The long-term evolution of ultra-faint dwarf galaxies and observational implications

¹ Dipartimento di Fisica, Sapienza, Universitá di Roma, P.le A.Moro 2, I-00185 Roma, Italy
² Department of Physics, New York University Abu Dhabi, PO Box 129188 Abu Dhabi, UAE
³ Center for Astrophysics and Space Science (CASS), New York University Abu Dhabi, PO Box 129188 Abu Dhabi, UAE
⁴ Astronomisches Rechen-Institut, Zentrum für Astronomie, University of Heidelberg, Mönchhofstrasse 12–14, 69120 Heidelberg, Germany
⁵ INFN, Sezione di Roma, Sapienza, Universitá di Roma, P.le A.Moro 2, I-00185 Roma, Italy
⁶ INAF, Observatory of Roma at Monteporzio Catone, Via Frascati, 33, I-00078 Monteporzio Catone, Italy
⁷ Centro Ricerche Enrico Fermi, Piazza del Viminale 1, I-00184 Roma, Italy
⁸ Dipartimento di Fisica e Astronomia, Universitá di Padova, Vicolo Osservatorio 3, I-35122 Padova, Italy
⁹ Departamento de Astronomía, Facultad Ciencias Físicas y Matemáticas, Universidad de Concepción, Avenida Esteban Iturra, Casilla 160-C, Concepción, 4030000, Chile
¹⁰ National Institute for Nuclear Physics – INFN, Sezione di Trieste, I-34127 Trieste, Italy
¹¹ National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Rd., Chaoyang District, 100101 Beijing, China
¹² Kavli Institute for Astronomy and Astrophysics, Peking University, Yiheyuan Lu 5, Haidian Qu, 100871 Beijing, China

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Received: 16 January 2026
Accepted: 11 April 2026

Abstract

Context. In the Local Group, dwarf spheroidal galaxies and ultra-faint dwarf galaxies (UFDs) exhibit large velocity dispersions. These values are generally attributed to the presence of substantial amounts of dark matter (DM), in line with predictions of the standard model of galaxy formation. However, alternative explanations exist, such as non-virialized dynamical states induced by tidal interactions, the presence of stellar streams, and the artificial inflation of the velocity dispersion caused by binary-star orbital motion.

Aims. We studied the dynamical evolution of UFDs using purely stellar (“dry”) dynamics, without invoking DM. We dynamically evolved our systems up to a Hubble time and compared our results with observational studies and previous theoretical work.

Methods. We employed direct high-precision N-body simulations performed with the NBODY6++GPU code. We explored the role of binaries in inflating the velocity dispersion of low-mass host galaxies. We also present both the stellar and dynamical evolution of the stellar population, which is necessary to properly interpret our results.

Results. We find that, in all our models, the UFD remains globally quasi-stationary for approximately 3000 Myr. Subsequently, the system undergoes mass segregation and experiences a phase resembling core collapse. Red giants and white dwarfs are found to play significant, but distinct, roles. Red giants provide the dominant contribution to the luminosity, whereas white dwarfs constitute the largest fraction of the nonluminous component, accounting for approximately 13% of the total stellar population. Finally, if not properly taken into account, velocity dispersion measurements can be strongly biased by the presence of a significant binary population, which can lead to substantial overestimates of velocity dispersion in UFDs.