Baryons in the CosmicWeb of IllustrisTNG - I: Gas in Knots, Filaments, Sheets and Voids

Abstract: We analyze the IllustrisTNG simulations to study the mass, volume fraction and phase distribution of gaseous baryons embedded in the knots, filaments, sheets and voids of the Cosmic Web from redshift $z=8$ to redshift $z=0$. We find that filaments host more star-forming gas than knots, and that filaments also have a higher relative mass fraction of gas in this phase than knots. We also show that the cool, diffuse Intergalactic Medium (IGM; $T<10^5 \, {\rm K}$, $ n_{\rm H}<10^{-4}(1+z) \, {\rm cm^{-3}}$) and the Warm-Hot Intergalactic Medium (WHIM; $ 10^5 \, {\rm K} <T<10^7 \, {\rm K}$, $ n_{\rm H} <10^{-4}(1+z)\, {\rm cm^{-3}}$) constitute $\sim 39\%$ and $\sim 46\%$ of the baryons at redshift $z=0$, respectively. Our results indicate that the WHIM may constitute the largest reservoir of {\it missing} baryons at redshift $z=0$. Using our Cosmic Web classification, we predict the WHIM to be the dominant baryon mass contribution in filaments and knots at redshift $z=0$, but not in sheets and voids where the cool, diffuse IGM dominates. We also characterise the evolution of WHIM and IGM from redshift $z=4$ to redshift $z=0$, and find that the mass fraction of WHIM in filaments and knots evolves only by a factor $\sim 2$ from redshift $z=0$ to $z=1$, but declines faster at higher redshift. The WHIM only occupies $4-11\%$ of the volume at redshift $0\leq z \leq 1$. We predict the existence of a significant number of currently undetected OVII and NeIX absorption systems in cosmic filaments which could be detected by future X-ray telescopes like Athena.

Analysis of Baryons in the Cosmic Web Using IllustrisTNG Simulations

This paper investigates the distribution and phase states of baryonic matter within the Cosmic Web using the IllustrisTNG simulations. The Cosmic Web, consisting of knots, filaments, sheets, and voids, serves as the large-scale structure of the universe. The authors adopt a classification method based on the deformation tensor to identify these structures across a redshift range from $z=8$ to $z=0$, analyzing the cosmic evolution of baryons.

Key Findings

A salient finding of this research is the significant presence of the Warm-Hot Intergalactic Medium (WHIM) and the diffuse Intergalactic Medium (IGM) in filaments and knots at low redshifts. At $z=0$, the WHIM and IGM together account for around 85% of the baryonic mass, confirming that the WHIM may constitute the main reservoir of the so-called “missing” baryons. The WHIM's contribution is prominent in filaments and knots while remaining negligible in sheets and voids, indicating its formation via shock heating in regions with dense matter concentrations.

Simulations demonstrate a conditional density-temperature phase diagram showing distinctively how baryons transition between different phases in various cosmic structures. The mass and volume fractions of gas phases reveal that star-forming gas predominates in filaments, while halo gas exhibits a more widespread distribution, including sheets and voids.

Numerical Results and Observational Significance

The analysis reveals that at $z=0$, approximately 46% of baryons exist as WHIM and 39% as diffuse IGM, residing primarily in filaments. The WHIM shows minimal evolution post $z=1$, suggesting stable WHIM formation dynamics beyond this epoch. Additionally, the volume occupancy of WHIM is low, confined to small dense regions, while the diffuse IGM occupies a larger volume fraction across the Cosmic Web.

The paper identifies a critical comparative analysis with previous Illustris simulations, showing discrepancies in the distribution of WHIM and diffuse IGM due to different feedback implementations. This highlights the influence of sub-grid physics, specifically AGN feedback, on the heating and distribution of baryons.

The detection of WHIM via OVII and NeIX absorption features is addressed, with predictions for future X-ray telescope observations (e.g., Athena). This aligns with recent observations suggesting the presence of WHIM, supporting the simulation's insight into the distribution and detectability of WHIM through specific ion absorption lines.

Implications and Future Directions

The findings highlight the Cosmic Web as a repository of complex baryonic states, emphasizing the role of filaments in harboring “missing” baryons. This research provides a cornerstone in guiding observational strategies, leveraging high-energy spectroscopy to identify WHIM constituents reliably. The analysis underscores the evolving contribution of different baryonic phases, shaping our understanding of large-scale structure formation and baryon dynamics.

Future explorations could enhance these insights by investigating the effect of different baryonic models and increasing the computational volume or resolution. Additionally, in-depth studies of metallicity evolution and detailed ionization conditions within specific cosmic environments could further refine our theoretical understanding and observational capabilities regarding the intergalactic baryon budget.

In conclusion, the study offers a detailed exposition on the distribution of baryonic phases within the Cosmic Web, contributing significantly to resolving the "missing" baryons conundrum at low redshifts while laying foundational work for future observational inquiries and theoretical developments in galaxy and large-scale structure evolution.