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CIBER $\times$ galaxy cross-correlations reveal a bright, low-redshift NIR background

This paper presents the first tomographic cross-correlation of CIBER near-infrared background data with galaxy catalogs, revealing that low-redshift large-scale structure and intra-halo light significantly contribute to EBL fluctuations, thereby exceeding standard integrated galaxy light predictions and highlighting the importance of low-mass halos in explaining the observed signal.

Original authors: Richard M. Feder, Grigory Heaton, James J. Bock, Yun-Ting Cheng, Yi-Kuan Chiang, Phillip M. Korngut, Shuji Matsuura, Jordan Mirocha, Kohji Tsumura, Michael Zemcov

Published 2026-08-13

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: Richard M. Feder, Grigory Heaton, James J. Bock, Yun-Ting Cheng, Yi-Kuan Chiang, Phillip M. Korngut, Shuji Matsuura, Jordan Mirocha, Kohji Tsumura, Michael Zemcov

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a giant, glowing fog. For billions of years, every star that has ever lived and every galaxy that has ever formed has added a tiny bit of light to this cosmic soup. Astronomers call this the Extragalactic Background Light (EBL). It's like the faint, lingering glow of a campfire long after the logs have turned to ash. For a long time, scientists thought they could explain this glow just by adding up the light from all the individual galaxies they could see in their telescopes. It seemed like a simple math problem: count the stars, multiply by their brightness, and you get the total glow.

But there's a catch. The universe isn't just a random scattering of stars; it's a cosmic web. Galaxies don't live in isolation; they hang out in groups and clusters, held together by invisible gravitational scaffolding called "dark matter halos." Think of these halos as giant, invisible neighborhoods. Just as neighbors in a real neighborhood might share a fence or a driveway, galaxies in these cosmic neighborhoods share light. Some of this light comes from the main house (the central galaxy), but some comes from the "satellite" galaxies orbiting it, and some is even a diffuse, fuzzy glow left behind from stars that were stripped away during galactic tugs-of-war. This paper is about figuring out how much of that "neighborhood glow" is actually contributing to the cosmic fog, and whether our current math is missing a huge chunk of the light.

The Cosmic Glow-Up: Finding the Missing Light

A team of astronomers, led by Richard Feder, decided to play a game of cosmic "connect the dots." They took data from a rocket-borne camera called CIBER, which took pictures of the near-infrared sky (a type of light just beyond what our eyes can see), and cross-referenced them with massive catalogs of galaxies from two huge ground-based surveys: the DESI Legacy Survey and the Hyper-Suprime-Cam.

Think of it like this: The CIBER camera sees the "fog" of light, but it can't tell which specific galaxy is making which part of the glow. The galaxy catalogs, however, act like a street map, showing exactly where the galaxies are. By overlaying the map on the fog, the scientists could ask: "Does the fog get brighter right where the galaxies are?" If the fog is just random noise, the answer would be "no." But if the fog is made of light from those galaxies and their neighbors, the answer should be "yes."

The Big Surprise
The team found a massive "yes," but it was way bigger than anyone expected. When they looked at the light on large scales (covering areas of the sky about the size of a few full moons), the connection between the galaxies and the background glow was 10 times stronger than their standard models predicted. It was as if they were trying to explain the noise in a crowded room by counting the people, but the actual noise was coming from a secret, invisible choir singing in the corners that no one had heard before.

This excess light was concentrated in the "low-redshift" universe, which is a fancy way of saying "relatively close to us," specifically within a distance where the light has traveled for less than about 6 billion years (redshift $z \lesssim 0.6$ ).

Who is Making the Noise?
The scientists then broke down the signal to see who was responsible. They found that:

Galaxy Clusters are Big Players: About 15–20% of this extra glow comes from massive galaxy clusters and the galaxies living inside them. These are the "rich neighborhoods" of the universe.
The "One-Halo" Mystery: The rest of the signal comes from smaller groups and individual galaxies. The team detected a specific type of clustering called "one-halo" power. Imagine a single dark matter halo as a house. The "one-halo" signal is the light coming from everything inside that one house: the main galaxy, its satellite companions, and the fuzzy, diffuse light that hangs around between them.
It's Not Just Bright Galaxies: The data suggests that this extra light isn't just coming from the biggest, brightest galaxies. Instead, it seems to be amplified by contributions from lower-mass halos (smaller neighborhoods) that are packed with satellite galaxies and diffuse light.

