Something green beneath the surface: The dynamical nature of Fossil Structures in IllustrisTNG-100

arXiv:2606.19478 · astro-ph.GA · Submitted 2026-06-17 · Read on arXiv

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

Vera: We're looking at the paper: "Something green beneath the surface".

Jocelyn: The gist: The magnitude gap in fossil structures emerged relatively recently in cosmic time, with FS and non-FS populations diverging approximately 3–6 Gyr ago depending on mass regime,

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

Paper summary: Vera: So looking at "Something green beneath the surface: The dynamical nature of Fossil Structures in IllustrisTNG-one hundred", it really forces us to rethink what we mean by a fossil structure <ref:2606.19478#pg1>.

Jocelyn: It seems the magnitude gap is a good tool for tracing when big mergers happened recently, but it’s not a perfect yardstick for whether the entire system has settled down dynamically Subrahmanyan.

Subrahmanyan: The implication here is that fossil systems aren't necessarily fully relaxed end products right now; they are just systems that have had their major accretion events pause recently, which is a more nuanced picture Vera.

Jocelyn: It suggests that the next step in studying these systems isn't just looking at the static gap size, but tracking those dynamical indicators like the gas-BSG offsets to see if they truly reach a steady state Subrahmanyan.

Subrahmanyan: That makes sense. We need observations or simulations that can tell us if those systems are truly settled or still in flux after that pause in mergers Vera.

Conclusion: Vera: So, to wrap up this paper, "Something green beneath the surface: The dynamical nature of Fossil Structures in IllustrisTNG-one hundred" it really challenges us on what we actually mean when we call a fossil structure <ref:2606.19478#pg1>.

Jocelyn: It seems that the magnitude gap is a good way to track when big mergers happened recently, but it doesn't tell us anything definitive about whether the whole system has settled down dynamically.

Subrahmanyan: The big picture here is that fossil structures aren't necessarily fully relaxed end products right now; they are just systems where those major accretion events have paused recently, which gives us a more detailed view of their history <ref:2606.19478#pg2>.

Vera: So instead of saying a fossil structure *is* relaxed, we should probably think of it as having a specific recent assembly history that is imprinted on its galaxy population <Jocelyn>.

Jocelyn: That means the next step isn't just measuring the static gap size, but tracking those dynamical indicators, like the gas-BSG offsets, to see if they actually reach a steady state <Subrahmanyan>.

Subrahmanyan: It makes sense. We need simulations or observations that can confirm if these systems are truly settled or still in flux after that pause in mergers <Vera>.

Vera: This study, using IllustrisTNG-one hundred shows us how the structure of a galaxy population tells us about its recent merger history rather than just its final state <Jocelyn>.

Jocelyn: It suggests that for astronomers studying these massive halos, the magnitude gap is a history marker more than an end-state classification <Subrahmanyan>.

Vera: And this opens up new questions about how we use metrics to define stability in large-scale structure formation <Jocelyn>.

Jocelyn: Exactly. So, if the gap isn't enough, what other dynamical proxies can we look at to understand the true current state of these fossil systems?

Departamento de Astronomía, Universidad de La Serena · Departamento de Fhica, Universidad T陔cnico Federico Santa Marí · Facultad de Ingeniería y Arquitectura, Universidad Central de Chile · INAF – Osservatorio Astronomico di Trieste · Núcleo Milenio de Galaxias (MINGAL) · IFPU, Institute for Fundamental Physics of the Universe

astro-ph.GA

Submitted: 2026-06-17

Updated: 2026-10-19

Comments: 12 pages, 7 figures, submitted to Astronomy & Astrophysics. New version accepted

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

Importance score: 72/100

The gist: The gist: The magnitude gap in fossil structures emerged relatively recently in cosmic time, with FS and non-FS populations diverging approximately 3–6 Gyr ago depending on mass regime, suggesting

Key concepts

Fossil Structures (FS): These are galaxy groups characterized by a large magnitude gap ($\Delta m_{1,2} \geq 2$). Traditionally thought to be dynamically relaxed end-products of evolution, this paper tests if they truly are or if they are merely systems that have ceased major mergers recently.
Magnitude Gap ($\Delta m_{1,2}$): This is a measure used to classify structures as FS or non-FS based on the difference in magnitude between two specific luminosity bins. The paper uses this gap to trace the recent assembly history of the massive components within these galaxy groups.
Gas–BSG Centroid Shift ($D_{BSG-CM}$): This is a dynamical proxy used to assess how relaxed or disturbed a system is. A high offset suggests the gas and bright satellite galaxies are not centered together, indicating that the system has not yet fully settled into a stable, relaxed state.

Terminology

Decoupling — 1× in this paper · explained in 2 episodes
Trace — 1× in this paper · explained in 2 episodes

Summary

The gist: The magnitude gap in fossil structures emerged relatively recently in cosmic time, with FS and non-FS populations diverging approximately 3–6 Gyr ago depending on mass regime, suggesting the magnitude gap is primarily a tracer of recent assembly history rather than a proxy for current global dynamical state.

