PAPER / ARXIV:2609.14535
T. Saifollahi (1 and 2), A. Lançon (1), A. M. N. Ferguson (3), P. Boldrini (4), M. Gatto (5), J. M. Howell (3), N. F. Martin (1 and 6), A. C. Robin (7), F. Annibali (8), M. Baes (9), E. Balbinot (10 and 11), G. Battaglia (12 and 13), M. Bellazzini (8), Michele Cantiello (14), M.-R. L. Cioni (15), C. Crociati (3), V. Dornan (3), P.-A. Duc (1), A. Genina (3), C. M. Gutierrez (12 and 13), R. Habas (14), P. Jablonka (16), S. S. Larsen (17), M. Libralato (18), F. R. Marleau (19), D. Martínez-Delgado (20 and 21), S. Martocchia (22), D. Massari (8), F. Niederhofer (15), J. D. Sakowska (23), M. Urbano (24 and 1), L. R. Bedin (18), M. Griggio (25), A. Mohandasan (7), B. Altieri (26), S. Andreon (27), N. Auricchio (8), C. Baccigalupi (28 and 29 and 30 and 31), M. Baldi (32 and 8 and 33), S. Bardelli (8), P. Battaglia (8), A. Biviano (29 and 28), M. Bolzonella (8), E. Branchini (34 and 35 and 27), M. Brescia (36 and 5), S. Camera (37 and 38 and 39), V. Capobianco (39), C. Carbone (40), J. Carretero (41 and 42), M. Castellano (43), G. Castignani (8), S. Cavuoti (5 and 44), K. C. Chambers (45), A. Cimatti (46), C. Colodro-Conde (12), G. Congedo (3), C. J. Conselice (47), L. Conversi (48 and 26), Y. Copin (49), A. Costille (22), F. Courbin (50 and 51 and 52), H. M. Courtois (53), M. Cropper (54), H. Degaudenzi (55), G. De Lucia (29), H. Dole (56), F. Dubath (55), X. Dupac (26), M. Farina (57), R. Farinelli (8), F. Faustini (43), S. Ferriol (49), M. Frailis (29), E. Franceschi (8), M. Fumana (40), L. Gabarra (58), S. Galeotta (29), K. George (59), B. Gillis (3), C. Giocoli (8 and 33), J. Gracia-Carpio (60), A. Grazian (18), F. Grupp (60 and 61), S. V. H. Haugan (62), H. Hoekstra (11), W. Holmes (63), I. M. Hook (64), F. Hormuth (65), A. Hornstrup (66 and 67), K. Jahnke (6), M. Jhabvala (68), S. Kermiche (69), A. Kiessling (63), B. Kubik (49), K. Kuijken (11), M. Kümmel (61), M. Kunz (70), H. Kurki-Suonio (71 and 72), A. M. C. Le Brun (73), S. Ligori
RESUMO
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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