🔭 astrophysics

SKYSURF IX -- The Cosmic Optical and Infrared Background from Integrated Galaxy Light Measurements

The SKYSURF program utilizes a massive, uniformly processed archive of 82,752 Hubble Space Telescope images to establish precise measurements of the integrated galaxy light and cosmic optical background, confirming that these emissions originate almost exclusively from within galaxies and leaving minimal room for diffuse extragalactic sources.

Original authors: Scott A. Tompkins, Simon P. Driver, Aaron S. G. Robotham, Rogier A. Windhorst, Delondrae Carter, Timothy Carleton, Zak Goisman, Daniel Henningsen, Luke J. Davies, Sabine Bellstedt, Jordan C. J. D'Silv

Published 2026-01-30

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CC BY 4.0

Original authors: Scott A. Tompkins, Simon P. Driver, Aaron S. G. Robotham, Rogier A. Windhorst, Delondrae Carter, Timothy Carleton, Zak Goisman, Daniel Henningsen, Luke J. Davies, Sabine Bellstedt, Jordan C. J. D'Silva, Juno Li, Seth H. Cohen, Rolf A. Jansen, Rosalia O'Brien, Anton M. Koekemoer, Norman Grogin, John MacKenty

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

The Big Picture: Counting the Stars to Measure the Light

Imagine you are standing in a massive, dark forest at night. You want to know how much total light is coming from the trees, but you can't see the individual leaves clearly, and there's a lot of fog (glow from the moon, streetlights, and dust) making it hard to see the true darkness of the forest.

This paper is about a team of astronomers who acted like super-precise forest rangers. They used the Hubble Space Telescope (HST) to take thousands of photos of the sky, not to find new planets or black holes, but to do something much more fundamental: count every single galaxy they could see and add up their light.

Their goal was to measure the Cosmic Optical Background (COB). Think of the COB as the "glow" of the entire universe. It's the faint, leftover light from every star that has ever formed in every galaxy, stretching back to the beginning of time.

The Challenge: The "Fog" in the Sky

Measuring this faint glow is incredibly difficult because the sky isn't truly black. It's filled with "foreground noise" that outshines the distant galaxies. The paper identifies three main types of this noise:

Zodiacal Light: Sunlight bouncing off dust in our own solar system (like a car headlight hitting dust in the air).
Diffuse Galactic Light: Starlight from our own Milky Way scattering off dust clouds nearby.
Stray Light: Glare from the Sun, Earth, or Moon hitting the telescope.

The authors had to be like digital photo editors, carefully subtracting all this "fog" and "glare" to reveal the faint, true light coming from deep space.

The Method: The "Sky Surf" Project

The team used a massive archive of Hubble images called SKYSURF.

The Scale: They processed over 82,000 individual images and stitched them together into nearly 17,000 large mosaics (like a giant puzzle).
The Coverage: This covers an area of sky about 19.6 square degrees. To visualize that, imagine the full moon is about 0.2 square degrees. They covered an area roughly 100 times the size of the full moon.
The Filter: Hubble wasn't originally designed to take wide-area photos; it's usually pointed at tiny, specific targets. The team had to be very strict, throwing away images that were too close to the Milky Way or had technical glitches, to ensure their sample was a fair representation of the whole universe.

The Discovery: Adding Up the Light

Once they had clean images, they didn't just look at the pictures; they counted the galaxies.

Counting: They identified millions of galaxies.
Summing: They calculated how much light each galaxy contributed.
Integrating: They added up all that light to get the Integrated Galaxy Light (IGL).

The Key Finding:
They found that the total light from all the galaxies they could see adds up to almost exactly the same amount as the total "glow" of the universe measured by other methods (like the New Horizons spacecraft, which is far out in the solar system away from Earth's glare).

The Analogy:
Imagine you are trying to figure out how much water is in a giant ocean.

Method A (Direct): You try to scoop up the water directly.
Method B (Indirect): You count every single raindrop that fell and every river that flowed in, then add them up.

For a long time, Method A and Method B gave very different answers. Scientists thought there must be a "hidden ocean" of light coming from somewhere else (like invisible galaxies or exotic particles).

This paper says: "No, the math works out perfectly." When you count the galaxies (Method B), the total matches the direct measurement (Method A). This means almost all the light in the universe comes from inside galaxies. There is no "hidden ocean" of extra light floating in the empty space between galaxies.

Why This Matters

The paper concludes that the universe is a very efficient place. The light we see is almost entirely the result of stars forming and dying inside galaxies. There is very little "leftover" light from mysterious, diffuse sources outside of galaxies.

By combining Hubble's deep vision with ground-based surveys (like WAVES and DEVILS), the team reduced the "statistical noise" (the uncertainty caused by looking at too small a piece of the sky) to less than 2%. This gives us a very precise census of the universe's light history.

Summary in One Sentence

The SKYSURF team meticulously counted billions of galaxies across a huge patch of sky, subtracted all the local "glare," and proved that the total light of the universe comes almost exclusively from the stars inside galaxies, leaving very little room for any mysterious, invisible sources of light.

