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

@misc{saifollahi2026euclid,
  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}},
  author = {T. Saifollahi and A. Lançon and A. M. N. Ferguson and P. Boldrini and M. Gatto and J. M. Howell and N. F. Martin and A. C. Robin and F. Annibali and M. Baes and E. Balbinot and G. Battaglia and M. Bellazzini and Michele Cantiello and M. -R. L. Cioni and C. Crociati and V. Dornan and P. -A. Duc and A. Genina and C. M. Gutierrez and R. Habas and P. Jablonka and S. S. Larsen and M. Libralato and F. R. Marleau and D. Martínez-Delgado and S. Martocchia and D. Massari and F. Niederhofer and J. D. Sakowska and M. Urbano and L. R. Bedin and M. Griggio and A. Mohandasan and B. Altieri and S. Andreon and N. Auricchio and C. Baccigalupi and M. Baldi and S. Bardelli and P. Battaglia and A. Biviano and M. Bolzonella and E. Branchini and M. Brescia and S. Camera and V. Capobianco and C. Carbone and J. Carretero and M. Castellano and G. Castignani and S. Cavuoti and K. C. Chambers and A. Cimatti and C. Colodro-Conde and G. Congedo and C. J. Conselice and L. Conversi and Y. Copin and A. Costille},
  howpublished = {arXiv},
  year = {2026},
  month = sep,
  doi = {10.48550/arxiv.2609.14535},
  eprint = {2609.14535},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2609.14535},
  note = {CC BY 4.0},
}

TY  - UNPB
TI  - 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
AU  - T. Saifollahi
AU  - A. Lançon
AU  - A. M. N. Ferguson
AU  - P. Boldrini
AU  - M. Gatto
AU  - J. M. Howell
AU  - N. F. Martin
AU  - A. C. Robin
AU  - F. Annibali
AU  - M. Baes
AU  - E. Balbinot
AU  - G. Battaglia
AU  - M. Bellazzini
AU  - Michele Cantiello
AU  - M. -R. L. Cioni
AU  - C. Crociati
AU  - V. Dornan
AU  - P. -A. Duc
AU  - A. Genina
AU  - C. M. Gutierrez
AU  - R. Habas
AU  - P. Jablonka
AU  - S. S. Larsen
AU  - M. Libralato
AU  - F. R. Marleau
AU  - D. Martínez-Delgado
AU  - S. Martocchia
AU  - D. Massari
AU  - F. Niederhofer
AU  - J. D. Sakowska
AU  - M. Urbano
AU  - L. R. Bedin
AU  - M. Griggio
AU  - A. Mohandasan
AU  - B. Altieri
AU  - S. Andreon
AU  - N. Auricchio
AU  - C. Baccigalupi
AU  - M. Baldi
AU  - S. Bardelli
AU  - P. Battaglia
AU  - A. Biviano
AU  - M. Bolzonella
AU  - E. Branchini
AU  - M. Brescia
AU  - S. Camera
AU  - V. Capobianco
AU  - C. Carbone
AU  - J. Carretero
AU  - M. Castellano
AU  - G. Castignani
AU  - S. Cavuoti
AU  - K. C. Chambers
AU  - A. Cimatti
AU  - C. Colodro-Conde
AU  - G. Congedo
AU  - C. J. Conselice
AU  - L. Conversi
AU  - Y. Copin
AU  - A. Costille
PY  - 2026
DA  - 2026/09/13
PB  - arXiv
DO  - 10.48550/arxiv.2609.14535
UR  - https://arxiv.org/abs/2609.14535
AB  - 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 ∼ 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 ≤ [M/H] ≤ −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±1.9) 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 [M/H]=−1.4±0.3, 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.
ER  -