IllustrisTNG50 angular momentum maps: tracing the morpho-kinematic evolution of galaxies

Abstract

Following the first observational study of the two-dimensional spatial distribution of stellar specific angular momentum (sAM) in late-type galaxies, we quantify the morpho-kinematic diversity of galaxy simulations using the newly proposed j-types classification. We analyse the stellar sAM surface density (sAMSD) of $\sim$8000 TNG50 stellar discs spanning $0 \leq z \leq 3.5$ and $9.5 \leq \log(M_\star/\mathrm{M}_\odot) \leq 11.2$, selected from the TNG50 MW/M31 parent sample. We characterize their j-substructures using four morpho-kinematic metrics derived from comparisons with the Freeman sAMSD distribution and the Fourier decomposition of the galaxies in the sAMSD space. A Gaussian mixture model with four fully covariant components assigns each galaxy a probability of belonging to one of four j-types. We find that TNG50 discs exhibit a morpho-kinematic diversity consistent with observations, redistributing stellar angular momentum through four dominant j-substructures that evolve with redshift as follows: j-irregulars ($\bar{z}=0.91$), j-spirals ($\bar{z}=0.76$), j-rings ($\bar{z}=0.62$), and j-bars ($\bar{z}=0.39$). The gas fraction and stellar rotational support ($V/\sigma$) drive this evolution: gas-rich galaxies preferentially host j-irregulars and j-spirals, whereas gas-poor systems favour j-rings and j-bars. At fixed gas fraction, higher $V/\sigma$ favours j-spirals and j-rings, respectively. We conclude that there is a canonical pathway for the redistribution of angular momentum within galactic discs undergoing secular evolution in TNG50, accessible only through their morpho-kinematic description. The sAMSD analysis links variations in stellar dynamics to their consequences for mass redistribution, enabling the reconstruction of comprehensive galactic evolutionary histories.