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. M. N. Ferguson; P. Boldrini; M. Gatto; J. M. Howell; N. F. Martin; A. C. Robin; F. Annibali; M. Baes; E. Balbinot; G. Battaglia; M. Bellazzini; Michele Cantiello; M. -R. L. Cioni; C. Crociati; V. Dornan; P. -A. Duc; A. Genina; C. M. Gutierrez; R. Habas; P. Jablonka; S. S. Larsen; M. Libralato; F. R. Marleau; D. Martínez-Delgado; S. Martocchia; D. Massari; F. Niederhofer; J. D. Sakowska; M. Urbano; L. R. 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. C. Chambers; A. Cimatti; C. Colodro-Conde; G. Congedo; C. J. Conselice; L. Conversi; Y. Copin; A. Costille; F. Courbin; H. M. 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. Gracia-Carpio; A. Grazian; F. Grupp; S. V. H. Haugan; H. Hoekstra; W. Holmes; I. M. 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. M. C. Le Brun; S. Ligori

arXiv · 2609.14535

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. M. N. Ferguson·P. Boldrini·M. Gatto·J. M. Howell·N. F. Martin·A. C. Robin·F. Annibali·M. Baes·E. Balbinot·G. Battaglia·M. Bellazzini·Michele Cantiello·M. -R. L. Cioni·C. Crociati·V. Dornan·P. -A. Duc·A. Genina·C. M. Gutierrez·R. Habas·P. Jablonka·S. S. Larsen·M. Libralato·F. R. Marleau·D. Martínez-Delgado·S. Martocchia·D. Massari·F. Niederhofer·J. D. Sakowska·M. Urbano·L. R. 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. C. Chambers·A. Cimatti·C. Colodro-Conde·G. Congedo·C. J. Conselice·L. Conversi·Y. Copin·A. Costille·F. Courbin·H. M. 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. Gracia-Carpio·A. Grazian·F. Grupp·S. V. H. Haugan·H. Hoekstra·W. Holmes·I. M. 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. M. C. Le Brun·S. Ligori

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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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T. Saifollahi, A. Lançon, A. M. N. Ferguson, P. Boldrini, M. Gatto, J. M. Howell, N. F. Martin, A. C. Robin, F. Annibali, M. Baes, E. Balbinot, G. Battaglia, M. Bellazzini, Michele Cantiello, M. -R. L. Cioni, C. Crociati, V. Dornan, P. -A. Duc, A. Genina, C. M. Gutierrez, R. Habas, P. Jablonka, S. S. Larsen, M. Libralato, F. R. Marleau, D. Martínez-Delgado, S. Martocchia, D. Massari, F. Niederhofer, J. D. Sakowska, M. Urbano, L. R. 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. C. Chambers, A. Cimatti, C. Colodro-Conde, G. Congedo, C. J. Conselice, L. Conversi, Y. Copin, A. Costille, F. Courbin, H. M. 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. Gracia-Carpio, A. Grazian, F. Grupp, S. V. H. Haugan, H. Hoekstra, W. Holmes, I. M. 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. M. C. Le Brun, S. Ligori. 2026-09-13. 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. https://arxiv.org/abs/2609.14535

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