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

TL;DR

This study uses cosmological simulations to show that a major merger early in the Universe's history can produce an ultra-faint dwarf galaxy with an extended stellar halo and metallicity properties similar to Tucana II, shedding light on galaxy formation.

Contribution

It demonstrates that early major mergers can form extended stellar halos in ultra-faint dwarf galaxies, matching observed properties of Tucana II, a novel insight into galaxy evolution.

Findings

Simulated galaxy has a de Vaucouleurs profile with extended stellar distribution.

The galaxy exhibits a metallicity gradient with [Fe/H]=-2.7 on average.

Results closely match properties of the observed Tucana II galaxy.

Abstract

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.