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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

T. Saifollahi A. Lançon A. Ferguson P. Boldrini M. Gatto J. M. Howell N. Martin A. Robin F. Annibali M. Baes E. Balbinot G. Battaglia M. Bellazzini M. Cantiello L. Cioni C. Crociati V. Dornan P. Duc A. Genina C. M. Gutierrez R. Habas P. Jablonka S. Larsen M. Libralato F. Marleau D. Martínez-Delgado S. Martocchia D. Massari F. Niederhofer J. Sakowska M. Urbano L. Bedin M. Griggio A. Mohandasan B. Altieri S. Andreon N. Auricchio C. Baccigalupi M. Baldi S. Bardelli P. Battaglia A. Biviano M. Bolzonella E. Branchini M. Brescia S. Camera V. Capobianco C. Carbone J. Carretero M. Castellano G. Castignani S. Cavuoti K. Chambers A. Cimatti C. Colodro-Conde G. Congedo C. Conselice L. Conversi Y. Copin A. Costille F. Courbin H. Courtois M. Cropper H. Degaudenzi G. de Lucia H. Dole F. Dubath X. Dupac M. Farina R. Farinelli F. Faustini S. Ferriol M. Frailis E. Franceschi M. Fumana L. Gabarra S. Galeotta K. George B. Gillis C. Giocoli J. Graciá-Carpio A. Grazian F. Grupp S. Haugan H. Hoekstra W. Holmes I. Hook F. Hormuth A. Hornstrup K. Jahnke M. Jhabvala S. Kermiche A. Kiessling B. Kubik K. Kuijken M. Kümmel M. Kunz H. Kurki-Suonio A. L. Le Brun S. Ligori P. Lilje V. Lindholm I. Lloro M. Magliocchetti G. Mainetti O. Mansutti O. Marggraf M. Martinelli N. Martinet F. Marulli R. Massey E. Medinaceli M. Meneghetti E. Merlin G. Meylan P. Monaco A. Mora Michele Moresco C. Moretti L. Moscardini R. Nakajima C. Neissner S. Niemi C. Padilla S. Paltani F. Pasian W. Percival V. Pettorino A. Pezzotta G. Polenta M. Poncet L. Popa F. Raison A. Renzi J. Rhodes G. Riccio I. Risso E. Romelli M. Roncarelli B. Rusholme R. Saglia Z. Sakr D. Sapone M. Schirmer P. Schneider A. Secroun E. Sihvola C. Sirignano G. Sirri L. Stanco P. Tallada-Crespí A. Taylor I. Tereno S. Toft R. Toledo-Moreo F. Torradeflot A. Tsyganov I. Tutusaus J. Valiviita T. Vassallo Y. Wang J. Weller A. Zacchei F. Zerbi I. Zinchenko E. Zucca J. García-Bellido K. T. U. D. Strasbourg Cnrs Observatoire Astronomique de Strasbourg Umr 67000 Strasbourg France C. S. D. Toulouse 18 avenue Edouard Belin 9. 31401ToulouseCedex I. F. Astronomy U. Edinburgh Royal Observatory B. Hill Edinburgh EH9 3HJ Uk Observatoire de Paris 61 PSLResearchUniversity Avenue de l'Observatoire 75005 Paris INAF-Osservatorio Astronomico di Capodimonte 16 ViaMoiariello 80131 Napoli Italy M. Astronomie 17 Konigstuhl 69117 Heidelberg Germany UniversitehLouis Pasteur Observatoire Des Sciences de l'Univers Theta Franche-Comte Bourgogne Institut Utinam Observatoire de Besançon Bp 25010 Besanccon Cedex INAF-Osservatorio di Astrofisica e Scienza Dello Spazio di Bologna V. Gobetti 40129 Bologna U. Gent D. Physics Astronomy N. Proeftuinstraat 9000 Ghent Belgium K. Institute University of Groningen P. Box 9700 AV Groningen T. Netherlands Leiden Observatory Leiden University 55 Einsteinweg 2333 CC Leiden Instituto de Astrofísica de Canarias E. 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Sep 2026 · 0 citations · 17 references

Physics

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.

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