Ultradiffuse Galaxies in the EAGLE Simulation

Abstract We use the highest-resolution EAGLE simulation, Recal-L025N0752, to study the properties and formation of ultradiffuse galaxies (UDGs). We identify 181 UDGs at z = 0 and find their properties are consistent with observations. The total masses of the EAGLE UDGs range from ∼5 × 10 8 M ⊙ to ∼2 × 10 11 M ⊙ , indicating that they are dwarf galaxies rather than failed L ⋆ galaxies. The EAGLE UDGs are not a distinct population, but rather a subset of dwarf galaxies, as their properties generally form a continuous distribution with those of normal dwarf galaxies. Unlike most of the situations in previous studies, the extended sizes of the field UDGs in EAGLE are not driven by high halo spins or by supernova-induced stellar expansion, but instead largely arise from high spins in their star-forming gas and thus the newly formed stars at large radii. We hypothesize that this might be attributed to galactic fountains, via which star-forming gas is launched to large halocentric distances and acquires additional angular momentum through interactions with the circumgalactic medium. For satellite UDGs, ∼60% of them were already UDGs before falling into the host galaxy, while the remaining ∼40% were normal galaxies prior to infall and subsequently transformed into UDGs due to tidal effects after infall.