Pinpointing the cosmic web between massive galaxy clusters I. The incidence of HI and OVI as WHIM tracers

arXiv:2610.11203 · astro-ph.CO · Submitted 2026-10-08 · Read on arXiv

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

Vera: I'm Vera, and with me are Jocelyn and Subrahmanyan, guest researcher.

Jocelyn: Today's paper: "Pinpointing the cosmic web between massive galaxy clusters I. The incidence of HI and OVI as WHIM tracers".

Vera: The gist: The incidence rate of H I and O VI as WHIM tracers in inter-cluster filaments indicates that warm-hot gas is prevalent in these environments,

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

Title and authors: Vera: So we’ve established the focus on finding WHIM signatures in cluster filaments using H I and O VI tracers. Now, let's talk about the paper's summary to get a clearer picture of what they actually did in this study >

Jocelyn: Okay. The summary says they constructed a catalog of intervening putative filaments first by identifying cluster pairs from a galaxy cluster catalog and then cross-matching those pairs with the UV bright QSO catalog, which was done using specific criteria like minimum richness lambda of ten for the clusters > <ref:2610.11203#pg1>

Subrahmanyan: And for these cluster pairs, they required each cluster to be at a redshift between z = zero point one and z = zero point five and the redshift difference between them to be less than two thousand km s−one >

Vera: That’s setting up the geometric space they are interested in, linking these specific massive structures together across that defined range of redshifts >

Jocelyn: Then they selected QSO sightlines such that their redshift had to be larger than the redshift of the cluster pair plus three thousand km s−one in rest-frame, and the sightline itself had to be within an impact parameter of three Mpc from that inter-cluster axis >

Subrahmanyan: That spatial constraint is key because it targets the volume between these clusters where they predict high probabilities of filamentary structures exist, based on cosmological dark matter simulations >

Vera: It's all about defining a very specific region in space and time to search for that warm-hot gas component, which is what the paper is doing by focusing on these cluster outskirts and the intergalactic medium between them >

Jocelyn: So, if you think about it, they are essentially creating a targeted survey volume around known cosmic web structures to see if those predicted WHIM signatures show up in reality >

The paper's summary: Vera: Now that we know the setup, let’s look at how the paper suggests improving or refining this approach, because they always have suggestions for next steps >

Jocelyn: They highlight a few key areas. First, they focus their analysis only on absorption features with reliabilities 'a' and 'b', excluding the most uncertain absorbers in order to make their measurements more robust >

Subrahmanyan: That’s smart because it directly relates to how we try to characterize the gas temperature, which brings us to the Doppler parameter fitting using Voigt profiles >

Vera: Right. They use Voigt profile fitting with Veeper9 to characterize observational parameters like redshift z, column density log N, and the Doppler parameter b >

Jocelyn: Then they restrict their analysis only to absorption features where 'b' is greater than or equal to forty km s−one because those broad absorbers trace gas at temperatures above one hundred five K without a critical dependence on the value of alpha, which accounts for turbulence > <ref:2610.11203#pg3,105 K without a critical dependence on the value>

Subrahmanyan: That finding about the Doppler parameter being robust under reasonable variations in the turbulent contribution is important because it means identifying broad absorbers, which they call BLAs, is not significantly affected by how much turbulence we assume there is >

Vera: So they’re essentially saying that this method for identifying WHIM tracers doesn't get completely messed up by how turbulent the gas actually is in those filaments >

Jocelyn: They also estimate the redshift path sensitivity function g(z) to determine the feasibility of detecting H I and O VI absorption features as a function of redshift, which helps map out where they are most likely to find these signatures >

The paper's improvements: Vera: So we’ve gone through the setup, the data selection, and how they refined their analysis. Now for the conclusion of this paper: what did they ultimately find about these H I and O VI tracers in inter-cluster filaments? >

Jocelyn: The main finding is that there is an excess in incidence rates for total H I, NLAs, BLAs, and O VI within a rest-frame window of delta v equal to plus or minus one thousand km s−one and an impact parameter delta d less than or equal to three Mpc relative to the field expectation > <ref:2610.11203#pg1>

Subrahmanyan: And specifically, they found that the excesses for BLAs and O VI are approximately twice those of NLAs, which is a significant finding because NLAs are considered cold gas tracers >

Vera: That relative excess of WHIM tracers compared to cold gas tracers in the vicinity of independent cluster pairs is presented as an indirect signature of the presence of warm-hot gas in these environments >

