A TeV-based Determination of the Local Extragalactic Background Light and its Consistency with Galaxy Counts and Direct Measurements

Using a sample of 268 very-high-energy gamma-ray spectra from 45 sources, this study determines the local extragalactic background light intensity, finding it consistent with integrated galaxy counts and deep-space measurements while ruling out the near-infrared excess reported by IRTS and CIBER.

The Gist

The Big Picture: The Cosmic "Fog"

Imagine the universe is a giant, dark room. You know there are people in there (stars and galaxies) because you can see their flashlights. But there is also a faint, glowing fog filling the entire room. This fog is made of all the light that has ever been emitted by every star and galaxy since the beginning of time. Astronomers call this the Extragalactic Background Light (EBL).

For a long time, scientists have been trying to measure exactly how thick this "fog" is. They have two main ways to do it:

1. The Direct Look: Trying to see the fog directly with a super-sensitive camera. (Problem: The "fog" of our own solar system—sunlight bouncing off dust—gets in the way and blinds the camera.)
2. The Galaxy Count: Counting every single star and galaxy we can see and adding up their light. (Problem: What if there are faint, invisible stars we missed?)

The New Detective Tool: Cosmic "Fog" Sensors

This paper introduces a clever new way to measure the fog. Instead of looking at the fog, the scientists look at how the fog affects things passing through it.

Imagine you are in a room full of thick fog. If you shout, your voice gets muffled. If you shine a very bright, high-energy laser beam through the fog, the beam gets weaker the further it travels because the fog particles absorb the light.

In space, the "laser beams" are Gamma Rays (super-high-energy light) coming from distant black holes (blazars). The "fog" is the EBL. When a Gamma Ray hits a photon from the EBL, they smash together and disappear, turning into matter (electrons and positrons). This is called pair production.

The key rule is: The thicker the fog, the more the laser beam gets dimmed.

What the Scientists Did

The team, led by J. Baxter, acted like cosmic detectives. They gathered data from 45 distant black holes using powerful ground-based telescopes (like H.E.S.S., MAGIC, and VERITAS). They looked at 268 different "snapshots" (spectra) of the light coming from these black holes.

They asked a simple question: "How much of this light was eaten by the cosmic fog?"

By measuring exactly how much the light was dimmed, they could calculate the density of the fog (the EBL) right here in our local neighborhood of the universe.

The "Fog" vs. The "Galaxy Count"

Once they calculated the fog density, they compared it to the "Galaxy Count" method (adding up all the known stars).

• The Result: The two methods matched almost perfectly!
• The Analogy: Imagine you are trying to guess how much water is in a swimming pool.
  • Method A: You count every single raindrop that fell into the pool.
  • Method B: You measure how much the water level rose.
  • The Paper's Finding: The water level rise matched the raindrop count exactly.

This is huge news because it tells us that we have found almost all the light in the universe. There aren't many "hidden" stars or mysterious sources of light that we missed. The "fog" is made almost entirely of the light from the galaxies we can already see.

The "Excess" Mystery

However, there was one weird glitch. Some older measurements (from satellites like IRTS and CIBER) suggested there was more fog than the galaxy count could explain. It was like the water level rising higher than the raindrops accounted for.

The new study says: "No, those older measurements were probably wrong."

Why? Because the older measurements were taken from inside our solar system, where sunlight bouncing off dust (Zodiacal light) creates a "glare" that makes the fog look brighter than it really is. The new study, using the Gamma Ray "laser" method, proves that the fog is actually much dimmer and matches the galaxy count perfectly. The "extra" light reported by the old satellites was likely just a measurement error caused by the glare of our own solar system.

The Bottom Line

1. We know the fog: By using high-energy gamma rays as a "probe," the scientists measured the local cosmic background light with high precision.
2. Galaxies rule: The light from all the known galaxies accounts for nearly 100% of this background light. There is very little room for any "mystery light" from unknown sources.
3. Old data was noisy: The confusing "extra light" seen in previous studies was likely just an illusion caused by sunlight in our own solar system.

In short: The universe is exactly as bright as we thought it would be if we just added up all the stars we can see. The cosmic fog is real, but it's not hiding any secrets.

Technical Summary

1. Problem Statement

The Extragalactic Background Light (EBL) represents the cumulative radiation from all stars and galaxies outside the Milky Way, spanning ultraviolet to far-infrared wavelengths. It is a critical tracer of cosmic star formation history, galaxy assembly, and dust reprocessing. However, measuring the EBL directly is challenging due to overwhelming foregrounds, primarily Zodiacal Light (scattered sunlight within the Solar System).

While Integrated Galaxy Light (IGL) counts and direct photometric measurements (e.g., from New Horizons or HST) provide estimates, discrepancies have historically existed between these methods and indirect constraints derived from high-energy $\gamma$-ray attenuation. High-energy $\gamma$-rays ($E_\gamma > 100$ GeV) traveling through the universe interact with EBL photons via pair production ($\gamma + \gamma_{EBL} \to e^+ + e^-$), causing an energy-dependent attenuation of the observed $\gamma$-ray flux. This paper aims to provide an updated, precise determination of the local ($z=0$) EBL intensity using Very-High-Energy (TeV) $\gamma$-ray data, test the consistency of modern EBL models, and quantify the "missing" diffuse component beyond resolved galaxies.

2. Methodology

Data Selection:

• Source: The analysis utilizes the STeVECat catalog, a compilation of 403 VHE $\gamma$-ray spectra from 73 extragalactic sources (blazars) observed by ground-based Imaging Atmospheric Cherenkov Telescopes (H.E.S.S., MAGIC, VERITAS).
• Sample: After applying cuts (redshift $z > 0.01$, minimum 4 flux points per spectrum, removing duplicate flux states), the final sample consists of 268 spectra from 45 sources, extending to $z \approx 0.94$. This is a significant expansion over previous works (e.g., Desai et al. 2019).

