A New Measurement of the Extragalactic Background Light using 15\,yr of {\it Fermi}-Large Area Telescope Data

Using 15 years of Fermi-LAT data and 1576 blazars, this study achieves the most precise determination of the extragalactic background light to date by detecting its attenuation with ~23σ significance across 19 redshift bins, thereby confirming general consistency with recent models of star formation and galaxy evolution.

The Gist

The Cosmic Fog: Measuring the Universe's "Background Noise"

Imagine you are standing in a dark forest at night, looking at a distant campfire. If the air is perfectly clear, you see the fire bright and sharp. But if there is fog, mist, or smoke between you and the fire, the light gets dimmer and redder. The "fog" scatters the light, making it harder to see the source clearly.

In our universe, there is a similar kind of "fog," but it's not made of water droplets. It's made of light itself. This is called the Extragalactic Background Light (EBL).

Think of the EBL as the "glow" of the entire universe. It's the combined light from every single star, galaxy, and black hole that has ever existed, stretched out over billions of years. It fills the empty space between galaxies like a faint, invisible blanket of photons (light particles).

The Problem: We Can't See the Fog Directly

Measuring this cosmic fog directly is incredibly hard. It's like trying to measure the brightness of a faint candle in a room while a blinding spotlight (our own Sun and the Milky Way) is shining right next to you. The "glare" from our local neighborhood drowns out the faint glow of the rest of the universe.

The Solution: Using Blazars as Flashlights

Instead of trying to measure the fog directly, the scientists in this paper used a clever trick. They used Blazars as their flashlights.

• What is a Blazar? Imagine a giant cosmic lighthouse. It's a supermassive black hole at the center of a galaxy, shooting a powerful beam of energy (gamma rays) straight at Earth.
• The Experiment: These blazars are incredibly bright and far away. As their high-energy gamma-ray beams travel across the universe to reach us, they have to pass through that "fog" of background light (the EBL).
• The Interaction: When a high-energy gamma ray hits a low-energy photon from the background fog, they annihilate each other, turning into a pair of particles (an electron and a positron). It's like two cars crashing and disappearing, leaving no car behind.
• The Result: The further the gamma rays have to travel, the more "fog" they hit, and the more of them get destroyed. By the time they reach Earth, the beam is dimmer than it was when it left.

What This Paper Did

The team, led by Anuvab Banerjee and colleagues, looked at 15 years of data from the Fermi-LAT telescope (a space telescope that watches for gamma rays).

1. More Data, Better Picture: Previous studies looked at about 750 blazars over 10 years. This study looked at 1,576 blazars over 15 years. It's like upgrading from a blurry, low-resolution photo to a sharp, 4K high-definition video.
2. The Measurement: They measured exactly how much the light from these blazars dimmed. Because they knew how bright the blazars should be (based on how they behave when there is no fog), they could calculate exactly how much "fog" was in the way.
3. The Result: They detected this cosmic fog with 23-sigma significance. In the world of science, 5-sigma is usually enough to claim a discovery. 23-sigma is like rolling a six on a die and getting it right 23 times in a row. It is an incredibly solid, undeniable proof that the fog exists.

What They Found

By measuring how much the light dimmed at different distances (redshifts), they mapped out the history of the universe's light:

• The "Horizon": They found the "Cosmic Gamma-Ray Horizon." This is the point in the universe where the fog is so thick that gamma rays can't get through at all. It's the edge of the "transparent" universe.
• Star Formation History: Since the fog is made of light from stars, measuring the fog tells us how many stars were being born at different times in history. Their results match up very well with what we know about how galaxies and stars have evolved over the last 13 billion years.
• No Hidden Secrets: They also checked if there was any "extra" light coming from stars that got stripped out of galaxies and are floating freely in the space between them (called Intra-Halo Light). They found that while there might be a tiny bit of this, it's not a huge component. The universe's light is mostly coming from the galaxies we can already see.

Why Does This Matter?

Think of the universe as a giant library.

• The Books: The galaxies and stars are the books.
• The Dust: The EBL is the dust on the books.

If you want to understand the history of the library (how stars formed, how galaxies grew), you need to know how much dust is on the shelves. If you don't account for the dust, your measurements of the books will be wrong.

This paper gives us the most precise "dust map" of the universe to date. It helps astronomers:

1. Test Physics: It helps check if the laws of physics work the same way across the whole universe.
2. Find New Particles: It helps rule out or find exotic particles (like axions) that might interact with light.
3. Understand the Past: It tells us exactly how bright the universe was when it was a baby, helping us understand the "re-ionization" era (when the first stars turned on the lights of the universe).

The Bottom Line

This paper is a massive success story of patience and technology. By waiting 15 years and watching over 1,500 cosmic lighthouses, the scientists have finally mapped the "fog" of the universe with incredible precision. They confirmed that our current theories about how stars and galaxies form are correct, and they've set a new gold standard for how we measure the history of light in the cosmos.

Technical Summary

1. Problem Statement

The Extragalactic Background Light (EBL) is the integrated electromagnetic radiation emitted by all stars, galaxies, and accreting black holes throughout cosmic history, spanning ultraviolet (UV) to infrared (IR) wavelengths. Precise measurement of the EBL is critical for:

• Probing models of star formation and galaxy evolution.
• Understanding the propagation of gamma rays across cosmological distances.
• Constraining cosmological parameters and testing exotic physics (e.g., axion-like particles, Lorentz invariance violation).
• Estimating the redshifts of distant blazars.

