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

TL;DR

This work analyzes ultra-diffuse galaxies (UDGs) in the TNG50-1 simulation alongside LSBs, HSBs, and dwarfs, combining scaling relations, intrinsic morphology, kinematics, and mock integral field spectroscopy to elucidate their origin. It finds that UDGs and dwarfs share similar scaling relations and are consistent with a dwarf-like, cored dark matter halo, while UDGs exhibit environment-dependent morphology from prolate in isolation to prolate/oblate when tidally influenced. The stellar and dark matter kinematics indicate slow-rotator, dispersion-dominated systems for UDGs and dwarfs, contrasting with disc-dominated, fast-rotating LSBs/HSBs, suggesting a common dynamical lineage between UDGs and dwarfs but distinct pathways from LSBs/HSBs. Overall, the study supports formation scenarios in which UDGs arise from low-mass halos with tidal processing, yielding a spectrum of intrinsic shapes and kinematic states shaped by environment.

Abstract

The formation and evolution of the ultra-diffuse galaxies (UDGs) continues to remain a puzzle. Similarities and differences in the morphological and the kinematical properties of the UDGs with their possible precursors, namely low-surface brightness (LSBs), L*-type high-surface brightness (HSBs) and dwarf galaxies, may provide crucial constraints on their origin and evolution. We selected samples of UDGs, LSBs, HSBs and dwarfs from TNG50-1. We first obtained a few possible scaling relations involving some mass properties to analyse if the regression fits for UDGs are in compliance with those of the other samples. Then, we studied individual galaxy cutouts to evaluate the intrinsic shapes of their dark-matter (DM) and stellar components, orbital and kinematical properties related to their stellar velocity dispersion. Finally, we constructed the mock IFU data using the SimSpin code to extract the stellar kinematic moment maps. We observe that the UDGs and the dwarf galaxies have nearly similar regression fits in a. stellar-to-gas mass ratio vs gas mass, b. stellar-to-gas mass ratio vs total dynamical mass, c. stellar central surface density vs ratio of stellar-to-total dynamical mass, and d. total baryonic mass vs total dynamical mass parameter spaces. Next, we find that the isolated UDGs are prolate rotators similar to the dwarf population, while the tidally-bound UDGs can exhibit both prolate and oblate-rotating shapes. The DM and stellar velocity anisotropy properties of the UDGs suggest that they reside in a cored, dwarf-like halo and may be classified by early-type galaxies. Finally, the stellar kinematic properties suggest that both the UDGs and the dwarfs are slow-rotators having low to nearly no-rotations in contrast to the late-type, disc-dominated, fast-rotating LSBs and HSBs. Therefore, we may conclude that the UDGs and the dwarfs possibly have a common dynamical lineage.

Figures (13)

• Figure 1: Distribution of the TNG50-1 UDGs selected in our sample in the (left) M$_g$-R$_e$, (middle) M$_*$-M$_\text{dyn}$, and (right) M$_*$-M$_\text{gas}$ space marked in teal stars. To compare with the observed populations, the UDGs obtained from 2017Leisman_ALFALFA, 2017RomanTrujillo_UDG, 2017Shi_UDG, 2019Janowiecki_UDG, 2021Marleau_UDG, 2021Kadowaki_UDG, 2022Poulain_UDG+Dwarf_Selection, and 2023Jones_UDG are superposed on the left panel. Similarly, on the middle and the right panel, the stellar, dynamical, and gas masses of the UDGs taken from 2020Guo_UDG, 2020PinaHI_UDG, 2020KarunakaranHI_UDG, 2021Marleau_UDG, 2022Kong_UDG, and 2022KadoFong_UDG are shown for comparison. The teal dots represent the remaining UDGs identified in TNG50-1 while the grey dots denote the whole sample space from which the UDGs were selected.
• Figure 2: Distribution of the tidal index, $\Theta$, for our galaxy samples: UDGs in teal, LSBs in yellow, HSBs in blue and the dwarfs in pink, along with their box plots. The blue region shows the buffer region which we rejected from our study.
• Figure 3: Histograms of (a) M$_*$, (b) M$_{\rm gas}$, (c) M$_{\rm dyn}$ and (d) M$_g$ of the UDGs, LSBs, HSBs, and the dwarfs considered in our study presented in teal, yellow, blue and pink colour, respectively. The solid histograms represent the isolated galaxies and the dotted histograms denote the tidally bound subsamples. The medians of the isolated and tidally bound subsamples are denoted with, respectively, solid and hollow triangle on the top following the same colour scheme.
• Figure 4: Possible scaling relations of (a) stellar-to-gas mass ratio versus gas mass (log$_{10}$ M$_*$/M$_{\rm gas}$ vs. log$_{10}$ M$_{\rm gas}$), (b) stellar-to-gas mass ratio versus total dynamical mass (log$_{10}$ M$_*$/M$_{\rm gas}$ vs. log$_{10}$ M$_{\rm dyn}$), and (c) total baryonic mass versus total dynamical mass log$_{10}$ M$_{\rm b}$ vs. log$_{10}$ M$_{\rm dyn}$). The UDGs, LSBs, HSBs, and the dwarfs are shown in teal, yellow, blue and magenta colour, respectively. Each of the galaxy samples are divided in two subsamples - isolated (denoted with filled circles) and tidally bound (denoted with empty circles). The scaling relations are obtained for the two subsamples of each galaxy classes separately and plotted with solid and dashed lines, respectively.
• Figure 5: Distribution of the (top) intermediate-to-major and minor-to-major ((b/a)$_{\rm DM}$-(c/a)$_{\rm DM}$) and (bottom) elongation and flattening ($e_{\rm DM}$-$f_{\rm DM}$) of the DM component of our galaxy samples. The histograms of the $x$ and $y$ data are plotted on the top and right sides of each panel, respectively, with equal bin widths, showing the fraction of galaxies in each bin. The isolated galaxies are shown with filled circles and filled histograms, while the empty circles and empty histograms represent the tidally bound subsamples. The same colour scheme as in Figure \ref{['fig:basic_regression']} is adopted.