The long-term evolution of Ultra Faint Dwarf Galaxies and observational implications

TL;DR

The paper addresses why ultra-faint dwarf galaxies exhibit large velocity dispersions, testing whether purely stellar dynamics without dark matter can account for kinematics. Using high-precision direct N-body simulations with varied binary fractions and stellar evolution, the authors evolve a UFD-like system up to a Hubble time and analyze multiple velocity-dispersion definitions. They find a two-phase evolution: an initial quasi-stationary period followed by mass segregation and core contraction, with red giants dominating luminosity and white dwarfs forming a sizable non-luminous component; importantly, unresolved binaries can inflate measured velocity dispersions, potentially biasing virial mass estimates by substantial factors. These results suggest that, for the faintest satellites, the need for large dark-matter content may be reduced and emphasize careful interpretation of velocity dispersions; future work will extend the model to include a dark-matter halo to test its dynamical impact.

Abstract

Context. In the Local Group, dwarf spheroidal galaxies (dSphs) 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 the predictions of the standard model of galaxy formation. However, alternative, more conservative explanations exist, such as non-virialized dynamical states induced by tidal interactions, the presence of stellar streams, and artificial inflation of the velocity dispersion caused by binary-star orbital motion. Aims. We study the dynamical evolution of UFDs using purely stellar ("dry") dynamics, without invoking DM. We dynamically evolve our systems up to a Hubble time and compare our results with observational studies and previous theoretical work. Methods. We employ direct high precision NBODY simulations performed with the NBODY6++GPU code. We explore 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 (WD) are found to play significant, but distinct, roles. Red giants provide the dominant contribution to the luminosity, whereas WDs constitute the largest fraction of the non-luminous component, accounting for approximately 13% of the total stellar population. Finally, if not taken into account properly, 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

Figures (13)

• Figure 1: Initial relaxation (blue) and dynamical friction (orange) times of our UFD Plummer model as functions of radius. The red and black vertical dashed lines denote the initial core and half-mass radius of the adopted Plummer model. For reference, the grey horizontal line marks the Hubble time.
• Figure 2: Lagrangian radii evolution of the UFD model U30 over one Hubble time, using the time-updated value of the total mass. (Top) For both single and binary stars; (Middle) for single stars; (Bottom) for binary stars.
• Figure 3: Average mass within the different Lagrangian radii of the U30 model over one Hubble time.
• Figure 4: From top to bottom: (i) Fraction of mass present in the UFD, compared to the total mass of each component over simulation time. and (ii) Evolution of the members of dynamical binaries surviving in the UFD (blue, right-side ordinate) and fraction of primordial binaries surviving in the UFD (orange, left-side ordinate).
• Figure 5: Evolution of the average semi-major axis for the binaries in model U30. The orange curve corresponds to the sample of primordial binaries, the blue accounts also for the dynamical binaries.