Understanding the Formation and Evolution of Dark Galaxies in a Simulated Universe

We study the formation and evolution of dark galaxies using the IllustrisTNG cosmological hydrodynamical simulation. We first identify dark galaxies with stellar-to-total mass ratios, M */M tot, smaller than 10-4, which differ from luminous galaxies with M */M tot >= 10-4. We then select the galaxies with a dark matter halo mass of similar to 109 h -1 M circle dot for mass completeness and compare their physical properties with those of luminous galaxies. We find that, at the present epoch (z = 0), dark galaxies are predominantly located in void regions without star-forming gas. We also find that dark galaxies tend to have larger sizes and higher spin parameters than luminous galaxies. In the early universe, dark and luminous galaxies show small differences in the distributions of spin and local environment estimates, and the difference between the two samples becomes more significant as they evolve. Our results suggest that, unlike luminous galaxies, dark galaxies tend to be initially formed in less dense regions and could not form stars because of heating from cosmic reionization and few interactions and mergers with other systems containing stars. This study based on numerical simulations can provide important hints for validating dark galaxy candidates in observations and for constraining galaxy formation models.