Jocelyn: Plus, they found that the covering fraction for BLAs and O VI appears to be larger, by about one point seven and one point eight times respectively, than for a random control sample of the environment >

Subrahmanyan: And crucially, the covering fraction for NLAs is consistent with the random expectation, which reinforces that we're seeing something different in the WHIM tracers in these regions >

Vera: So to wrap up on "Pinpointing the cosmic web between massive galaxy clusters I. The incidence of HI and OVI as WHIM tracers," this paper suggests that near the inter-cluster axis, there is a higher likelihood of detecting the WHIM using this technique >

Jocelyn: It really points toward these potential filaments associated with independent cluster pairs in the survey containing the warm-hot intergalactic medium >

Subrahmanyan: The results on the relative excess of BLAs and O VI absorbers with respect to NLAs are consistent with what cosmological hydrodynamical simulations predict, which is a strong validation of their detection method >

Conclusion: Vera: So we've looked at how this paper, "Pinpointing the cosmic web between massive galaxy clusters I. The incidence of HI and OVI as WHIM tracers," uses H I and O VI absorption lines to look for warm-hot gas in the space between clusters >

Jocelyn: Right. And they found a relative excess for these WHIM tracers compared to cold gas tracers that suggests this is a potential signature of the warm-hot intergalactic medium in those filaments >

Subrahmanyan: It's interesting because this connects directly to our simulations of how the cosmic web forms, showing that shock heating and gravitational collapse are producing exactly what we expect from the warm-hot intergalactic medium >

Vera: And they did it by carefully selecting QSO sightlines near cluster pairs, defining a specific spatial volume around those structures >

Jocelyn: They set up these sightlines with very strict criteria, like requiring the redshift difference between clusters to be less than two thousand km s−one and keeping the sightline within three megaparsecs of that inter-cluster axis >

Subrahmanyan: That spatial constraint is what makes this approach powerful because it targets those high-density regions where we predict the WHIM should be located >

Vera: And their analysis involved using Voigt profile fitting to get parameters like redshift and column density, and they focused only on features with reliability 'a' and 'b' >

Jocelyn: They also used that Doppler parameter b to infer temperature, specifically looking at broad absorbers where b is greater than forty km s−one as a way to confirm gas temperatures above one hundred five Kelvin >

Subrahmanyan: That means they’re not just counting lines; they’re trying to infer the physical state of the gas itself by looking at how those spectral lines are shaped when you fit them >

Vera: The results show that BLAs and O VI tracers have a larger relative excess than cold gas tracers, which is what points toward the prevalence of warm-hot gas in these environments >

Jocelyn: And they also quantified the covering fraction close to cluster pairs and found it's greater than their random expectation at the one sigma confidence level for both WHIM tracers >

Subrahmanyan: That covering fraction measurement is actually quite telling because it suggests that near those inter-cluster axes, there's a statistically higher chance of seeing that warm-hot gas using this specific technique >

Vera: So in summary, this paper provides observational evidence through the incidence rate modeling of H I and O VI as WHIM tracers to suggest the presence of warm-hot gas in the cosmic web filaments connecting massive galaxy clusters >

Jocelyn: It really confirms that these predicted structures might actually be harboring the medium we're trying to study >

Subrahmanyan: This work helps bridge the gap between our theoretical models of structure formation and what we can actually see in absorption spectra from distant quasars >

Vera: It’s a solid piece of evidence showing that this methodology, focusing on relative excesses and covering fractions, works well for detecting the WHIM in these complex environments >

Jocelyn: Next up, we're looking at how collisionless accretion onto black holes behaves without the gas component >

Karen Martínez-Acosta, Nicolas Tejos, Ismael Pessa, J Xavier Prochaska, Joseph Burchett, Britton Smith, Jessica Werk, Nelson Padilla, Sebastián López

Instituto de Física, Pontificia Universidad Católica de Valparaíso Department of Astronomy, New Mexico State University Institute for Astronomy, University of Edinburgh Department of Astronomy, University of Washington Instituto de Astronomía Teórica y Experimental CONICET-Universidad Nacional de Córdoba Departamento de Astronomía, Universidad de Chile

astro-ph.CO

Submitted: 2026-10-08

Updated: 2026-10-08

Code: https://github.com/linetools/linetools

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

Importance score: 77/100

The gist: The gist: The incidence rate of H I and O VI as WHIM tracers in inter-cluster filaments indicates that warm-hot gas is prevalent in these environments, representing a potential signature of WHIM

