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Detection of HI filament: Pair Stacking vs. Filament Stacking

By comparing pair and filament stacking methods using EAGLE and IllustrisTNG simulations, this study demonstrates that filament stacking is a more promising technique for detecting faint neutral hydrogen in cosmic filaments, as it achieves significantly higher column densities and avoids the massive structure contamination that severely limits pair stacking.

Original authors: Yuxi Meng, Jie Wang, Yingjie Jing, Hongxiang Chen, Zerui Liu

Published 2026-04-21

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CC BY 4.0

Original authors: Yuxi Meng, Jie Wang, Yingjie Jing, Hongxiang Chen, Zerui Liu

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 Web: Hunting for Invisible Threads

Imagine the universe not as a collection of isolated islands (galaxies), but as a giant, three-dimensional spiderweb. The "knots" of this web are massive galaxy clusters, and the thin threads connecting them are cosmic filaments.

Inside these filaments flows a ghostly gas called neutral hydrogen (H I). It's the fuel for new stars, but it's so thin and faint that it's almost invisible. Trying to see it with a telescope is like trying to spot a single strand of spider silk in a hurricane from a mile away.

To find this invisible gas, astronomers use a trick called "stacking." Instead of looking at one filament and hoping to see it, they take thousands of pictures of different parts of the web, line them up perfectly, and blend them together. This amplifies the faint signal, making the "ghost" visible.

This paper asks a simple but crucial question: What is the best way to line up these pictures?

The authors tested two methods: "Pair Stacking" and "Filament Stacking."

Method 1: The "String Between Two Rocks" (Pair Stacking)

The Idea:
Imagine you have two heavy rocks (massive galaxies) sitting in a field. You assume there's a string (a filament) connecting them. So, you find thousands of pairs of rocks, draw a straight line between them, and stack those lines on top of each other to see if a string appears.

The Problem:
The universe is messy.

False Positives: Sometimes, you pick two rocks that aren't actually connected by a string; there's just empty space (a void) between them. When you stack these, you dilute your signal.
The "Garbage" Problem: The biggest issue is that massive rocks are surrounded by a huge, messy pile of debris (halos of gas and dark matter). When you draw a line between two rocks, you accidentally include a lot of this messy debris in your picture.
The Cleanup: The authors tried to "mask" (cover up) the messy debris around the rocks to see just the string. But once they did that, the signal almost vanished. It turned out that most of what they were seeing wasn't the thin string at all; it was just the messy piles around the rocks.

The Verdict: It's an easy method to set up, but it's like trying to hear a whisper in a room full of shouting people. Even if you tell the shouters to be quiet, the whisper is still too faint to hear.

Method 2: The "Map of the Web" (Filament Stacking)

The Idea:
Instead of guessing where the strings are by connecting two rocks, this method uses a sophisticated map (created by an algorithm called DisPerSE) to find the strings first. It looks at the distribution of all the stars and galaxies to trace the actual shape of the web. Once the strings are identified, the astronomers stack the gas right along those specific lines.

The Advantage:
This is like using a metal detector to find a buried treasure chest rather than guessing where it might be.

Precision: Because the map knows exactly where the "spine" of the filament is, the stacking is much tighter. The signal isn't spread out; it's concentrated right in the center.
Resilience: Even when the authors covered up the messy debris around the massive galaxies (masking), a strong signal remained. The gas in the filaments is distinct enough to be seen even without the "noise" of the big galaxies.
The Power of Numbers: This method works best when you have a very detailed map with lots of small galaxies (high density). The more "dots" you have on your map, the better you can trace the web, and the clearer the signal becomes.

The Verdict: This method is much more powerful. Even after removing the "noise," the signal is 10 to 100 times stronger than the first method.

The Big Picture: What Does This Mean for Us?

