Stellar halos tracing the assembly of ultra-faint dwarf galaxies

Context. Ultra-faint dwarfs (UFDs) are expected to be relics of the earliest galaxies to have formed in the Universe. Observations show the presence of a stellar halo around UFDs, which can give precious insights into UFD evolution. Indeed, stellar halos can form via tidal interaction, early supernova feedback, or merging events. Aims. This work investigates how merger properties impact the formation of stellar halos around UFDs, focusing on Tucana II, the most promising UFD assembled through mergers. Methods. We developed N -body simulations of dry isolated mergers between two UFDs, resolving their stellar component down to 1 M ⊙ . We built a suite of simulations by varying: the merger-specific i) angular momentum, l , and ii) kinetic energy k , iii) the merger mass ratio, M 1 / M 2 , iv) the dark-to-stellar mass ratio, M DM / M ⋆ , of the progenitors, and v) their stellar size, R 1/2 . To fully explore such a five-dimensional parameter space, we trained a neural network to emulate the properties of the resulting “post-merger” UFD, by quantifying the half-mass radius ( R ⋆ ) and the fraction of stars at radii > 5 R ⋆ ( f 5 ). Results. Our principal component analysis clearly shows that f 5 ( R ⋆ ) is primarily determined by M 1 / M 2 ( R 1/2 ), with R 1/2 ( M 1 / M 2 ) playing a secondary role. Both f 5 and R ⋆ show almost no dependence on k , l , and M DM / M ⋆ in the explored range. Using our emulator, we find that to form the stellar halo observed in Tucana II; that is, f 5 = 10 ± 5% and R ⋆ = 120 ± 30 pc, we need to merge progenitors with M 1 / M 2 = 8 −3 +4 , the size of the more massive one being R 1/2 = 97 −18 +25 pc. Such findings are corroborated by the consistency ( χ 2 ≃ 0.5 − 2) between stellar density profiles observed for Tucana II and those of simulations that have M 1 / M 2 and R 1/2 close to the values predicted by the emulator. Conclusions. The stellar halos of UFDs contain crucial information about the properties of their smaller progenitor galaxies. Ongoing and planned spectroscopic surveys will greatly increase the statistics of observed stars in UFDs, and thus of their associated stellar halos. By interpreting such observations with our simulations, we will provide new insights into the assembly history of UFDs, and thus the early galaxy formation process.