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The Three Hundred project: cosmic web identification from 2D gas and Compton- $y$ maps of galaxy clusters outskirts

This study uses "The Three Hundred" simulations to demonstrate that 2D reconstructions of the cosmic web from gas and Compton- $y$ maps accurately represent the underlying 3D filamentary structures, with filaments contributing approximately 80% of the integrated Compton- $Y$ signal in cluster outskirts.

Original authors: Sara Santoni, Marco De Petris, Gustavo Yepes, Weiguang Cui, Daniel de Andrés, Antonio Ferragamo, Raphaël Wicker

Published 2026-04-28

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CC BY 4.0

Original authors: Sara Santoni, Marco De Petris, Gustavo Yepes, Weiguang Cui, Daniel de Andrés, Antonio Ferragamo, Raphaël Wicker

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 Cosmic Highway System: Mapping the Invisible Bridges of the Universe

Imagine you are looking at a map of a massive country at night. You see bright, glowing cities (these are the galaxy clusters) scattered across the landscape. But if you look closely, you’ll notice these cities aren't just floating randomly in the dark; they are connected by glowing highways and transit lines (this is the Cosmic Web).

These "highways" are made of gas and dark matter, and they act as the plumbing system of the universe, funneling matter into the big cities. The problem? These highways are incredibly dim and hard to see. They are like faint, dusty roads in the middle of a desert at twilight.

This scientific paper, titled "The Three Hundred project," is essentially a "test drive" to see if our current telescopes and mathematical tools are good enough to map these cosmic highways accurately.

The Three Big Questions

The researchers wanted to solve three main mysteries:

1. The "Flat Map" Problem (2D vs. 3D)

Imagine you are looking at a 3D model of a highway interchange from a drone, but then you switch to a flat, 2D photograph taken from directly above. Some roads might overlap, making it look like one giant intersection when it’s actually two separate levels.

The scientists asked: "If we only see the universe in 2D (like a flat photo), how much of the real 3D highway system are we missing or misinterpreting?"

The Result: They found that while we do lose some detail—specifically, we tend to underestimate how many "roads" connect to the big "cities"—the 2D maps are still remarkably good. Most of the major highways show up clearly, even if the map looks a bit simpler than reality.

2. The "Heat Signature" Trick (The SZ Effect)

Since the gas in these highways is too faint to see with regular light, astronomers use a trick called the Sunyaev-Zel’dovich (SZ) effect. Think of this like using a thermal imaging camera to find a warm animal in a dark forest. Instead of looking for light, they look for the "heat" (pressure) left by the gas.

The scientists asked: "If we use this 'thermal camera' instead of a regular camera, will we still see the same highways?"

The Result: Yes! The "heat maps" matched the "gas maps" very closely. This is great news because it means we can use these thermal signatures to successfully map the invisible structure of the universe.

3. The "Hidden Tolls" (The Impact on Cluster Measurements)

When astronomers measure a galaxy cluster, they often try to calculate its total "weight" (mass) by looking at its brightness. However, if a massive highway is feeding directly into the city, that highway's light might get mixed up with the city's light.

The scientists asked: "Are these highways 'polluting' our measurements of the cities?"

The Result: They discovered that the highways are a huge deal! In the outskirts of these clusters, about 80% of the signal actually comes from the filaments (the highways) rather than the cluster itself. It’s like trying to weigh a city, but realizing that 80% of the weight you're measuring is actually the massive traffic jams on the roads leading into it.

Why Does This Matter?

If we want to understand how the universe grew from a smooth soup of particles into the complex web of galaxies we see today, we have to understand the "roads."

This paper proves that even though our view is a bit flattened and we are using "thermal" shortcuts, we have the right tools to map the cosmic web. It gives astronomers the confidence to say: "We can see the invisible bridges, and we know how to account for them when we weigh the giants of the universe."