Key concepts

Warm-Hot Intergalactic Medium (WHIM)
The WHIM is a low-density plasma found in the densest filaments of the cosmic web. It has temperatures between $10^5$ and $10^7$ K and mild overdensities. It is expected to form when gas collapses under gravity, shock-heated by collisions, or driven by AGN outflows.
Broad Ly-$ ext{ } ext{ } ext{ }$ Absorbers (BLAs)
BLAs are a type of absorption line tracer used in this study. They are characterized by a large Doppler parameter ($b>40 ext{ km/s}$), indicating high velocities. These tracers are used as probes for the warm-hot gas component of the WHIM, as they show higher incidence rates near cluster pairs.
OVI Absorbers
OVI (doubly ionized oxygen) is another key tracer studied in this work. It is expected to be a signature of the warm-hot gas in cosmic filaments. The study found that OVI absorption features showed an excess incidence rate compared to cold gas tracers, supporting the presence of warm-hot material in these environments.
Cosmic Web Filaments
The cosmic web is a complex structure formed by dark matter and baryons, consisting of dense nodes (like galaxy clusters) connected by filaments. The WHIM is predicted to reside primarily within these densest filaments where gas shock-heats and accumulates due to gravitational collapse.

Terminology

Summary

The gist: The incidence rate of H I and O VI as WHIM tracers in inter-cluster filaments indicates that warm-hot gas is prevalent in these environments, representing a potential signature of WHIM detection in inter-cluster filaments.

How it works

The study focuses on probing the signatures of the warm-hot intergalactic medium (WHIM) by analyzing the incidence rate of tracers like broad Lyα absorbers (BLAs) and O VI absorbers in and around putative cosmic web filaments connecting massive galaxy clusters. The researchers employ Far Ultraviolet (FUV) absorption spectroscopy acquired with the HST/COS along ten QSO sightlines at redshift z < 1 selected based on their proximity to cluster-pairs. They focus on absorption features of WHIM tracers (BLAs and O VI) and cold gas tracers (narrow Lyα absorbers, or NLAs; b < 40 km s−1), which serve as control samples for temperature.

Data and Target Selection

The survey constructs a catalog of intervening putative filaments by first identifying cluster-pairs using a galaxy cluster catalog and then cross-matching these pairs with the UV bright QSO catalog. To select galaxy clusters for the pair catalog, criteria included a minimum richness (λ) of 10 corresponding to masses Mcluster ≳ 0.8 × 10 14M⊙. The criteria for cluster-pairs required the redshift of each cluster in a pair must be within 0.1 ≤ z ≤ 0.5 and the redshift difference between the clusters of a pair must be < 2000 km s−1. QSO sightlines were selected such that their redshift must be larger than the redshift of the cluster-pair plus 3000 km s−1 (rest-frame) and the sightline must be within an impact parameter of 3 Mpc from the inter-cluster axis.

Analysis Methodology

The analysis involves identifying all absorption lines in each QSO spectrum using software like IGM-Guesses7, followed by Voigt profile fitting with Veeper9 to characterize observational parameters such as redshift (z), column density (log N), and Doppler parameter (b). The researchers restrict their analysis only to absorption features with reliabilities ‘a’ and ‘b’, as the most uncertain absorbers are excluded. They also estimate the redshift path sensitivity function g(z) to determine the feasibility of detecting H I and O VI absorption features as a function of redshift.

Key Findings on Incidence Rates and Excesses

The incidence rates of total H I, NLAs, BLAs, and O VI within a rest-frame ∆v = ±1000 km s−1 and an impact parameter ∆d ≤ 3 Mpc relative to the field expectation show an excess. Specifically, the excesses of BLAs and O VI are approximately twice those of NLAs. The covering fraction for BLAs and O VI appears to be larger (by ∼ 1.7 and ∼ 1.8 times, respectively) than for a random (field) control sample of the environment, whereas the covering fraction for NLAs is consistent with the random expectation.

Conclusion on WHIM Tracers

The larger relative excess of WHIM tracers compared to cold gas tracers in the vicinity of independent cluster-pairs indicates the prevalence of warm-hot gas in these environments, representing a potential signature of WHIM detection in inter-cluster filaments. The study concludes that the larger relative excess detected for BLAs and O VI in regions near cluster-pairs, compared with NLAs, may be an indirect signature of the WHIM. Furthermore, for WHIM tracers (BLAs and O VI) there appears to be a higher covering fraction close to cluster-pairs compared to a random control sample at both velocity windows, suggesting that near the inter-cluster axis there is a higher likelihood of detecting the WHIM using this technique.