Analysis Framework:

• Attenuation Modeling: The observed spectrum $\Phi_{obs}$ is modeled as the intrinsic spectrum $\Phi_{int}$ attenuated by an optical depth $\tau$: $\Phi_{obs} = \Phi_{int} \exp(-\alpha \tau)$.
• Model-Dependent Analysis: Seven existing EBL models were tested (ranging from semi-analytical forward evolution to backward evolution based on galaxy counts). For each model, a scale factor $\alpha$ was fitted to the data.
• Template-Marginalized Optical Depths: To derive model-independent constraints, the authors selected a subset of four diverse EBL models (Inoue et al. 2013; Franceschini & Rodighiero 2017; Saldana-Lopez et al. 2021; Finke et al. 2022). They calculated a stacked optical depth $\tau(E_\gamma, z)$ by averaging results across these templates, effectively marginalizing over specific evolutionary prescriptions.
• EBL Reconstruction: Two distinct approaches were used to reconstruct the EBL intensity spectrum at $z=0$ using the derived TeV optical depths combined with GeV optical depths (from Abdollahi et al. 2018):
  1. Physically Motivated Framework: Based on PEGASE spectral libraries, incorporating star formation history (Madau & Dickinson 2014), dust attenuation, and metallicity evolution.
  2. Empirical Reconstruction: A semi-empirical approach modeling luminosity density as a sum of log-normal distributions with fixed pivot wavelengths, allowing independent evolution parameters for UV/optical vs. IR components.

Systematics:

• Energy scale uncertainties (simulated as $\pm 15\%$) and intrinsic spectral shape degeneracies (specifically excluding simple power-law models) were evaluated. The total systematic uncertainty on the EBL scale factor is estimated at $\sim 7\%$.

3. Key Contributions

1. Expanded Dataset: The study doubles the number of redshift bins (from 2 to 4) and extends the redshift range to $z \sim 0.94$ compared to previous TeV-based EBL studies, leveraging the larger STeVECat catalog.
2. Model Validation: It provides a rigorous test of seven modern EBL models, showing that most require only minor rescaling ($\le 10\%$) to fit the observed $\gamma$-ray attenuation.
3. Joint Reconstruction: It combines TeV optical depths (sensitive to optical/near-IR EBL) with GeV optical depths (sensitive to UV/optical EBL) to produce a coherent, VHE-anchored reconstruction of the local EBL spectrum.
4. Quantifying the Diffuse Component: It places strict upper limits on any additional diffuse EBL component beyond resolved galaxy counts, addressing long-standing tensions with near-IR excess measurements.

4. Key Results

• EBL Model Consistency:

  • The Saldana-Lopez et al. (2021) model showed the best agreement with data ($\alpha_{best} = 0.99 \pm 0.05 \pm 0.06$), requiring almost no rescaling.
  • All tested models were consistent with the data within $\le 10\%$ rescaling, indicating that current models calibrated on galaxy counts and direct measurements accurately reproduce the TeV attenuation pattern.
  • The Kneiske & Dole (2010) model was rejected at $2.4\sigma$ for the fiducial normalization ($\alpha=1$), but this is expected as it represents a strict lower limit.

• Reconstructed EBL Spectrum:

  • Both the physical and empirical reconstruction methods yielded highly consistent results for the local ($z=0$) EBL intensity.
  • The reconstructed spectrum follows the Integrated Galaxy Light (IGL) to within 2–3 nW m$^{-2}$ sr$^{-1}$ (typically $< 25\%$) over the 0.5–30 $\mu$m range.
  • The results are consistent with low-zodiacal-light direct measurements, specifically New Horizons (LORRI) and Dark Cloud observations.

• Resolution of the Near-IR Excess:

  • The study finds a significant ($3\text{--}5\sigma$) discrepancy with the near-IR excess reported by IRTS and CIBER.
  • The $\gamma$-ray derived optical depths are incompatible with the high intensities implied by IRTS/CIBER. If the IRTS/CIBER excess were real, it would require an additional diffuse component of $\sim 5\text{--}10$ nW m$^{-2}$ sr$^{-1}$, which is ruled out by the TeV attenuation data.
  • The authors conclude that the IRTS/CIBER excess likely stems from underestimated systematic errors in foreground modeling (Zodiacal Light) rather than a true diffuse EBL component.

• Upper Limits on Diffuse Light:

  • Combining residuals across 0.8–5 $\mu$m, the study sets a 95% upper limit on any additional diffuse component of $\Delta I \lesssim 2.95$ nW m$^{-2}$ sr$^{-1}$, or $\lesssim 24.1\%$ of the IGL level.

5. Significance

This work provides a robust, VHE-anchored determination of the local EBL intensity. By demonstrating that the EBL intensity derived from $\gamma$-ray attenuation aligns closely with the light from resolved galaxies (IGL) and deep-space direct measurements, the paper concludes that known galaxy populations account for the vast majority of the optical and near-infrared background.

The findings effectively rule out the existence of a large, unaccounted-for diffuse component (such as from decaying dark matter or intra-halo light) that would explain the historical near-IR excesses seen in IRTS and CIBER data. This reinforces the standard cosmological picture where the EBL is dominated by the integrated light of stars in galaxies, with only limited room for additional diffuse sources. The study also serves as a crucial complement to GeV-based analyses (e.g., Banerjee et al. 2026), providing a complete picture of EBL evolution from the local universe to $z \simeq 4$.