Direct measurement of the EBL is hindered by foreground contamination (Zodiacal Light, Diffuse Galactic Light). While galaxy counts provide a lower limit, they miss faint, unresolved populations. An alternative, powerful indirect method involves measuring the attenuation of high-energy gamma rays from distant blazars caused by pair production ($\gamma + \gamma_{EBL} \to e^+ + e^-$) with EBL photons. Previous measurements using Fermi-LAT data (e.g., Fermi-LAT Collaboration et al. 2018) were limited by exposure time and sample size. This paper aims to update these measurements using a significantly larger dataset to improve precision and extend the redshift coverage.

2. Methodology

Data Sample

• Source: The study utilizes 15 years of Fermi-LAT data (August 2008 – September 2023), representing a $\sim$1.7x increase in exposure compared to the previous 101-month analysis.
• Object Selection: The sample is drawn from the 4LAC-DR3 catalog.
  • Selection Criteria: Only blazars (FSRQs and BL Lacs) with measured redshifts and significant detection ($TS \ge 25$) above 1 GeV were retained.
  • Final Sample: 1,576 blazars (752 FSRQs and 822 BL Lacs) with redshifts ranging from $z = 0.03$ to $z = 4.31$.
• Analysis Technique:
  • Energy Range: 1 GeV to 1 TeV.
  • Spectral Modeling: Intrinsic spectra were modeled using a log-parabola or a power-law with exponential cutoff, selected based on curvature tests ($TS_{c,1}$ and $TS_{c,2}$).
  • EBL Attenuation: The observed spectrum is related to the intrinsic spectrum via an attenuation factor $e^{-b\tau_{model}(E,z)}$, where $b$ is a renormalization factor fitted to the data.
  • Joint-Likelihood Fit: A global fit was performed across all blazars to determine the collective detection significance of the EBL.

Optical Depth Derivation

• The optical depth ($\tau_{\gamma\gamma}$) was measured in 19 redshift bins and 6 energy bins (logarithmically spaced between 1 GeV and 1 TeV).
• To minimize model dependence, the final $\tau_{\gamma\gamma}$ was calculated as the average of three leading EBL models:
  1. Finke et al. (2022)
  2. Saldana-Lopez et al. (2021)
  3. Franceschini & Rodighiero (2017)
• Systematic uncertainties (intrinsic spectrum choice, $E_{max}$, IRF) were accounted for by adding a 20% uncertainty in quadrature to statistical errors.

Reconstruction Methods

Two methods were used to reconstruct the EBL intensity and luminosity density from the measured optical depths:

1. Empirical Model: Fitting the luminosity density with a sum of 7 log-normal distributions evolving with redshift (based on Abdollahi et al. 2018).
2. Physically Motivated Model: Using the framework of Finke et al. (2022), which incorporates star formation history, dust extinction, and metallicity evolution, constrained by MCMC fitting.

3. Key Contributions

• Unprecedented Dataset: The first EBL measurement using 15 years of Fermi-LAT data and a sample nearly double the size of previous studies (1,576 vs. 759 blazars).
• Extended Redshift Coverage: The analysis extends the measurement of EBL optical depth up to $z \approx 4.3$, compared to $z \approx 3.1$ in previous works.
• High Significance Detection: Achieved a collective detection of EBL attenuation with $\sim 23\sigma$ significance ($TS_{EBL} \approx 540$), an 80% increase in statistical significance over previous results.
• Time-Resolved Analysis: Conducted a specific test on highly variable sources to confirm that intrinsic source variability does not bias the EBL attenuation measurement.

4. Results

EBL Detection and Optical Depth

• Significance: The renormalization factor $b$ is consistent with 1.0 within $2\sigma$ for all three tested models, confirming the EBL intensity matches current theoretical predictions.
• Optical Depth: The study provides the most precise determination of $\tau_{\gamma\gamma}$ to date in the GeV range, covering 19 redshift epochs.
• Cosmic Gamma-Ray Horizon (CGRH): The energy at which $\tau_{\gamma\gamma} = 1$ (the transition from a transparent to an opaque universe) was measured with unprecedented accuracy. The results align well with the Saldana-Lopez (2021), Finke (2022), and Franceschini (2017) models.

EBL Intensity and Evolution

• Empirical Reconstruction: The reconstructed EBL intensity at $z=0$ is consistent with independent measurements (e.g., New Horizons, HST) but slightly lower in the 4–20 $\mu$m range compared to previous Fermi-LAT results, likely due to updated VHE constraints.
• Physical Model: The physically motivated model yields star formation rate (SFR) parameters consistent with Finke et al. (2022). The model successfully constrains the luminosity density and SFR out to $z \approx 5$ (when the Universe was $\sim 1$ Gyr old).
• Diffuse Components (IHL): The study tested for Intra-Halo Light (IHL), a potential diffuse component from tidally stripped stars. A 95% upper limit of $\alpha_{IGL} < 0.23$ was derived, indicating that IHL contributes no more than ~23% of the local EBL intensity. This disfavors claims that IHL contributes up to 30% of the near-infrared background.

5. Significance

• Cosmological Tool: The results solidify gamma-ray attenuation as a robust cosmological tool for constraining star formation history and galaxy evolution, particularly at early epochs ($z > 3$).
• Benchmark for Future Observations: The refined CGRH measurements provide a stringent benchmark for theoretical models and will serve as a reference for the Cherenkov Telescope Array (CTAO), which will probe higher energies (multi-TeV) with greater sensitivity.
• Future Prospects: The paper highlights that the next major leap in EBL characterization requires:
  1. Extending measurements to $z \gtrsim 5$ (requiring bright high-z GRBs).
  2. Determining redshifts for the $\sim 1,600$ BL Lacs in the current Fermi-LAT catalog that lack them, which would dramatically improve the signal-to-noise ratio due to the hard spectra of BL Lacs.

In conclusion, this work represents the most precise determination of the EBL using GeV $\gamma$-rays to date, confirming the consistency of current EBL models and providing a refined map of the Universe's luminosity density evolution.