The authors ran these tests using super-computer simulations of the universe (EAGLE and IllustrisTNG). Here is the takeaway in plain English:

Don't just connect the dots: Simply connecting two big galaxies to find the gas between them is a flawed strategy because you get too much "background noise" from the galaxies themselves.
Map the whole web: Using advanced algorithms to trace the actual shape of the cosmic web is the winning strategy. It isolates the faint gas much better.
Resolution matters: Because the gas is concentrated in a very thin line (the spine of the filament), we need telescopes with high "resolution" (sharp eyes) to see it clearly. If our telescopes are too blurry, we miss the peak of the signal.
The future is bright: With upcoming radio telescopes (like FAST) and massive galaxy surveys (like DESI), we will soon have the sharp eyes and detailed maps needed to finally "see" this invisible gas.

The Analogy:
Think of Pair Stacking as trying to find a specific conversation in a crowded stadium by standing between two loud fans and hoping their voices cancel out the noise. It rarely works.
Think of Filament Stacking as using a directional microphone that can pinpoint the exact location of the conversation based on the layout of the stadium. It cuts through the noise and hears the whisper clearly.

Conclusion: To find the building blocks of our universe hidden in the cosmic web, we need to stop guessing and start mapping. The "Filament Stacking" method is the key to unlocking the secrets of the universe's invisible gas.

1. Problem Statement

Detecting neutral hydrogen (H I) in cosmic filaments via 21 cm intensity mapping is a critical goal for understanding baryon distribution and galaxy formation. However, the signal is extremely faint due to the low density of H I in the intergalactic medium (IGM).

The Challenge: Current observational techniques struggle to isolate the filamentary signal from the much brighter H I emission in galaxies and massive halos (clusters/groups).
The Context: While "pair stacking" (stacking galaxy pairs to trace the filaments connecting them) has been used for other components (e.g., dark matter, WHIM), recent simulations suggest it yields very low signals for H I ( $\sim 0.3 \mu K$ ) due to contamination from massive structures and the low intrinsic column density of filaments.
The Question: Is there a more effective stacking technique than pair stacking to detect H I in cosmic filaments?

2. Methodology

The authors utilized two state-of-the-art cosmological hydrodynamical simulations, EAGLE and IllustrisTNG (TNG100), to compare two distinct stacking methodologies.

A. Simulation Setup

Data: Redshift $z=0$ snapshots from EAGLE (100 $h^{-1}$ Mpc box) and TNG100 (106.5 $h^{-1}$ Mpc box).
Environment Classification: Space was classified into Knots (halos $M_h \ge 10^{13} M_\odot h^{-1}$ ), Filaments (regions within 1 $h^{-1}$ Mpc of filaments identified by galaxies with $M_* \ge 10^8 M_\odot h^{-1}$ ), and Voids.
H I Mass: Derived using post-processing based on the Rahmati et al. (2013) model.

B. Stacking Techniques

Pair Stacking:
- Principle: Assumes filaments connect massive halos. Stacks the region between pairs of central galaxies ( $M_* \ge 10^{10} M_\odot h^{-1}$ ) with projected separations of 6–14 $h^{-1}$ Mpc.
- Contamination Removal: Fits and subtracts a spherical profile around the galaxy pair ends to remove the contribution of the galaxies themselves, leaving a residual "bridge" signal.
- Variations: Tested subsets based on galaxy mass ( $M_* \ge 10^{11} M_\odot h^{-1}$ ) and separation distance ( $<8$ $h^{-1}$ Mpc).
Filament Stacking:
- Principle: Directly identifies cosmic filaments using the DisPerSE algorithm (based on discrete Morse theory) applied to the galaxy distribution.
- Process: Stacks H I column density pixels based on their perpendicular distance to the identified filament spines.
- Variables: Tested with varying galaxy number densities (by lowering the stellar mass threshold from $10^{11}$ down to $10^7 M_\odot h^{-1}$ ) to assess the impact of catalog completeness.

C. Masking Strategy

To isolate the diffuse IGM signal, the authors applied halo masking. They excluded pixels within $2 \times R_{200c}$ of halos above specific mass thresholds ( $M_h \ge 10^{11}$ to $10^{15} M_\odot h^{-1}$ ). This simulates the removal of bright galaxies and their immediate surroundings, which dominate the H I signal.

