Andromeda XXXVI: Discovery of a new ultra-faint dwarf galaxy towards M31

¹ Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
² Centro de Estudios de Física del Cosmos de Aragón (CEFCA), Unidad Asociada al CSIC, Plaza San Juan 1, 44001 Teruel, Spain
³ ARAID Foundation, Avda. de Ranillas, 1D, E-50018 Zaragoza, Spain
⁴ Department of Physics, University of Surrey, Guildford GU2 7XH, UK
⁵ INAF-Osservatorio Astronomico d’Abruzzo, via Mentore Maggini s.n.c., 64100 Teramo, Italy
⁶ IAC- Instituto de Astrofísica de Canarias, Calle Vía Lactea s/n, E-38205 La Laguna, Tenerife, Spain
⁷ Departmento de Astrofísica, Universidad de La Laguna, E-38206 La Laguna, Tenerife, Spain
⁸ UAI – Unione Astrofili Italiani/P.I. Sezione Nazionale di Ricerca Profondo Cielo, 72024 Oria, Italy
⁹ Institute for Computational Cosmology, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
¹⁰ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, D-14482 Potsdam, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 30 March 2026
Accepted: 26 May 2026

Abstract

Aims. We present deep imaging of Andromeda XXXVI (And XXXVI), a dwarf galaxy discovered through visual inspection of the Pan-Andromeda Archaeological Survey using observations obtained with the OSIRIS+@GTC instrument.

Methods. The colour-magnitude diagram of And XXXVI shows a well-defined red giant branch (RGB). However, constraining a distance is challenging because the tip of the RGB is sparsely populated, and no horizontal branch stars have been found. The RGB is nevertheless well matched by an old (12.5 Gyr) metal-poor ([Fe/H = −2.5) isochrone shifted to the distance of Andromeda (776 kpc). With a projected distance of ∼119 kpc from M31, And XXXVI is most likely a satellite of Andromeda.

Results. With M_V ∼ −5.9 ± 0.1, a half-light radius of r_h ∼ 64₋₁₉⁺³⁰ pc, and an ellipticity of ϵ ∼ 0.015_−0.012^+0.032, And XXXVI is one of the faintest of ultra-faint M31 dwarfs discovered to date.

Conclusions. The discovery of And XXXVI adds to the faint end of M31’s satellite luminosity function, suggesting the presence of an even larger population of very faint satellites. Deeper space-based imaging and/or spectroscopic observations are needed to better constrain its position within M31’s halo. Combined with a detailed star formation history, such data would help determine whether And XXXVI’s old metal-poor stellar population indicates early quenching, which would be similar to the trends seen in Milky Way satellites, and whether And XXXVI could be considered a reionisation fossil.