The dynamical lineage of ultra-diffuse galaxies from TNG50-1

Indian Institute of Science Education and Research Tirupati 517619, India

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Received: 26 June 2025
Accepted: 12 December 2025

Abstract

Context. The formation and evolution of ultra-diffuse galaxies (UDGs) continue to be a puzzle. Broadly, the formation scenarios of UDGs can be classified into two categories: a massive yet failed L_*-type and a dwarf-like origin. The similarities and differences in the morphological and kinematical properties of the UDGs with their possible precursors may provide important constraints on their origin and evolutionary history.

Aims. We compared and contrasted structural, orbital, and kinematical properties of the UDGs with other galaxy populations, namely, low-surface brightness galaxies (LSBs), L_*-type or high-surface brightness galaxies (HSBs), and the dwarf galaxies.

Methods. We selected a sample of UDG, LSB, HSB, and dwarf galaxies from the TNG50-1 box of the IllustrisTNG simulation. We first obtained a few possible scaling relations involving their mass properties and conducted Spearman’s rank correlation tests to analyse if the regression fits for UDGs are in compliance with those of the other galaxy samples. Then, we studied the cut-outs of the individual galaxies to investigate the intrinsic shapes of their dark matter (DM) and stellar components. We also investigated their orbital and kinematical properties by evaluating a few parameters composed of velocity dispersion components. Finally, we constructed mock integral field spectroscopic data using the publicly available software SimSpin to extract the kinematic moment maps of the line-of-sight velocity distribution and probe the stellar kinematic properties of our galaxy samples. In all the cases, we divided the samples in two subpopulations: isolated and tidally bound to study the effect of the local environment.

Results. We observed that the UDGs and the dwarf galaxies have nearly similar regression fits in the following parameter spaces: (a) stellar-to-gas mass ratio versus gas mass, (b) stellar-to-gas mass ratio versus total dynamical mass, and (c) total baryonic mass versus total dynamical mass. Further, we can infer that the isolated UDGs can be classified as prolate, while the tidally bound UDGs can exhibit both prolate and oblate shapes. The DM and stellar velocity anisotropy of the UDGs suggest that they reside in a cored low-mass halo and can be classified as early-type galaxies. Finally, their stellar kinematic properties suggest that the UDGs are slow-rotators exhibiting low to nearly no rotation.

Conclusions. The UDGs and the dwarf galaxies share similarities as far as the aforementioned possible scaling relations are concerned. Both the isolated UDGs and dwarf population can be characterised by prolate shapes, unlike other galaxy populations. However, the tidally bound UDGs exhibit both prolate as well as oblate shapes. The velocity anisotropy of the UDGs and the dwarfs hint at the fact that they may have originated in a dwarf-like halo, as opposed to the LSBs or the HSBs. Moreover, the UDGs and the dwarfs can be classified as early-type slow-rotating galaxies, in contrast to the late-type, disc-dominated, and fast-rotating LSBs and the HSBs. Therefore, we conclude that the UDGs and the dwarfs possibly have a common dynamical lineage.