3. Key Results

A. Performance of Pair Stacking

Low Signal-to-Noise: The unmasked signal is dominated by background and the spherical profiles of the galaxies. The intrinsic filament signal is only $\sim 10\%$ of the total.
Sensitivity to Masking: The signal is highly sensitive to halo contamination.
- Masking halos with $M_h \ge 10^{12} M_\odot h^{-1}$ (Milky Way mass) reduces the filament signal to 27% of its unmasked value.
- Masking halos with $M_h \ge 10^{11} M_\odot h^{-1}$ reduces the signal to <5% ( $< 10^{15} \text{ cm}^{-2}$ ).
Selection Bias: Selecting only massive or short-separation pairs increases the raw signal, but this enhancement is almost entirely due to increased halo contamination, not the filament itself.

B. Performance of Filament Stacking

High Signal Retention: Even after masking halos with $M_h \ge 10^{11} M_\odot h^{-1}$ $M_{h} \geq 1 0^{11} M_{⊙} h^{- 1}$ , the filament stacking signal retains a significant portion of the original value.
- At a radius of 10 kpc $h^{-1}$ , the masked signal is $\sim 1.32 \times 10^{17} \text{ cm}^{-2}$ (compared to $1.71 \times 10^{18} \text{ cm}^{-2}$ unmasked).
Dependence on Resolution and Density:
- The signal peaks sharply at the filament spine.
- Crucial Finding: Higher galaxy number density (lower stellar mass threshold) significantly improves the signal. Using a dense catalog ( $M_* \ge 10^7 M_\odot h^{-1}$ ) allows for precise spine localization, resulting in a column density 1–2 orders of magnitude higher than pair stacking.
- Sparse catalogs (e.g., $M_* \ge 10^{11} M_\odot h^{-1}$ ) fail to identify true filaments, leading to uniform, low-density profiles.

C. Direct Comparison

Magnitude Difference: Filament stacking produces a signal 1.5 to 2 orders of magnitude higher than pair stacking across all masking thresholds.
Robustness: Under realistic observational conditions (DESI-like galaxy density, FAST-like resolution), filament stacking yields a signal of $\sim 2.44 \times 10^{17} \text{ cm}^{-2}$ (masked), whereas pair stacking yields only $\sim 5.31 \times 10^{15} \text{ cm}^{-2}$ .
Simulation Consistency: While EAGLE and TNG100 predict different absolute H I densities (TNG is $\sim 0.5$ dex higher), the relative superiority of filament stacking over pair stacking is consistent in both simulations.

4. Key Contributions

Methodological Shift: Demonstrates that directly identifying filaments via galaxy distributions (Filament Stacking) is superior to the indirect "bridge" approach (Pair Stacking) for H I detection.
Quantification of Contamination: Provides a rigorous quantification of how massive halos contaminate pair stacking, showing that the method fails to isolate the diffuse IGM signal once halos are masked.
Optimization Guidelines: Establishes that the success of filament stacking is contingent on high galaxy number density (completeness of the catalog) and high spatial resolution to resolve the sharp density peak at the filament spine.
Feasibility Assessment: Confirms that with upcoming optical surveys (e.g., DESI) and radio telescopes (e.g., FAST), detecting H I in filaments is feasible using filament stacking, provided massive halos are masked.

5. Significance

This study resolves a critical bottleneck in 21 cm intensity mapping. It suggests that the failure of previous pair-stacking attempts to detect H I filaments was likely due to the method's inherent vulnerability to halo contamination and its inability to isolate the diffuse component.

The paper provides a clear roadmap for future observational campaigns:

Strategy: Prioritize filament stacking over pair stacking.
Requirements: Utilize deep, high-completeness galaxy catalogs to accurately trace the cosmic web topology.
Outcome: This approach offers a realistic path to detecting the "missing baryons" in the warm-hot intergalactic medium (WHIM) and understanding the gas reservoirs fueling galaxy formation.