Cosmic gas accretion from filaments onto galaxy clusters using the IllustrisTNG simulation

arXiv:2604.24852 · astro-ph.CO, astro-ph.GA · Submitted 2026-04-27 · Read on arXiv

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Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: Today's paper: "Cosmic gas accretion from filaments onto galaxy clusters using the IllustrisTNG simulation".

Jocelyn: Galaxy clusters grow by accreting matter along cosmic filaments,

Vera: First, who's behind it and why it matters.

Paper summary: Vera: So Jocelyn, I've been looking over the paper "Cosmic gas accretion from filaments onto galaxy clusters using the IllustrisTNG simulation," and it really paints a picture of how these massive structures actually grow by pulling in gas along those cosmic filaments. The central thesis seems to be that these filaments aren't just passive channels; they imprint a strong directional dependence on the gas properties, shaping their thermodynamic structure out to several virial radii.

Jocelyn: That's exactly what caught my attention, Vera; it sounds like the paper is trying to move beyond treating cluster environments as simple spherical systems and instead focusing on how that anisotropic accretion affects things. What they claim is that filaments are the key driver for these directional influences on gas properties.

Subrahmanyan: From a theoretical standpoint, this focus on anisotropic inflows is critical because it directly addresses the complex interplay between large-scale structure formation and the thermodynamics of galaxy clusters. If gas accretion isn't isotropic, then our models of how energy is deposited and how structures evolve need to account for that directional flow into the cluster environment.

Vera: Right, and they use the IllustrisTNG-three hundred simulation at z = zero to test this, analyzing four hundred fifteen clusters based on specific mass criteria like M200 greater than five times ten to the power of thirteen solar masses per h cubed and being more distant than five virial radii from the box edges <ref:2604.24852#pg0>. They're setting up a very specific sample size for their study.

Jocelyn: It’s interesting that they are sorting those clusters into ninety-five relaxed and ninety-five unrelaxed groups based on proxies like the center-of-mass offset normalized by R200, denoted as delta r, and the sub-halo mass fraction, fsub. That classification seems important for understanding how cluster maturity affects accretion.

Subrahmanyan: The researchers are essentially trying to link the dynamical state of a cluster—how relaxed or disturbed it is—to the specific way gas is being fed into it along those filaments. That connection between structure and accretion dynamics is where I see the biggest theoretical payoff for cosmology.

Vera: What I found most compelling in this paper, "Cosmic gas accretion from filaments onto galaxy clusters using the IllustrisTNG simulation," is their analysis comparing isotropic profiles against anisotropic profiles to see how much of that gas volume actually lies within filaments near the cluster centers. They highlight that below zero point seven five R200, the ratio of anisotropic to isotropic profiles is close to unity, which suggests a large portion of the inner cluster volume at those radii is occupied by these converging filaments.

Paper summary: Jocelyn: That finding about the profile ratios being close to one below zero point seven five R200 tells us that we need to pay very close attention when we look at gas distributions in that inner region of a cluster, as they aren't uniformly distributed spherically. It really validates the idea that filaments dominate the local environment over other structures like isolated halos in those scales.

Subrahmanyan: That observation supports the concept of filamentary channeling, suggesting that even within what we consider a virialized system, the accretion process is highly structured and not a smooth, isotropic infall from all directions. This has significant implications for how we model cluster evolution over cosmic time (Cosmic gas accretion from filaments onto galaxy clusters using the IllustrisTNG simulation).

Vera: Moving into the thermal aspects, they found three distinct temperature regimes influenced by these filamentary structures. At large distances, around three point five to five times R200, filaments are hotter than the isotropic average and can reach temperatures up to twice as high, which suggests they might be isothermal at that scale and consistent with warm gas.

Jocelyn: And then there's the transition between three point five and two times R200 where the radial temperature gradient along the filaments is shallower than in their surroundings, leading to temperatures that are up to about twenty percent lower around three times R200 compared to the ambient medium. That sounds like gas rapidly penetrating into that warmer circumcluster medium.

Subrahmanyan: That rapid penetration suggests a dynamic process where the gas cools or mixes differently when flowing along a filament compared to simply being ambiently heated, which is something we need to incorporate more finely in our simulations of structure growth.

Vera: Then around two times R200, the filaments become hotter than the intracluster medium and show an inverse tangential gradient where the filament center is hotter than its outskirts. At this interface, their temperature exceeds the isotropic average by up to about ten percent, which points toward heating induced by interaction with that hot intracluster medium.

Jocelyn: So, so they've mapped out these thermal shifts directly tied to the geometry of the accretion flow; it's fascinating how the physical structure dictates the resulting temperature profiles in such a detailed way.

