{
  "abstract": "Fossil structures (FS) have traditionally been considered dynamically relaxed end-products of group evolution, characterized by a large magnitude gap ($Δm_{1,2} \\geq 2$). 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 ($M_{200} > 10^{13}\\,M_{\\odot}$) 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 ($μ^{\\rm{FS}}{\\star}$=0.17 vs. $μ^{\\rm{NFS}}{\\star}$=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 ($D_{BSG-CM} \\approx 0.15 R/R_{200}$), 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.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2606.19478",
  "author_total": 5,
  "authors": [
    {
      "name": "Mary Verdugo-Santos"
    },
    {
      "name": "Facundo A. Gómez"
    },
    {
      "name": "Diego Pallero"
    },
    {
      "name": "Franklin Aldás"
    },
    {
      "name": "Cristian A. Vega-Martínez"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/eq3v8jv672ht3.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2606.19478",
  "fulltext_url": null,
  "id": "eq3v8jv672ht3",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2606.19478",
  "language": null,
  "licence": "LicenseRef-arXiv-nonexclusive-1.0",
  "licence_label": "arXiv licence",
  "licence_source": "article",
  "licence_url": "http://arxiv.org/licenses/nonexclusive-distrib/1.0/",
  "metadata_licence": "CC0-1.0",
  "native_subjects": [
    [
      "arxiv",
      "astro-ph.GA"
    ]
  ],
  "openalex": null,
  "pdf_url": "https://arxiv.org/pdf/2606.19478",
  "pmcid": null,
  "pmid": null,
  "published": "2026-10-19",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/eq3v8jv672ht3.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:30:52Z",
      "native_id": "2606.19478",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2606.19478"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.GA",
      "subfield": "galaxies"
    }
  ],
  "title": "Something green beneath the surface: The dynamical nature of Fossil Structures in IllustrisTNG-100",
  "title_text": "Something green beneath the surface: The dynamical nature of Fossil Structures in IllustrisTNG-100",
  "type": "preprint",
  "updated_at": "2026-10-30T12:33:41Z",
  "url": "https://research.draconitic.com/paper/eq3v8jv672ht3",
  "venue": "arXiv",
  "versions": []
}