CIBER-2: Measuring Near-IR Cosmic Background

• CIBER-2 is a dedicated experiment that maps near-infrared extragalactic background light across 0.5–2.0 μm to probe cosmic history.
• Its broadband intensity mapping and multi-band configuration enable precise absolute photometry and fluctuation spectrum analysis.
• The results refine models of cosmic star formation, reionization, and local zodiacal light, guiding future cosmological surveys.

The Cosmic Infrared Background Experiment 2 (CIBER-2) is a dedicated sounding-rocket payload developed to measure the intensity and spatial fluctuations of the near-infrared extragalactic background light (EBL) over the wavelength range 0.5–2.0 μm. As the EBL is the integrated emission from all astrophysical sources throughout cosmic history, CIBER-2 targets the unresolved light from galaxies at all epochs, as well as diffuse or faint components not accessible to deep point-source surveys. By combining precision absolute photometry with broadband intensity mapping, CIBER-2 provides novel constraints on cosmic star formation, reionization, and hierarchical structure formation, as well as insights into local foregrounds such as zodiacal light. CIBER-2’s methodology, instrument architecture, and science objectives expand significantly on its predecessor, enabling multi-band measurements with high sensitivity and wide sky coverage.

1. Scientific Rationale and Objectives

CIBER-2’s science case is grounded in the need to empirically determine the spectral energy distribution and spatial structure of the EBL in the near-infrared. The experiment addresses several major themes:

• EBL Intensity Mapping: The EBL encapsulates the cumulative radiative output from resolved and unresolved galaxies, proto-galaxies, intrahalo light (IHL), and potential exotic sources over cosmic time. Direct measurement of the EBL provides integral constraints on cosmic star formation history and the contributions of diffuse sources absent in galaxy counts.
• Spatial Fluctuations and Intensity Mapping: By mapping the angular power spectrum of EBL fluctuations, CIBER-2 aims to distinguish the signatures of clustering due to first-light galaxies during the epoch of reionization, emission from low-redshift galaxies, and diffuse components such as IHL. This broadband intensity mapping (BBIM) methodology is sensitive to faint and diffuse sources, and complements traditional point-source extraction (Zemcov et al., 6 Oct 2025).
• Epoch of Reionization: Leveraging near-IR spectral coverage, CIBER-2 targets signatures of redshifted UV light from the epoch of reionization, including both the absolute background and spatial power spectrum, with the goal of constraining the timing and duration of early star and galaxy formation (Zemcov et al., 2011).
• Foreground Characterization: Accurate models of foregrounds, particularly the zodiacal light (scattered sunlight from interplanetary dust), are essential. CIBER-2’s multi-band approach and in-flight calibrations are critical for foreground subtraction, directly addressing the limitations imposed by bright and variable foregrounds in previous direct EBL measurements (Matsuura et al., 2017).

2. Instrument Architecture and Spectral Configuration

CIBER-2 utilizes a high-etendue optical system optimized for absolute calibration and sky-mapping applications:

• Telescope and Cryogenic Assembly: The instrument features a 28.5 cm primary mirror (aluminum), a Cassegrain optical train (f/3.26, f = 930 mm), providing a 2.3° × 2.3° field of view per camera. The optical assembly, including the detectors, is cooled below ~90 K in a liquid nitrogen cryostat to minimize thermal background and dark current (Zemcov et al., 6 Oct 2025).
• Spectral Bandpass and Filtering: CIBER-2 divides the focal plane into three arms using dichroic beam splitters: Arm S (0.5–0.9 μm), Arm M (1.0–1.4 μm), and Arm L (1.45–2.0 μm). Each camera combines a broadband dual-band windowpane filter and a linear variable filter (LVF) for slitless, low-resolution spectroscopy (R ≈ 20).
• Detectors and Readout: Each arm employs a 2048 × 2048 Teledyne HAWAII-2RG HgCdTe detector, with 4″ pixel scale. Custom electronics provide clocking, high-speed readout, and “row-chopping” (row skipping to mitigate 1/f noise at spatial frequencies corresponding to EBL fluctuations).
• Baffling and Stray Light Control: Deployable baffles and an actuatable optical shutter reduce contamination from Earthshine, thermal emission, and rocket skin scattering; a pop-up baffle system provides additional stray light rejection during flight.
• Calibration Hardware: In-flight self-calibration is achieved with internal calibration lamps and a mechanical shutter that allows direct measurement of dark current in flight. Preflight laboratory calibration uses standardized light sources and integrating spheres (Zemcov et al., 6 Oct 2025).

3. Methodology: Absolute Photometry and Broadband Intensity Mapping

CIBER-2’s measurement approach combines:

• Absolute Surface Brightness Calibration: The instrument’s spectral response and throughput are calibrated using the relation

$$C_{\textrm{flux}} = \frac{\lambda F_{\textrm{ref}}(\lambda_p)}{\sum i_i}$$

with color corrections

$$K = \frac{\int F_\lambda T_\lambda \lambda d\lambda}{F_\lambda(\lambda_p)\,\lambda_p \int T_\lambda d\lambda}$$

This allows conversion of measured photocurrent into surface brightness units (nW m⁻² sr⁻¹), accounting for filter transmission and the spectral energy distribution of reference sources (Zemcov et al., 6 Oct 2025).