Paper summary: Subrahmanyan: These thermal gradients and heating mechanisms are vital because they tell us about energy transfer processes occurring at these interfaces, which is essentially how we understand non-gravitational physics in cluster environments (Cosmic gas accretion from filaments onto galaxy clusters using the IllustrisTNG simulation).

Vera: When we look at the impact of cluster properties on this anisotropy, they found a strong dependence with cluster mass. The temperature drop inside filaments around clusters, between one point five and three point five times R200, is significantly stronger for massive clusters, reaching up to thirty-five percent below the isotropic temperature.

Jocelyn: That means that in more massive systems, the influence of the filamentary accretion flow on the gas thermodynamics is much more pronounced than in less massive halos. It suggests that larger structures are linked to filaments that are denser and hotter overall.

Subrahmanyan: I agree, because a more massive halo generally implies a deeper potential well and thus a more intense interaction with the surrounding cosmic web environment, leading to these stronger thermal contrasts when accretion occurs along specific pathways (Cosmic gas accretion from filaments onto galaxy clusters using the IllustrisTNG simulation).

Vera: Furthermore, they noted that relaxed massive clusters exhibit the strongest temperature contrast overall, with filament temperatures being colder up to about forty percent than the surrounding cluster outskirts at three times R200. This really ties in their dynamical state classification with the thermal structure we saw earlier.

Jocelyn: That contrast is quite a bit, so a forty percent difference in temperature between what's in the filament and what’s outside it at that radius is substantial evidence of this filamentary channeling effect at work within these mature systems.

Subrahmanyan: This finding suggests that the more dynamically settled clusters are those embedded in more well-established, denser filaments, which is a key piece of evidence for how assembly history dictates the current thermodynamic state of the ICM (Cosmic gas accretion from filaments onto galaxy clusters using the IllustrisTNG simulation).

Vera: Finally, looking at gas dynamics and accretion signatures, their radial velocity analysis shows clear preferential flow. Beyond three times R200, warm gas predominantly accretes along these filaments, while between one and three times R200, velocities are bimodal: high-velocity inflows occur mainly along the filaments.

Paper summary: Jocelyn: That preference for high-velocity inflow along the filament path is a direct observational signature we can look for in actual X-ray observations of cluster outskirts to confirm this simulation's findings. It gives us a target for what to search for in real data.

Subrahmanyan: The bimodal velocity distribution between one and three times R200 implies that the accretion process isn't just a smooth funnel but involves both organized filamentary flows and some more diffuse, isotropic infall from the general surrounding medium (Cosmic gas accretion from filaments onto galaxy clusters using the IllustrisTNG simulation).

Vera: They also identified potential shock mechanisms in the cluster outskirts between one and two times R200, where entropy outside filaments increases by up to fifty percent compared to inside them, supported by a Mach number distribution showing a shock front near R200 with M approximately two in most of the clusters.

Jocelyn: A Mach number around two means we're talking about some fairly significant shocks happening when that fast infalling gas hits the intracluster medium, which is a very tangible physical process to visualize.

Subrahmanyan: These shock fronts are crucial because they represent where kinetic energy from the infall is being converted into thermal energy, providing a mechanism for heating and mixing that we need to account for when calculating the overall mass and temperature profiles of these systems (Cosmic gas accretion from filaments onto galaxy clusters using the IllustrisTNG simulation).

Vera: So, to wrap up this paper on "Cosmic gas accretion from filaments onto galaxy clusters using the IllustrisTNG simulation," what we've seen is a comprehensive look at how filamentary accretion dictates the thermal and dynamical structure of galaxy clusters across various scales.

Jocelyn: It really shows that these structures are not isolated; they are continuously influenced by the cosmic web, and that this influence leaves a very distinct thermodynamic fingerprint on the gas within them.

Subrahmanyan: The implication here is that understanding cluster growth requires moving past simple spherical models and incorporating the anisotropic, filament-driven processes we've detailed in this work into our broader cosmological frameworks (Cosmic gas accretion from filaments onto galaxy clusters using the IllustrisTNG simulation).

Vera: And for those of us looking at observational data, this paper provides a strong framework to look for these specific thermal and velocity anomalies when we examine X-ray observations of cluster outskirts.

Jocelyn: Exactly; it gives us something concrete to search for in the sky that aligns with the simulation's predictions about gas accretion pathways and shock heating.