What They Ruled Out
The team was very careful to make sure this wasn't a trick of the light. They checked if the glow was just caused by stars in our own Milky Way or by errors in their equipment. They cross-checked their data against a map of stars from the Gaia satellite and found no correlation. This means the extra glow is definitely coming from outside our galaxy, not from local stars or instrument glitches. They also confirmed that the standard "Integrated Galaxy Light" (IGL) model—which simply adds up the light from known galaxies—underestimates the actual brightness by a huge margin, even if they assume the galaxies are much more biased (clumped) than usual.

How Sure Are They?
The results are statistically very strong. The team detected this extra signal with a significance of up to 13-sigma (a measure of certainty where 5-sigma is usually considered a discovery). In plain English, the odds of this being a random fluke are essentially zero. They measured the signal across different wavelengths (1.1 and 1.8 micrometers) and different galaxy catalogs, and the result was consistent every time.

The Bottom Line
This paper suggests that the universe is brighter in the near-infrared than we thought, specifically because of the "fuzzy" light from galaxies living in groups and clusters at relatively low distances. The standard models of how galaxies form and shine are missing a significant piece of the puzzle. It's likely that there is a lot more "diffuse intra-halo light" (stars stripped from their galaxies and floating in the dark matter neighborhoods) and more satellite galaxies than our current simulations predict.

While the team hasn't solved the entire mystery of the cosmic background light, they have firmly established that the "neighborhood glow" from low-redshift structures is a major, previously unappreciated contributor to the cosmic fog. This discovery sets the stage for future missions, like CIBER-2 and SPHEREx, to go back with better maps and figure out exactly what kind of stars are hiding in the shadows of these cosmic neighborhoods.

Technical Summary: CIBER × Galaxy Cross-Correlations Reveal a Bright, Low-Redshift NIR Background

Problem Statement
Measurements of the Extragalactic Background Light (EBL) in the near-infrared (NIR) have historically exhibited significant tension. While absolute intensity measurements from the Cosmic Infrared Background ExpeRiment (CIBER) and DIRBE imply a background intensity roughly three times higher than estimates derived from integrated galaxy light (IGL) in deep catalogs, fluctuation-based measurements have offered a potential resolution by bypassing absolute calibration issues. However, previous CIBER fluctuation analyses revealed excess power on large angular scales ( $\ell < 2000$ ) that exceeded standard IGL models by an order of magnitude at high significance. The physical origin of this excess remained ambiguous: it could stem from an enhanced population of unresolved galaxies, a higher intensity bias ( $b_I$ ), or contributions from intra-halo light (IHL) and satellite galaxies that are not captured in standard galaxy counts. Distinguishing between these scenarios using auto-power spectra alone is difficult due to degeneracies between clustering and Poisson noise.

Methodology
This work presents the first tomographic analysis of NIR EBL anisotropies by cross-correlating CIBER 1.1 and 1.8 $\mu$ m imager data (from the fourth flight) with photometric galaxy catalogs from the DESI Legacy Survey (DR8) and the Hyper-Suprime-Cam (HSC) Ultra-Deep Survey.

