{
  "abstract": "We report the discovery of an ultra-faint stellar system in the vicinity of the Fornax dwarf spheroidal (dSph) galaxy in Euclid DR1. This system, which we designate Fornax-7, is located $1.1$ deg from the centre of the Fornax dSph at about $145$ kpc, corresponding to a projected distance of $\\sim 2.8$ kpc assuming the distance of the Fornax dSph. We characterise Fornax-7 using constraints from its colour-magnitude diagram (CMD), luminosity function (LF), and integrated optical colours from ground-based data. These observables are modelled jointly through a forward-modelling framework to constrain its distance modulus, stellar population properties, and total stellar mass. Assuming a low metallicity of $-2.2 \\leq {\\rm [M/H]} \\leq -2.0$, similar to known ultra-faint systems around the Milky Way and the LMC, we derive a distance modulus consistent with that of the Fornax dSph. We estimate an age of $(10.4\\pm1.9)$ Gyr and infer a stellar mass of $170^{+50}_{-62}$ solar masses. Relaxing the metallicity constraint and allowing for a wider range that covers the metallicity of the known old Fornax globular clusters (GCs), we find a metal-poor population but with a higher metallicity ${\\rm [M/H]}=-1.4\\pm0.3$, while the age, distance modulus, and mass remain mostly the same. These results suggest a likely physical association with the Fornax dSph. Fornax-7 may represent either an extremely remote star cluster associated with Fornax, a dwarf satellite of Fornax (i.e., a \"satellite of a satellite\"), or an independent ultra-faint satellite in the outer halo of the Milky Way. In all cases, it constitutes an exceptional system probing the lowest-mass regime of galaxy and star cluster formation, and the structure of dwarf galaxy haloes. Deeper photometric and spectroscopic follow-up observations are required to confirm both the nature of the system and its dynamical connection to Fornax.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2609.14535",
  "author_total": 168,
  "authors": [
    {
      "name": "T. Saifollahi"
    },
    {
      "name": "A. Lançon"
    },
    {
      "name": "A. M. N. Ferguson"
    },
    {
      "name": "P. Boldrini"
    },
    {
      "name": "M. Gatto"
    },
    {
      "name": "J. M. Howell"
    },
    {
      "name": "N. F. Martin"
    },
    {
      "name": "A. C. Robin"
    },
    {
      "name": "F. Annibali"
    },
    {
      "name": "M. Baes"
    },
    {
      "name": "E. Balbinot"
    },
    {
      "name": "G. Battaglia"
    },
    {
      "name": "M. Bellazzini"
    },
    {
      "name": "Michele Cantiello"
    },
    {
      "name": "M. -R. L. Cioni"
    },
    {
      "name": "C. Crociati"
    },
    {
      "name": "V. Dornan"
    },
    {
      "name": "P. -A. Duc"
    },
    {
      "name": "A. Genina"
    },
    {
      "name": "C. M. Gutierrez"
    },
    {
      "name": "R. Habas"
    },
    {
      "name": "P. Jablonka"
    },
    {
      "name": "S. S. Larsen"
    },
    {
      "name": "M. Libralato"
    },
    {
      "name": "F. R. Marleau"
    },
    {
      "name": "D. Martínez-Delgado"
    },
    {
      "name": "S. Martocchia"
    },
    {
      "name": "D. Massari"
    },
    {
      "name": "F. Niederhofer"
    },
    {
      "name": "J. D. Sakowska"
    },
    {
      "name": "M. Urbano"
    },
    {
      "name": "L. R. Bedin"
    },
    {
      "name": "M. Griggio"
    },
    {
      "name": "A. Mohandasan"
    },
    {
      "name": "B. Altieri"
    },
    {
      "name": "S. Andreon"
    },
    {
      "name": "N. Auricchio"
    },
    {
      "name": "C. Baccigalupi"
    },
    {
      "name": "M. Baldi"
    },
    {
      "name": "S. Bardelli"
    },
    {
      "name": "P. Battaglia"
    },
    {
      "name": "A. Biviano"
    },
    {
      "name": "M. Bolzonella"
    },
    {
      "name": "E. Branchini"
    },
    {
      "name": "M. Brescia"
    },
    {
      "name": "S. Camera"
    },
    {
      "name": "V. Capobianco"
    },
    {
      "name": "C. Carbone"
    },
    {
      "name": "J. Carretero"
    },
    {
      "name": "M. Castellano"
    },
    {
      "name": "G. Castignani"
    },
    {
      "name": "S. Cavuoti"
    },
    {
      "name": "K. C. Chambers"
    },
    {
      "name": "A. Cimatti"
    },
    {
      "name": "C. Colodro-Conde"
    },
    {
      "name": "G. Congedo"
    },
    {
      "name": "C. J. Conselice"
    },
    {
      "name": "L. Conversi"
    },
    {
      "name": "Y. Copin"
    },
    {
      "name": "A. Costille"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/2x27nz9824j02.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2609.14535",
  "fulltext_url": null,
  "id": "2x27nz9824j02",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2609.14535",
  "language": null,
  "licence": "CC-BY-4.0",
  "licence_label": "CC BY 4.0",
  "licence_source": "article",
  "licence_url": "http://creativecommons.org/licenses/by/4.0/",
  "metadata_licence": "CC0-1.0",
  "native_subjects": [
    [
      "arxiv",
      "astro-ph.GA"
    ],
    [
      "arxiv",
      "astro-ph.IM"
    ],
    [
      "openalex",
      "3103|Galaxies: Formation, Evolution, Phenomena"
    ]
  ],
  "openalex": "W7213274198",
  "pdf_url": "https://arxiv.org/pdf/2609.14535",
  "pmcid": null,
  "pmid": null,
  "published": "2026-09-13",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/2x27nz9824j02.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:21:25Z",
      "native_id": "2609.14535",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2609.14535"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.GA",
      "subfield": "galaxies"
    },
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.IM",
      "subfield": "instrumentation-methods"
    }
  ],
  "title": "Euclid: Data Release 1 (DR1) -- Fornax-7, an ultra-faint companion to the Fornax dwarf spheroidal galaxy? A remote star cluster or dwarf satellite of the Fornax dSph",
  "title_text": "Euclid: Data Release 1 (DR1) -- Fornax-7, an ultra-faint companion to the Fornax dwarf spheroidal galaxy? A remote star cluster or dwarf satellite of the Fornax dSph",
  "type": "preprint",
  "updated_at": "2026-10-30T12:21:25Z",
  "url": "https://research.draconitic.com/paper/2x27nz9824j02",
  "venue": "arXiv",
  "versions": []
}