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Ultra-faint dwarf galaxies (UFDs) are promising observable proxies tobuilding blocks of galaxies formed in the early Universe. We study theformation and evolution of UFDs using cosmological hydrodynamic simulations. Inparticular, we show that a major merger of two building block galaxies with3,900 Msun and 7,500 Msun at the cosmic age of 510 Myr results in a system withan extended stellar distribution consistent with the de Vaucouleurs profile.The simulated galaxy has an average stellar metallicity of [Fe/H]=-2.7 andfeatures a metallicity gradient. These results closely resemble the propertiesof a recently discovered UFD, Tucana II, which is extremely metal-poor and hasa spatially extended stellar halo with the more distant stars being moremetal-poor. Our simulation suggests that the extended stellar halo of Tucana IImay have been formed through a past major merger. Future observational searchesfor spatially extended structures around other UFDs, combined with furthertheoretical studies, will provide tangible measures of the evolutionary historyof the ancient, surviving satellite galaxies.

Formation of an extended stellar halo around an ultra-faint dwarf galaxy following one of the earliest mergers from galactic building blocks