Something green beneath the surface: The dynamical nature of Fossil Structures in IllustrisTNG-100
Abstract
Fossil structures (FS) have traditionally been considered dynamically relaxed end-products of group evolution, characterized by a large magnitude gap (Δm1,2 ≥ 2). However, recent observations and simulations suggest this picture is incomplete. We investigate 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. Using \textsc{IllustrisTNG-100}, we select 182 structures (M200 > 1013 M⊙) at z = 0, classifying them as FS/non-FS based on Δm1,2 in the r-band. We track Δm1,2 evolution over 9\,Gyr and analyze: (1) the emergence of Δm1,2, (2) the fraction of quenched galaxies (sSFR < 10−11\,yr−1), (3) the distribution of galaxies in color--stellar mass space, and (4) the gas--BSG centroid shift as a dynamical proxy. The magnitude gap in FS is primarily driven by the absence of massive recent accretion: FS exhibit significantly lower BSG-to-satellite stellar mass ratios (μFS⋆=0.17 vs. μNFS⋆=0.39) for the most massive satellite accreted within the last 6\,Gyr. FS also host a more prominent red sequence and marginally higher quenched fractions than non-FS. Our findings indicate that while the magnitude gap effectively identifies systems that have ceased major mergers in the last 3-6 Gyr, it is a poor proxy for their current global dynamical state. Both FS and non-FS populations exhibit intermediate gas-BSG offsets (DBSG−CM ≈ 0.15 R/R200), failing to reach full relaxation. This decoupling suggests that the magnitude gap traces the assembly history of massive components rather than the overall stability of the intra cluster medium.
Related
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- Dynamical evolution and surface accretion of DART impact ejecta in the (65803) Didymos system
BibTeX
@misc{verdugosantos2026something,
title = {{Something green beneath the surface: The dynamical nature of Fossil Structures in IllustrisTNG-100}},
author = {Mary Verdugo-Santos and Facundo A. Gómez and Diego Pallero and Franklin Aldás and Cristian A. Vega-Martínez},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2606.19478},
eprint = {2606.19478},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2606.19478},
}
RIS
TY - UNPB
TI - Something green beneath the surface: The dynamical nature of Fossil Structures in IllustrisTNG-100
AU - Mary Verdugo-Santos
AU - Facundo A. Gómez
AU - Diego Pallero
AU - Franklin Aldás
AU - Cristian A. Vega-Martínez
PY - 2026
DA - 2026/10/19
PB - arXiv
DO - 10.48550/arxiv.2606.19478
UR - https://arxiv.org/abs/2606.19478
AB - Fossil structures (FS) have traditionally been considered dynamically relaxed end-products of group evolution, characterized by a large magnitude gap (Δm_1,2 ≥ 2). However, recent observations and simulations suggest this picture is incomplete. We investigate 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. Using \textsc{IllustrisTNG-100}, we select 182 structures (M_200 > 10^13 M_⊙) at z = 0, classifying them as FS/non-FS based on Δm_1,2 in the r-band. We track Δm_1,2 evolution over 9\,Gyr and analyze: (1) the emergence of Δm_1,2, (2) the fraction of quenched galaxies (sSFR < 10^−11\,yr^−1), (3) the distribution of galaxies in color--stellar mass space, and (4) the gas--BSG centroid shift as a dynamical proxy. The magnitude gap in FS is primarily driven by the absence of massive recent accretion: FS exhibit significantly lower BSG-to-satellite stellar mass ratios (μ^FS⋆=0.17 vs. μ^NFS⋆=0.39) for the most massive satellite accreted within the last 6\,Gyr. FS also host a more prominent red sequence and marginally higher quenched fractions than non-FS. Our findings indicate that while the magnitude gap effectively identifies systems that have ceased major mergers in the last 3-6 Gyr, it is a poor proxy for their current global dynamical state. Both FS and non-FS populations exhibit intermediate gas-BSG offsets (D_BSG−CM ≈ 0.15 R/R_200), failing to reach full relaxation. This decoupling suggests that the magnitude gap traces the assembly history of massive components rather than the overall stability of the intra cluster medium.
ER -