Technical Summary: The Three Hundred Project – Cosmic Web Identification from 2D Gas and Compton-y Maps

1. Problem Statement

Galaxy clusters reside at the nodes of the cosmic web, connected by a network of filamentary structures. While the 3D distribution of the cosmic web is well-studied in simulations, observational studies face significant challenges. Direct detection of the Warm-Hot Intergalactic Medium (WHIM) within filaments is difficult due to faint X-ray emissions and weak thermal Sunyaev-Zel’dovich (tSZ) signals. Furthermore, observational data are inherently 2D projections, which can lead to:

Projection Effects: The loss of 3D information, potentially causing spurious detections or the masking of real structures.
Tracer Bias: Differences in the reconstructed web depending on whether galaxies, dark matter, or gas (density vs. pressure/temperature) are used.
Parameter Bias: The impact of filamentary gas on the estimation of cluster properties, such as the integrated Compton- $Y$ parameter, which is often used as a proxy for cluster mass.

2. Methodology

The study utilizes data from The Three Hundred, a suite of 324 high-resolution "zoom-in" hydrodynamical simulations of massive galaxy clusters.

Benchmark: The authors use the 3D gas density distribution (from previous work by Santoni et al. 2024) as the "ground truth" to evaluate the accuracy of 2D reconstructions.
Filament Extraction: The DisPerSE (Discrete Persistent Structure Extractor) algorithm is used to identify the cosmic web skeleton (nodes and filaments) from both 3D grids and 2D projected maps.
Observational Mocks:
- 2D Gas Maps: Created by projecting gas particles onto a 2D grid.
- Compton- $y$ Maps: Generated using the PYMSZ4 package to simulate the tSZ effect, which traces electron pressure ( $P_e = n_e k_B T_e$ ).
Quantitative Metrics:
- Completeness and Purity: Used to assess the reliability of critical points (nodes) and filament segments.
- Geometric Distance: The distance between the spines of the 2D/SZ networks and the projected 3D networks is calculated to measure spatial agreement.
- Connectivity ( $k$ ): The number of filaments connected to a cluster, used to test if 2D projections underestimate the cluster's environmental complexity.
- Radial Profiles: Analysis of the Compton- $y$ signal inside vs. outside filaments to quantify their contribution to the cluster outskirts.

3. Key Contributions and Results

A. Reliability of 2D Gas Projections:

Nodes: The 2D gas maps provide a robust representation of the 3D network, with an average completeness of 81% and purity of 77%.
Filaments: The spatial agreement is high; the median distance between the spines of the 2D and projected 3D networks is only $\approx 0.22 \, h^{-1}$ Mpc, which is well within the typical filament radius ( $\sim 1\text{--}2 \, h^{-1}$ Mpc).
Connectivity Underestimation: A significant finding is that 2D projections systematically underestimate the connectivity of the most massive clusters. This is attributed to the loss of 3D information where filaments may overlap or be hidden in the projected density field.

B. The SZ Effect as a Tracer:

The Compton- $y$ maps (tracing pressure) are effective tracers of the cosmic web. The median distance between the SZ and gas skeletons is $\approx 0.24 \, h^{-1}$ Mpc.
The SZ network shows a completeness of 73% and purity of 63%, demonstrating that the tSZ effect can reliably identify filamentary structures.

C. Impact on Cluster Physics:

Filamentary Contribution: In the cluster outskirts ( $> 2R_{500}$ ), filaments are not just present; they are dominant. Approximately 80% of the integrated Compton- $Y$ parameter in the outskirts is contributed by filamentary structures.
Mass Proxy Implications: The study finds that the ratio $Y_{5R_{500}}/Y_{500}$ is approximately $1.90^{+0.68}_{-0.27}$ . This is consistent with existing Universal Pressure Profile (UPP) models but highlights that the outskirts are highly dynamic and deviate from simple spherical models due to the connected cosmic web.

4. Significance

This paper provides a critical validation for future observational cosmology. It proves that:

The cosmic web can be reconstructed from 2D gas and SZ observations, despite the inherent loss of 3D information.
SZ observations are a viable tool for studying the large-scale environment of galaxy clusters.
Cluster mass estimates must account for the environment: Because filaments contribute significantly to the Compton- $Y$ signal in the outskirts, ignoring the cosmic web could lead to systematic errors in mass proxies derived from integrated pressure profiles.