Ultra-Diffuse, Ultra-Different: Observed vs. Simulated Ultra-Diffuse Galaxies Live in Fundamentally Different Halos

TL;DR

The paper tackles why ultra-diffuse galaxies (UDGs) often exhibit high globular cluster (GC)–inferred halo masses yet yield modest dynamical masses in simulations. It matches observed UDG dynamical masses to best-fitting halos in the NIHAO and HESTIA suites and compares the resulting halo masses to GC-based inferences, uncovering a systematic discrepancy: simulated halos that reproduce inner dynamics are too light overall. It finds that HESTIA halos tend to lie above standard stellar mass–halo mass relations, while NIHAO halos generally track these relations, yet many observed UDGs require more massive halos than those simulated in the same dynamical-mass context. The authors argue that resolving this tension requires greater diversity in dwarf-galaxy halo profiles, including large, extended cores, implemented within self-consistent cosmological simulations to reconcile observed halo masses with simulated inner dynamics.

Abstract

In this work, we compare galaxies from the NIHAO and HESTIA simulation suites to ultra-diffuse galaxies (UDGs) with spectroscopically measured dynamical masses. For each observed UDG, we identify the simulated dark matter halo that best matches its dynamical mass. In general, observed UDGs are matched to simulated galaxies with lower stellar masses than they are observed to have. These simulated galaxies also have halo masses much less than would be expected given the observed UDG's stellar mass and the stellar mass -- halo mass relationship. We use the recently established relation between globular cluster (GC) number and halo mass, which has been shown to be applicable to UDGs, to better constrain their observed halo masses. This method indicates that observed UDGs reside in relatively massive dark matter halos. This creates a striking discrepancy: the simulated UDGs are matched to the dynamical masses of observed ones, but not their total halo masses. In other words, simulations can produce UDGs in halos with the correct inner dynamics, but not with the massive halos implied by GC counts. We explore several possible explanations for this tension, from both the observational and theoretical sides. We propose that the most likely resolution is that observed UDGs may have fundamentally different dark matter halo profiles than those produced in NIHAO and HESTIA. This highlights the need for a simulation that self-consistently produces galaxies of a stellar mass of $\sim 10^8 M_\odot$ in dark matter halos that exhibit the full range of large dark matter cores to cuspy NFW-like halos.

Figures (4)

• Figure 1: Dynamical mass enclosed within a radius vs that radius. Red points are observed UDGs with markers corresponding to their being in a field (triangle), group (squares) or cluster (circles) environment. On the left we compare the observed dynamical masses to mass profiles from the NIHAO simulation, and on the right we compare the observed dynamical masses to mass profiles obtained from the HESTIA simulation. For both simulations, their mass profiles are colour-coded by their total halo mass, and we do not limit the plotted halos to just those that contain UDGs. For both simulations, we exclude any gas mass when plotting, as the observed UDGs are largely gas-free. UDGs are matched to the halo that best matches their observed dynamical mass for comparison in Figure \ref{['fig:smhm']}. It is of note that while many UDG dynamical masses prefer massive halos ($>10^{11}~M_{\odot}$), none reside in halos as massive as the Milky Way ($\sim 10^{12}~M_\odot$).
• Figure 2: The stellar mass -- halo mass relationship. On the left we show galaxies in the NIHAO simulation (blue squares). On the right we show galaxies in the HESTIA simulation (blue triangles). We plot UDGs using their observed stellar masses and the total halo mass from their best-fitting dark matter halo after matching them with simulations, as shown in Figure \ref{['fig:mass_radius']} (red points, with markers per Figure \ref{['fig:mass_radius']}). We connect the observed UDG to its best-fitting simulated galaxy using a dotted grey line. On the left of the plot, there are 7 UDGs that are connected to a NIHAO dark matter halo that did not form any stellar mass (possibly due to reionisation, a known effect at such low halo masses). As such, their dotted grey lines overlap and connect to a datum outside the plotted region. In both panels, we include an observed stellar mass -- halo mass relationship for normal galaxies from Brook2014 and Danieli2023. We define regions of >0.5 dex beyond the relationship of Brook2014 and label them as the regions of overbright galaxies (cyan; i.e., where galaxies have more stellar mass than expected given their total halo mass) and of failed galaxies (olive; i.e., where galaxies have less stellar mass than expected given their total halo mass). HESTIA tends to create galaxies that lie above this relationship, which suggests that their halos over-produce stars (i.e., the simulation suffers from over-cooling). NIHAO creates galaxies that largely follow the established relationship. In both simulations, observed UDGs tend to have best-fitting halos for their dynamical masses which host galaxies of markedly different (usually lower) stellar mass in the simulation. We take this as observational evidence for the need for increased scatter in the stellar mass -- halo mass relationship in the stellar mass range of dwarf galaxies ($M_\star \approx 10^8$ M$_\odot$).
• Figure 3: Observed halo mass based on UDG GC number vs. matched halo mass in the simulation from the UDG's dynamical mass. Markers follow Figure \ref{['fig:mass_radius']}.
• Figure 4: Normalised histograms of the total halo mass of the comparison halos from the simulations. NIHAO is plotted in grey and is distributed in a flat manner with halo mass. HESTIA is plotted in brown and has comparatively more low mass halos than high mass halos. In any given cosmological volume, there are more low-mass halos than high-mass halos (e.g., the HESTIA simulations) however, the NIHAO simulations are not reflective of cosmological halo abundances and display a flat distribution.