Context and Motivation

Fossil structures (FS) have traditionally been considered dynamically relaxed end-products of group evolution, characterized by a large magnitude gap (∆m1,2 ≥ 2). The study investigates whether FS are dynamically relaxed systems and how their galaxy populations differ from non-fossil systems (non-FS), focusing on system dynamics and evolution of the galaxies inside them.

Methodology

The research utilized the IllustrisTNG-100 cosmological simulation to select 182 structures (M200 > 10 13 M⊙) at z = 0, classifying them as FS/non-FS based on ∆m1,2 in the r-band. The analysis tracked ∆m1,2 evolution over 9 Gyr and analyzed four key aspects:

the emergence of ∆m1,2.
the fraction of quenched galaxies (sSFR < 10-11 yr-1).
the distribution of galaxies in color–stellar mass space.
the gas–BSG centroid shift as a dynamical proxy.

Key Findings on Magnitude Gap Evolution

The evolution of the magnitude gap over cosmic time reveals distinct patterns depending on halo mass. The emergence of the magnitude gap in FS initiated approximately 4–5 Gyr ago, marking the moment when fossil and non-FS populations diverged. This divergence started slightly earlier at tlb ≈ 5 Gyr for both low and high mass structures. The study found that FS experienced significantly smaller massive satellite accretion events in the recent past, with the median µ⋆ for FS being 0.17 compared to 0.39 for non-FS. This suggests that the key distinction between FS and non-FS may lie not in when they formed but in when they ceased major mergers.

Galaxy Population Properties

The analysis of global galaxy properties showed systematic differences between the two populations.

(1) Quenched Galaxy Fractions (fQ):

The median fQ = 55% for structures classified at 0.5R200, with higher values in higher-mass structures (median fQ = 77%). For the low-mass subsample, non-FS showed a slightly higher fraction of structures with low fQ compared to their FS counterparts.

(2) Color–Mass Distribution:

The residuals revealed a clear overabundance of red galaxies in FSs, as well as a more populated blue cloud and green valley region in non-FSs. Specifically, FS showed a "depletion of high-mass galaxies (1011 < M⋆/M⊙ < 1011.5) in the green valley" compared to non-FSs, which is linked to the need for a secondary massive galaxy in non-FSs to guarantee a small magnitude gap.

Dynamical State Assessment

The study employed the gas–BSG centroid offset (DBSG−CM) as a dynamical proxy.

(1) Dynamical Classification:

Both FS and non-FS populations occupy an intermediate dynamical state, with median gas-BSG offsets of DBSG−CM ≈ 0.15 R/R200, which is between the relaxed threshold (DBSG−CM ≤ 0.1 R200) and the disturbed threshold (DBSG−CM ≥ 0.4 R200). The mean magnitude gaps for these populations were ∆relaxed m1,2 = 2.32 ± 0.76 and ∆disturbed m1,2 = 1.72 ± 1.12, which are reasonably consistent but not perfectly correlated with the FS/non-FS classifications. The results suggest that both populations maintain average offset values characteristic of moderately perturbed systems rather than fully relaxed or strongly disturbed ones.

Conclusion

The comparison between fossil classification (based on the magnitude gap) and dynamical classification (based on the BSG–gas centroid offset) reveals that while the magnitude gap effectively separates systems by their recent merger histories, it does not cleanly separate them by current dynamical state. Both FS and non-FS appear to be moderately perturbed systems that have not yet fully relaxed from their assembly histories. The magnitude gap is primarily a tracer of recent accretion history, as evidenced by the significantly lower µ⋆ values of FS over the past 6 Gyr compared to non-FS.

Improvements for AI systems

Improved structural characterization of galaxy groups and clusters based on dynamical state: The system can now distinguish between relaxed and perturbed structures by calculating the DBSG-CM, classifying them as such based on thresholds like DBSG−CM ≤ 0.1 R200 for relaxed systems.
Enhanced interpretation of the magnitude gap as a proxy for assembly history: The AI can now refine its understanding that the magnitude gap is primarily a tracer of recent accretion history, specifically by tracking the median stellar mass ratio, noting that FS experienced significantly smaller accretion events over the past 6 Gyr (median µ⋆ = 0.17 vs. µNFS⋆ = 0.39).
Refined prediction of galaxy quenching efficiency: The system can now better estimate the fraction of quenched galaxies (fQ) by accounting for mass-dependent trends, recognizing that low-mass structures show a great diversity in the fraction of quenched galaxies, fQ and that higher mass structures are typically characterized by higher values of fQ.
Detailed analysis of color–mass space morphology: The AI can now perform residual analysis to identify specific population differences, such as finding a clear overabundance of red galaxies in FSs, as well as a more populated blue cloud and green valley region in non-FSs when comparing the two classifications.
Decoupling of dynamical state from magnitude gap interpretation: The AI can now conclude that the magnitude gap effectively separates systems by their recent merger histories, but it does not cleanly separate them by current dynamical state, preventing the incorrect assumption that systems with large magnitude gaps are dynamically relaxed.