Conclusion: Vera: The title itself, "Cosmic gas accretion from filaments onto galaxy clusters using the IllustrisTNG simulation," sounds pretty descriptive of what they did in a nutshell. It tells us exactly which physical process they were modeling and the tool they used to investigate it across different scales.

Jocelyn: I agree with Vera; it's very straightforward, which is good for understanding the core concept quickly, especially for those of us who are looking at observational data and trying to match up what we see in the sky. Who were the authors behind this paper?

Subrahmanyan: The authors are a solid team of researchers who really brought together hydrodynamics and large-scale structure modeling to tackle this complex accretion problem. Their approach is definitely rooted in established simulation techniques from the IllustrisTNG project, but their specific analysis focuses on extracting detailed thermodynamic signatures from that simulation data.

Vera: That's right, and when we look at the implications of this work, it really suggests that the way clusters grow isn't just about random gravity; it’s heavily influenced by these large-scale filamentary structures in a very organized way.

Jocelyn: I can see how that matters because if gas is flowing preferentially along filaments, then the chemical enrichment and temperature distribution inside a cluster aren't uniform; they should show some kind of directional bias that we could potentially measure in real X-ray observations.

Subrahmanyan: Exactly, and from a theoretical perspective, this helps us understand how energy gets deposited into the intracluster medium when it comes along these channels, which is fundamental to our models of how structure evolves over billions of years.

Vera: It really paints a clearer picture for us on what to look for in the sky—those specific thermal and velocity patterns that are imprinted by this accretion process.

Jocelyn: And it gives us a roadmap for interpreting those observations, moving beyond just seeing a hot gas blob to understanding the physics of how that gas got there.

Subrahmanyan: So, as we wrap up this discussion on the paper's core message and its potential impact on our understanding of cosmic structure assembly, we need to think about how these filament-driven accretion pathways connect to larger cosmological theories.

Université Paris-Saclay · Institut d’Astrophysique Spatiale

astro-ph.CO, astro-ph.GA

Submitted: 2026-04-27

Updated: 2026-10-07

Comments: 12 pages, 8 figures, submitted to A&A, comments welcome

License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/

Importance score: 90/100

The gist: Galaxy clusters grow by accreting matter along cosmic filaments, and this study characterizes gas accretion onto clusters using the IllustrisTNG-300 hydrodynamical simulation to build a coherent

Key concepts

Filaments
These are cosmic structures where matter collects along channels that feed material into galaxy clusters. The study shows they act as distinct pathways, strongly influencing how gas properties like temperature and pressure change as it falls toward the cluster center.
Isotropic vs. Anisotropic Profiles
Isotropic profiles measure gas properties averaged spherically around a cluster, ignoring directional flow. Anisotropic profiles specifically account for the directional influence of filaments. Comparing these two helps researchers determine how much of the observed gas structure is due to random distribution versus organized filamentary accretion.
Thermal Regimes
The study identified three temperature zones within clusters influenced by filaments: one far out where filaments are hot, a middle zone where they penetrate the cluster medium, and an inner zone near 2R200 where filament temperatures exceed the surrounding gas due to interaction.

Terminology

Summary

Galaxy clusters grow by accreting matter along cosmic filaments, and this study characterizes gas accretion onto clusters using the IllustrisTNG-300 hydrodynamical simulation to build a coherent picture of how gas is accreted and thermally processed during its infall into these structures.

The gist: Filaments act as distinct channels within the cluster environment, imprinting a strong directional dependence on gas properties that shapes their thermodynamic structure out to several virial radii.

Simulation and Sample Selection

The study utilizes the IllustrisTNG-300 magneto-hydrodynamical simulation at z = 0, analyzing 415 galaxy clusters selected from the Friends-of-Friends (FoF) halo catalog based on criteria: M200 > 5 × 1013M⊙h−1 and halos more distant than 5R200 from the box edges. The physical properties of the gas, including temperature, entropy, density, and pressure, are extracted from gas cells using equations (1) through (3), where entropy is defined as S = T n2/3e. To explore cluster properties, the dynamical state is assessed using two proxies: a center-of-mass offset normalized by R200 (∆r) and the sub-halo mass fraction (fsub). This selection process resulted in 95 relaxed clusters and 95 unrelaxed clusters.

Anisotropic Gas Property Analysis

The core methodology involves probing the radial distribution of gas properties as a function of distance to clusters by comparing isotropic profiles (spherically averaged) with anisotropic profiles, which account only for gas in filaments. The analysis uses volume-weighted averages for properties like temperature (T), entropy (S), and pressure (P) within 50 bins from 0.05 to 5R200. A key finding is that the ratio of anisotropic to isotropic profiles is close to unity below 0.75 R200, indicating that a large fraction of the cluster volume at these radii is occupied by filaments converging near cluster centers.

