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

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 >