• Fluctuation Power Spectrum Mapping: After masking detected sources, the spatial distribution of residual sky brightness is Fourier-transformed to derive the power spectrum of spatial fluctuations. The methodology is specifically sensitive to diffuse/unresolved components and enables separation of cosmological signal from local foregrounds.
• LVF Spectroscopy: The LVF, placed close to the detector, provides spatially multiplexed, low-resolution spectra across the field, enabling both broadband mapping and absolute EBL spectrum extraction.
• Noise Mitigation: Temporal slope fitting of the ramped detector signal (via non-destructive reads) and dedicated dark integrations (using the shutter) are used to model and subtract readout noise and dark current.
• Point Source Masking: The combination of high-resolution imaging and wide field enables effective masking of bright stars and galaxies, critical for accurate fluctuation analysis.

4. Flight History, Calibration, and Performance

CIBER-2’s development and operations have followed an iterative path:

• Flight Hardware Deployment: Three missions: engineering flight (2021), failed launch due to radar failure (2023, payload recovered and refurbished), and full science flight (2024) (Zemcov et al., 6 Oct 2025).
• Observing Campaigns: Successful flights achieved apogees above 315 km, securing several hundred seconds of uncontaminated integration above 200 km—above the bulk of atmospheric airglow. Fields such as COSMOS and Lockman Hole were targeted to optimize extragalactic coverage and minimize foregrounds.
• In-Flight Performance: Laboratory and in-flight measurements confirmed the throughput, focus, PSF, noise properties, and absolute calibration of the instrument. Noteworthy results include robust absolute photometric calibration using bright stars, assessment of PSF broadening during transient pointing drifts, and verification of row-chopping noise suppression.
• System Challenges: Minor issues (e.g., partial shutter leaks detected through persistent reference pixel signals, transient pointing drift, and small-scale PSF elongation) were identified and quantified, but overall systematics were controlled to allow robust background and fluctuation analysis.

5. Foreground Handling and Data Analysis Considerations

• Zodiacal Light Characterization: The ability to distinguish ZL (the dominant foreground at these wavelengths) from the cosmic EBL is a major challenge. The instrument’s spectral and spatial multiplexing allows ZL modeling and subtraction informed by its known spatial structure and spectrum (Matsuura et al., 2017, Zemcov et al., 6 Oct 2025).
• Cross-validation Techniques: The six-band coverage, combination of LVF spectroscopy and broad-band imaging, and differential field selection (high and low ecliptic latitude) provide data for direct empirical foreground modeling and identification of systematic errors due to scattering or airglow residuals.
• Source Masking and Sky Selection: The ~2.3° × 2.3° FOV per camera allows efficient sky coverage and the selection of regions minimally contaminated by bright stars or local cirrus, reducing the need for aggressive masking and mitigating power spectrum bias.

6. Scientific Impact and Implications

• Extragalactic Background Light Constraints: CIBER-2 is designed to measure both the mean intensity and angular power spectrum of the diffuse near-infrared background, providing constraints on the total radiative energy output from galaxies, stellar halos, and possible IHL or even exotic phenomena over cosmic time (Matsuura et al., 2017, Cheng et al., 2021).
• Epoch of Reionization and Early Galaxies: Fluctuation mapping across multiple bands enables discrimination between low-redshift and high-redshift (z > 6) component signatures, including the Lyman drop-out imprint expected from first-light populations [(Zemcov et al., 2011); (Feng et al., 2018)].
• Foregrounds and Local Dust: By providing new high-precision spectra, CIBER-2 offers data for improved modeling of interplanetary dust (IPD), local zodiacal light (including possible isotropic or red ZL components), and their wavelength-dependent scattering and absorption properties (Takimoto et al., 2021).
• Experimental Pathfinding: As a dedicated, high-sensitivity, multi-band BBIM pathfinder, CIBER-2 both complements and informs larger-scale missions (e.g., SPHEREx, Euclid, JWST)—demonstrating advanced calibration, background suppression, and intensity mapping methods required for future EBL experiments (Zemcov et al., 6 Oct 2025, Feng et al., 2018).
• Testing of Exotic Scenarios: The accuracy and broad coverage of CIBER-2 data contribute to tests of dark matter decay (e.g., axion-like particle scenarios), ALP-induced transparency of TeV gamma rays, and non-standard light sources or IHL that might contribute to the observed EBL excess (Kohri et al., 2017, Kalashev et al., 2018).

7. Outlook and Future Directions

• Data Analysis and Release: Post-flight data are subject to rigorous calibration, systematic assessment, and foreground subtraction pipelines. The multi-band, multi-field measurements will enable detailed EBL SED and fluctuation spectrum characterization. Publication and data release will provide benchmarks for future missions and theoretical studies.
• Complementarity in Wavelength and Technique: CIBER-2, with its combination of absolute photometry, slitless low-resolution spectra, and high-fidelity spatial mapping, bridges the gap between deep pencil-beam galaxy counts and wide-field low-resolution intensity mapping, advancing the field’s capacity to robustly measure cosmic backgrounds.
• Methodological Legacy: Advances in payload cryogenics, etendue-optimized optics, multi-arm dichroic implementation, LVF-based slitless spectroscopy, and real-time calibration serve as engineering and methodological blueprints for future cosmological background experiments operating above Earth's atmosphere.

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References:

The detailed technical and scientific context summarized here, including instrument description, flight performance, and scientific rationale, is based on "The Cosmic Infrared Background Experiment-2: An Intensity Mapping Optimized Sounding-rocket Payload to Understand the Near-IR Extragalactic Background Light" (Zemcov et al., 6 Oct 2025), with relevant background from related CIBER and EBL literature as cited above.