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

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.