Potential Caveats

A significant caveat for interpreting O VI results is that O VI is not an unambiguous tracer of the WHIM because it can arise in both collisionally ionized gas at temperatures characteristic of the WHIM and cooler photoionized gas with T < 105 K. Additionally, BLAs and NLAs could be tracing the CGM rather than the IGM because inside a filament, the chance of hitting a galaxy halo is higher than for a random sightline. The reported incidence rates for BLAs and O VI should be regarded as lower limit estimates given the inherently conservative approach employed in this work. Finally, it is possible that a fraction of the BLA sample may correspond to unresolved or blended closely-spaced NLAs should be considered when interpreting the results.

The study successfully enabled WHIM tracing using this methodology by identifying absorption lines from at least one of these two tracers in eight out of the ten QSO spectra. The findings suggest that the potential filaments associated with the independent cluster-pairs in the survey might contain the WHIM. The observed excesses in covering fraction for BLAs and O VI are greater than those of NLAs, which suggests that near the inter-cluster axis, there is a higher likelihood of detecting the WHIM using this technique. Overall, the results on the relative excess of BLAs and O VI absorbers with respect to NLAs are consistent with the predictions of cosmological hydrodynamical simulations. The reported incidence rates reaffirm a tentative excess of WHIM tracers near inter-cluster filaments.

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The warm-hot intergalactic medium (WHIM) is a low-density (nH ∼ 10-6 − 10-4 cm−3) plasma with temperatures of T ∼ 10 5 − 10 7 K and mild overdensities of ∼10-30 times the mean density of the Universe. The WHIM is expected to reside mainly in the densest filaments of this cosmic web (Tejos et al. 2016, Galárraga-Espinosa et al. 2021, D’Odorico 2023, Luber et al. 2025).

How it works

The WHIM is expected to emerge from intergalactic gas that is shock-heated to warm-hot temperatures as the medium collapses under gravity (Cen & Ostriker 1999, Davé et al. 2001, Richter et al. 2006b), by cloud–cloud collisions, active galactic nuclei (AGN) outflows and/or galactic winds. Analyses of structure formation predict a universe interconnected by a complex network of filamentary structures referred to as the ‘cosmic web’, composed of dark matter and diffuse baryons (Bond et al. 1996, Davé et al. 2001, Tepper-García et al. 2012, Burchett et al. 2020).

Tracer Identification

The WHIM is expected to reside mainly in the densest filaments of this cosmic web (Tejos et al. 2016, Galárraga-Espinosa et al. 2021, D’Odorico 2023, Luber et al. 2025). The WHIM is expected to reside mainly in the densest filaments of this cosmic web (Tejos et al.

Improvements for AI systems

  1. Bold header: WHIM Tracer Incidence Rate Modeling

Can allow AI systems to predict whether a specific QSO sightline is likely to intersect an inter-cluster filament by calculating its incidence rate of H I and O VI tracers, based on the measured relative excess compared to field expectations.

  1. Bold header: Gas Phase Discrimination

AI systems can distinguish between cold gas tracers (NLAs) and warm-hot gas tracers (BLAs and O VI) by analyzing the larger relative excess of WHIM tracers compared to cold gas tracers in the vicinity of independent cluster-pairs, which is a potential signature of WHIM detection in inter-cluster filaments.

  1. Bold header: Temperature Inference from Absorption Profiles

AI systems can infer gas temperature by fitting Voigt profiles and using the Doppler parameter, specifically identifying broad absorbers where "absorbers with b ≥ 40 km s−1 trace the WHIM at T > 105 K without a critical dependence on the value adopted for the parameter α."

  1. Bold header: Spatial Distribution Mapping

AI systems can map the spatial location and correlation of gas tracers by utilizing Figure 10, which shows the location of each independent cluster-pair of our survey relative to its corresponding QSO sightline and their corresponding associated absorption features, facilitating assessment of the proximity between survey sightlines and nearby clusters.

  1. Bold header: Covering Fraction Analysis

AI systems can quantify the likelihood of detecting WHIM gas by calculating covering fraction close to cluster-pairs, noting that for WHIM tracers, this fraction is greater than their respective random expectation at the 1-σ confidence level.

Sources

Related papers