We address the origin of Ultra-Diffuse Galaxies ( UDGs ) , which have stellar masses typical of dwarf galaxies but effective radii of Milky Way-sized objects .
Their formation mechanism , and whether they are failed L _ { \star } galaxies or diffuse dwarfs , are challenging issues .
Using zoom-in cosmological simulations from the NIHAO project , we show that UDG analogues form naturally in dwarf-sized haloes due to episodes of gas outflows associated with star formation .
The simulated UDGs live in isolated haloes of masses 10 ^ { 10 - 11 } M _ { \odot } , have stellar masses of 10 ^ { 7 - 8.5 } M _ { \odot } , effective radii larger than 1 kpc and dark matter cores .
They show a broad range of colors , an average Sérsic index of 0.83 , a typical distribution of halo spin and concentration , and a non-negligible HI gas mass of 10 ^ { 7 - 9 } M _ { \odot } , which correlates with the extent of the galaxy .
Gas availability is crucial to the internal processes that form UDGs : feedback driven gas outflows , and subsequent dark matter and stellar expansion , are the key to reproduce faint , yet unusually extended , galaxies .
This scenario implies that UDGs represent a dwarf population of low surface brightness galaxies and should exist in the field .
The largest isolated UDGs should contain more HI gas than less extended dwarfs of similar M ^ { \star } .