The Formation of Isolated Ultra-Diffuse Galaxies in Romulus25

Abstract

We use the Romulus25 cosmological simulation volume to identify the largest-ever simulated sample of field ultra-diffuse galaxies (UDGs). At z=0, we find that isolated UDGs have average star formation rates, colors, and virial masses for their stellar masses and environment. Although they tend to have slightly elevated HI masses, UDGs are consistent with the general isolated dwarf galaxy population and make up ~20% of all field galaxies with 10\(^7\)<M\(_\star\)/M\(_\odot\)<10\(^{9}\). The properties of our UDGs are consistent with existing observations of field UDGs, but we predict that many isolated UDGs have been missed by current surveys. Despite their isolation at z=0, the UDGs in our sample are the products of major mergers at z>1. Mergers are no more common in UDG than non-UDG progenitors, but mergers that create UDGs tend to happen earlier, produce a temporary boost in spin, and cause star formation to migrate to the outskirts of galaxies, resulting in lower central star formation rates. The centers of the galaxies fade as their central stellar populations age, but their global star formation rates are maintained through bursts of star formation at larger radii. This formation channel is unique relative to other proposals for UDG formation in isolated galaxies, or for UDGs in groups and clusters, demonstrating that UDGs can potentially be formed through multiple mechanisms.