The Formation of Isolated Ultra-Diffuse Galaxies in Romulus25

<p>Ultra-diffuse galaxies (UDGs) are a subset of the low surface brightness (LSB) galaxy population characterized by dwarf-like luminosities but light profiles that can extend as far as those of Milky-Way-mass galaxies. Although thousands of these objects have been discovered in cluster and group environments, fewer than 200 have thus far been confirmed in the field. We use the Romulus25 cosmological simulation to study the methods by which isolated UDGs form and evolve. Beyond their low central surface brightnesses and large effective radii, these galaxies have fairly typical global properties for their masses and environments. However, their central star formation rates are systematically low, leading to lower central stellar densities and older, fainter central stellar populations. We find that isolated UDGs are the products of early major mergers that produce a temporary boost in spin and cause star-forming gas to migrate from the centers of the galaxies to larger radii. Although our simulated UDGs are consistent with the few existent observations of isolated UDGs, we predict that many isolated UDGs have been missed by current surveys. The Nancy Grace Roman Telescope&rsquo;s ability to provide deep, wide-field photometry will make it an ideal instrument for future discovery and study of these extreme LSB galaxies.</p>