission lines from moderate- to low-mass galaxies out to z ~ 3.5 (Pirzkal et al., 2023).
Redshift Completeness: Approximately one quarter of galaxies have secure redshift measurements in simulated and real slitless surveys, limited primarily by spectral overlap and SNR (Wen et al., 8 Jan 2024).

5. Scientific Results and Early Discoveries

PASSAGE has demonstrated several key capabilities based on early data:

Measurement of the Lyα Luminosity Function During Reionization: Detection of four LAEs at z ~ 7.5–9.5 enabled direct measurement of the Lyα luminosity function and its decline compared with post-reionization, with the observed drop being a factor ≲10 (less than many pre-JWST predictions). Inferred bubble sizes around detected LAEs are ≳2 physical Mpc (Runnholm et al., 26 Feb 2025).
Evidence of Patchy Reionization: Counts of LAEs across fields (e.g., {3,1,0,0}) are consistent with strong field-to-field variance, supporting models of patchy reionization.
Discovery of High-Redshift Overdensities: WFSS data revealed an over-density of nine galaxies at z = 8.47 in the SAPPHIRES field, suggesting accelerated assembly of massive systems in dense environments and providing constraints for early structure formation models (Fudamoto et al., 19 Mar 2025).
Enabling Detailed Kinematic and Metallicity Analyses: Orthogonal grism data and extended spectral coverage facilitate kinematic modeling, metallicity gradient measurements, and emission-line mapping with sub-kiloparsec scale resolution when combined with lensing or parallel NIRCam imaging (Wang et al., 2019, Barišić et al., 15 Aug 2024).

6. Data Release, Legacy Value, and Future Prospects

PASSAGE data products—including processed grism spectra, extracted emission line catalogs, high-level data products, and summary tables—are released immediately, supporting community-driven science. The combination of unbiased selection, large area, and homogeneous reduction sets a legacy standard for spectroscopic surveys in the JWST era (Malkan et al., 30 Aug 2025).

Ongoing and future analyses will leverage the full multi-field dataset to map the evolution of emission line properties, especially at low mass and faint luminosity, calibrate the contribution of faint galaxies to reionization, and quantify the impact of cosmic variance on high-z galaxy demographics. PASSSAGE’s methodology and technical approach also inform the design and optimization of next-generation slitless surveys with Roman, Euclid, and CSST (Wen et al., 8 Jan 2024, Monaco, 2023).