  Ultra-faint dwarf galaxies (UFDs) are promising observable proxies to
building blocks of galaxies formed in the early Universe. We study the
formation and evolution of UFDs using cosmological hydrodynamic simulations. In
particular, we show that a major merger of two building block galaxies with
3,900 Msun and 7,500 Msun at the cosmic age of 510 Myr results in a system with
an extended stellar distribution consistent with the de Vaucouleurs profile.
The simulated galaxy has an average stellar metallicity of [Fe/H]=-2.7 and
features a metallicity gradient. These results closely resemble the properties
of a recently discovered UFD, Tucana II, which is extremely metal-poor and has
a spatially extended stellar halo with the more distant stars being more
metal-poor. Our simulation suggests that the extended stellar halo of Tucana II
may have been formed through a past major merger. Future observational searches
for spatially extended structures around other UFDs, combined with further
theoretical studies, will provide tangible measures of the evolutionary history
of the ancient, surviving satellite galaxies.