Data Processing: The authors utilize CIBER data covering 20 deg $^2$ and apply masks to remove bright stars and galaxies. They construct galaxy overdensity fields ( $\delta_g$ ) from DESI-LS and HSC samples, applying random catalog corrections to account for survey geometry and depth variations.
Power Spectrum Estimation: A pseudo- $C_\ell$ pipeline is employed to recover unbiased auto- and cross-power spectra. This includes corrections for mode-mixing (due to masking and filtering), beam transfer functions, and pixel window functions. Noise covariance is estimated via Monte Carlo realizations of instrument noise.
Halo Modeling: The analysis employs a halo model framework to decompose the cross-power spectra ( $C_{Ig}$ $C_{I g}$ ) into three components:
- Two-halo term: Tracing large-scale structure (LSS) correlations between separate halos.
- One-halo term: Tracing correlations of objects within the same dark matter halo (centrals and satellites).
- Poisson term: Arising from the discrete nature of the sources.
  The model is fitted using Markov Chain Monte Carlo (MCMC) sampling to constrain amplitudes for the two-halo ( $A_{2h}$ ) and one-halo ( $A_{1h}$ ) terms, as well as a population weighting parameter ( $f_{pop}$ ) to account for star-forming vs. quiescent central galaxies.
Tomographic Binning: The analysis splits the galaxy samples into redshift bins ( $\Delta z = 0.2$ ) spanning $0 < z < 1$ to probe the redshift evolution of the signal.

Key Contributions and Results

Significant Excess Cross-Power: The study detects cross-power between CIBER and galaxy tracers at high significance ( $>5\sigma$ to $>10\sigma$ ) on scales $\ell < 2000$ . The measured power exceeds standard IGL model predictions (based on the Ares semi-empirical model) by factors of 3 to 10, particularly at low redshifts ( $z \lesssim 0.6$ ).
Redshift Dependence: The excess signal is concentrated at low redshift ( $z < 0.6$ ). The cross-correlation coefficient ( $r_{Ig}$ ) peaks at $z \sim 0.2-0.4$ and declines toward $z=1$ .
Cluster Contributions: By isolating cluster member galaxies (CMGs) from the DESI-LS sample, the authors find that clusters and their associated structure account for 15–20% of the large-angle cross-power. This indicates that group- and galaxy-scale halos contribute the bulk of the signal, rather than just massive clusters.
Detection of One- and Two-Halo Clustering: Parametric fits robustly detect both one-halo and two-halo clustering components. The one-halo amplitude is found to be similar between the shallower DESI-LS and deeper HSC catalogs. This similarity suggests that the signal is not solely driven by the most massive halos (which would be more prominent in HSC) but likely involves lower-mass halos with satellites and/or diffuse intra-halo light.
Bias-Weighted Intensity Kernel: Converting the two-halo fits into estimates of the bias-weighted intensity kernel ( $b_I \times dI/dz$ ), the authors find that standard IGL predictions underestimate the measurements even when assuming an intensity bias as high as 3 (comparable to large Sunyaev-Zel'dovich clusters). This implies that a higher specific intensity evolution ($dI/dz$) is required to reconcile the observations with theory.
Reconstruction of Auto-Power: Using a coherence-based estimator, the authors reconstruct the NIR auto-power associated with correlated LSS at $z < 1$ . They find this reconstructed power exceeds IGL predictions by a factor of 5–10 on scales $\ell < 2000$ , accounting for a substantial fraction of the previously reported CIBER auto-power excess.

Significance and Claims
The paper claims to identify low-redshift large-scale structure (LSS) as a significant and previously unappreciated contributor to NIR EBL fluctuation measurements. The results suggest that the "missing" NIR background is not necessarily a population of entirely new, undetected high-redshift sources, but rather an enhancement in the clustering signal from low-redshift structures, potentially driven by:

Satellite Galaxies: A more luminous or numerous satellite population than standard Halo Occupation Distribution (HOD) models predict.
Diffuse Intra-Halo Light (IHL): Tidal stripping and other mechanisms producing diffuse stellar emission in group- and galaxy-scale halos.

The authors conclude that these findings set the stage for future cross-correlation science with CIBER-2, SPHEREx, and other LSS tracers. They emphasize that fluctuation measurements targeting scales near $\ell \sim 1000$ (e.g., for Epoch of Reionization searches) will require larger survey areas or improved foreground mitigation to achieve their stated statistical precision, as low-redshift clustering is a dominant systematic. The work motivates the development of more detailed models incorporating central+satellite prescriptions and IHL to interpret the rich dataset provided by upcoming intensity mapping experiments.

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