The formation and evolution of supermassive disks in IllustrisTNG

Context. Supermassive disks are outstanding galaxies whose formation and evolution are still poorly understood. They comprise a large variety of objects, from large low-surface brightness galaxies such as Malin-1 to the most spectacular superluminous spirals. However, we still do not know the physical mechanisms behind their formation, whether they will be long-lived objects, or whether their mass could destroy them in time. Aims. We investigated the formation and evolution of supermassive disks in the magnetohydrodynamic state-of-the-art simulation IllustrisTNG-100. We aim to characterize the main formation mechanisms behind these galaxies and to understand the role played by the environment in their formation. Methods. Supermassive disks were identified using morpho-kinematic criteria based on a relation between the spin-parameter ( λ R ) and its ellipticity ( ɛ ) with two different thresholds. We defined supermassive disks as galaxies with λ /√ ɛ ≥ 0.31 or 0.71, and with stellar mass log 10 M ⋆ /M ⊙ >10 11 . We studied their colors, merging histories, active galactic nucleus histories, and environments. Additionally, we studied galaxies individually to check how they formed. Results. Supermassive disk galaxies typically have a quiescent merging history, with 48% experiencing no significant mergers at z ≤1. Their stellar mass growth is driven mainly by star formation, unlike spheroidal galaxies, which require a significant number of mergers to form. Moreover, the mergers experienced by disk galaxies are generally rich in gas, irrespective of whether they are minor or major events. Supermassive disks exist across various environments, from in isolation to in clusters, with ∼60% in isolation or low-mass groups, ∼25% in massive groups, and ∼15% within galaxy clusters. When studying the evolution of supermassive disks at z = 0.5, we show that when they gain sufficient mass, the probability of them maintaining their disk-like structure up to z = 0 is relatively high (∼60%). Lastly, while active galactic nuclei significantly influence the regulation of star formation in galaxies, they do not directly alter their morphological structures.