Technical Summary: SKYSURF IX - The Cosmic Optical Background from Integrated Galaxy Light Measurements

Problem Statement
The Extragalactic Background Light (EBL), particularly the Cosmic Optical Background (COB), represents the integrated radiation from all sources outside the Milky Way. While the COB is intimately linked to the cosmic star formation history (CSFH) and galaxy evolution, its precise measurement has been hindered by significant systematic uncertainties. Direct measurements are complicated by bright foregrounds, primarily Zodiacal light (ZL), Diffuse Galactic Light (DGL), and scattered sunlight/moonlight, which often outshine the EBL. Conversely, indirect measurements via Integrated Galaxy Light (IGL)—integrating the flux of resolved galaxies—are sensitive to cosmic variance (due to limited survey areas) and magnitude zero-point discrepancies when combining data from different facilities. Historically, a tension existed between upper limits from direct measurements and lower limits from IGL studies, with discrepancies of factors of 2–4. Recent work suggests these tensions may be resolving, but a robust, high-precision IGL measurement across a wide wavelength range is required to confirm that the COB arises almost exclusively from processes within galaxies.

Methodology
This study, part of the SKYSURF (Sky Surface Brightness) HST Legacy Archival program, utilizes a unified processing pipeline to analyze a massive dataset of Hubble Space Telescope (HST) imaging.

Data Assembly: The authors processed 82,752 HST images across 23 filters into 16,686 mosaics. The data covers a combined area of approximately 19.6 deg², with individual filters ranging from 0.16 to 7.0 deg².
Processing Pipeline: Using the ProPane and ProFound software packages (consistent with the WAVES and DEVILS surveys), the team generated sky-subtracted science images, inverse variance maps, and masks. They employed a consistent pipeline to ensure uniformity across the diverse HST archival data.
Object Detection and Cataloging: Source detection was performed using ProFound. The team implemented a rigorous four-step filtering process to generate reliable galaxy number counts:
1. Galactic Extinction Correction: Applied using Planck dust maps and SVO extinction coefficients.
2. Star-Galaxy Separation: Utilized the relationship between apparent magnitude and half-light radius ( $R_{50}$ ), employing double-Gaussian fitting to distinguish populations, with specific boundaries defined for each filter.
3. Bright Star Masking: Defined circular exclusion zones around bright stars based on diffraction spike sizes to prevent fragmentation and photometric contamination.
4. Completeness Corrections: Addressed varying exposure times within mosaics by binning data by exposure time and calculating completeness limits for each bin.
Bias Mitigation: To address HST's historical bias toward observing clusters and nearby galaxies, the authors implemented statistical filtering. They compared field energy curves against a reference fit derived from ground-based WAVES and DEVILS data, removing overdense fields via sigma-clipping. They also removed fields with sub-regions exceeding the 99th percentile of object density to eliminate artifacts from nearby low-surface-brightness objects.
IGL Calculation: Galaxy number counts were integrated to calculate the IGL using standard equations converting AB magnitudes to intensity ( $nW m^{-2} sr^{-1}$ ). The analysis combined HST data with ground-based counts from WAVES (1150 deg²) and DEVILS (6 deg²) to extend the dynamic range. Magnitude zero-point corrections were applied to align ground-based data with HST filters.
Uncertainty Analysis: The study quantified four primary sources of error: Poisson statistical errors, cosmic variance (calculated using the cosvararea function), magnitude zero-point differences, and spline fitting errors. These were propagated using Monte-Carlo simulations (10,000 iterations).

Key Results

IGL Measurements: The authors present IGL measurements at 17 different wavelengths between 0.3 and 1.6 µm. For the first time, they establish a measurement of the IGL at 0.59 µm using HST data alone: 8.90 ± 0.29 $nW m^{-2} sr^{-1}$ .
Total COB: By integrating the IGL measurements across the optical range (0.1–8 µm) and combining them with previous works (Driver et al. 2016b; Koushan et al. 2021), the paper derives a total Cosmic Optical Background value of 25.15 ± 0.49 $nW m^{-2} sr^{-1}$ (an uncertainty of ~2%).
Error Budget: The combination of numerous independent sight-lines and large area coverage reduced cosmic variance uncertainties in deep number counts to 0.21%–1.8%. The total error on the IGL measurements ranges from 1.1% to 9%, depending on the filter and the availability of ground-based reference data.
Convergence: The measured IGL values are consistent with recent direct measurements from the New Horizons mission (Postman et al. 2024) and Very High Energy (VHE) gamma-ray constraints (Gréaux et al. 2024). Specifically, the IGL value at 0.59 µm aligns with the New Horizons detection and VHE estimates within their respective error bars.

Significance and Claims
The paper claims that the convergence of IGL-based measurements, direct New Horizons observations, and VHE constraints indicates that the COB arises almost exclusively from processes within galaxies. This leaves very little room for significant diffuse emission from outside the Milky Way (e.g., intra-cluster light or ultra-diffuse galaxies) to contribute to the optical background.

By reducing uncertainties to the ~1% level in specific bands, the authors argue that the COB can now serve as a powerful, precise constraint on models of galaxy formation and evolution, specifically the Cosmic Star Formation History (CSFH). The work demonstrates that with sufficient area coverage and rigorous bias correction, HST archival data can provide a near-complete census of photon production from baryon-photon decoupling to the present day. The authors note that while HST has limitations regarding survey area compared to future facilities like Euclid or Roman, the current dataset provides a finer sampling of the COB spectrum than previous studies.