Thermal Regimes and Gradients

The temperature profiles reveal three distinct thermal regimes influenced by filamentary structures:

  1. At large distances, r ≈ [3.5 – 5]R200, filaments are hotter than the isotropic average, reaching temperatures up to twice as high, suggesting filaments are isothermal at this scale and consistent with warm gas (T ∼ 105–7 K).

  2. Between 3.5 and 2 R200, the radial temperature gradient is shallower along filaments than in their surroundings, resulting in temperatures up to ∼ 20% lower around 3 R200 compared to the ambient medium, reflecting rapid penetration into the warm circumcluster medium.

  3. Around 2R200, filaments become hotter than the ICM and exhibit an inverse tangential gradient—i.e., the filament center is hotter than its outskirts. At this interface, filament temperature exceeds the isotropic average by up to ∼ 10%, suggesting heating induced by interaction with the hot ICM.

Impact of Cluster Properties

The study investigates how cluster mass and dynamical state impact gas anisotropy. The strong dependence with cluster mass is evident, as the temperature drop inside filaments around clusters (∼ 1.5–3.5R200) is significantly stronger for massive clusters, reaching up to 35% below the isotropic temperature. Furthermore, relaxed massive clusters exhibit the strongest temperature contrast, with filament temperatures being colder up to ∼ 40% than the surrounding cluster outskirts at 3R200. This suggests that massive halos are linked to more well-established filaments that are denser and hotter.

Gas Dynamics and Accretion Signatures

The radial velocity analysis, quantified by the conditional probability distribution P(vrad r), highlights preferential flow. Beyond 3R200, warm gas predominantly accretes along filaments, while between 1R200 and 3R200, velocities are bimodal: high-velocity inflows occur mainly along filaments, while lower-velocity outflows are almost only outside filaments. This indicates that gas accretion occurs through a combination of coherent filamentary inflows and more diffuse isotropic infall from the surrounding medium, with gas within filaments following preferential, dynamically organized pathways.

Shock Mechanisms

The analysis identifies potential shock mechanisms in the cluster outskirts. Between 1 and 2R200, the entropy outside filaments increases from ∼ 0% up to ∼ 50% higher than inside filaments, likely reflecting heating or mixing driven by shocks. A Mach number distribution around R200 shows a shock front near R200 with M ∼ 2 in most of the clusters, supporting the scenario where "fast infalling gas along filaments shocks against the intracluster medium.

Improvements for AI systems

Based on the scientific paper provided, here are specific improvements that could be made to AI systems by leveraging its findings:

  1. Improve Large-Scale Structure (LSS) Modeling and Baryonic Physics:

  2. Improve Galaxy Cluster Evolution Predictions and Diagnostics:

  3. Develop Observational Signatures for Cosmic Filaments:

  4. Refine Simulation Fidelity and Parameterization for Hydrodynamics:

Specific Improvements in Functionality:

  1. AI systems can be trained to accurately predict the thermodynamic state (temperature, entropy, density) of gas as it accretes from cosmic filaments into galaxy clusters by learning the complex radial gradients and anisotropic profiles described in Section 3.2 and Figure 1.

  2. AI systems can be improved to diagnose cluster dynamical states (relaxed vs. unrelaxed) with higher accuracy by utilizing the combination of center-of-mass offset and substructure mass fraction proxies (Section 2.3), leading to better predictions of how cluster mass and relaxation correlate with gas anisotropy (Section 3.3).

  3. AI systems can be developed to identify and classify anisotropic imprints in simulated or observational data by learning the characteristic temperature inversions (e.g., filament cores being hotter than outskirts) and tangential gradients across different cluster populations, as detailed in Section 4.1 and Figure 3.

  4. AI systems can perform predictive modeling of gas flow (accretion vs. ejection) by analyzing conditional probability distributions of radial velocity, allowing them to distinguish between fast infalling warm gas along filaments and slower, more diffuse isotropic infall from the surrounding medium (Section 5).

  5. AI systems can generate synthetic observational data that mimics the specific temperature profiles and velocity field signatures observed in different cluster environments (e.g., massive relaxed vs. low-mass unrelaxed clusters), enabling better calibration of X-ray and Sunyaev–Zel’dovich observations (Section 6).

  6. AI systems can be used to model the impact of cluster properties on shock physics, predicting the Mach number distribution around virial shocks based on whether a cluster is relaxed or unrelaxed, which is crucial for interpreting high-energy astrophysical data (Section 6.2).

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