Galaxy formation with wave/fuzzy dark matter: The core-halo structure and the solitonic imprint

TL;DR

This work investigates how different dark matter models shape stellar density profiles in dwarf galaxies by comparing CDM, WDM, and wave/fuzzy dark matter (ψDM) in high-resolution cosmological hydrodynamical simulations. It shows that ψDM naturally produces a core-halo stellar structure, with a central solitonic core and an extended halo, while CDM lacks such a two-regime imprint in DM, gas, and stars; WDM can mimic some features but does not generate a true solitonic core. Using Jeans modeling and MCMC fits to ψDM profiles, the study derives core radii and transition radii consistent with observed Local Group dSphs, and finds a distinctive asymmetry in ψDM cores caused by soliton motion. The results suggest that stellar core-halo signatures can serve as observational discriminants between DM models, with implications for interpreting core formation and for future tests with JWST and lensing studies. Overall, the findings favor ψDM as a promising alternative to CDM and WDM, though they acknowledge tensions with some boson-mass constraints and emphasize the need for broader simulations and multi-boson scenarios to fully assess observational viability.

Abstract

Dark matter-dominated cores have long been claimed for the well-studied local group dwarf galaxies. More recently, extended stellar halos have been uncovered around several of these dwarfs through deeper imaging and spectroscopy. Such core-halo structures are not a feature of conventional cold dark matter (CDM). In contrast, smooth and prominent dark matter cores are predicted for wave/fuzzy dark matter ($ψ$DM). The question arises as to what extent the visible stellar profiles should reflect this dark matter core structure. Here we compare cosmological hydrodynamical simulations of CDM, ``WDM'' (model used as a proxy for $ψ$DM) \& $ψ$DM, aiming to predict the stellar profiles for these three DM scenarios. We show that cores surrounded by extended halos are distinguishable for $ψ$DM, where the stellar density is enhanced in the core due to the presence of the relatively dense soliton. Our analysis demonstrates that, in our simulations, a distinctive core-halo structure does not appear in the case of CDM in the DM, gas, or stars. Whereas we do find a core-halo transition for DM, gas, and stars for $ψ$DM, and the scale of this transition is in line with the predicted core radius set by the soliton scale anticipated for the adopted boson mass of 2.5$\times10^{-22}$eV. The presence of a core-halo structure in the stellar profile for Galaxy 1 for $ψ$DM is visible for the most massive and the first galaxy to form in the simulation. Clearly, further simulations are needed to establish how strict this possible relationship is between the DM and stellar core-halo profile as a potential observational discriminator. Furthermore, we observe the anticipated asymmetry for $ψ$DM due to the soliton's motion (jumping and random walk), a distinctive characteristic not found in the symmetric distributions of stars in the warm and CDM models.

Figures (13)

• Figure 1: Logarithmic (comoving) projected densities along the line of sight from the study by Mocz:2019. The projection is performed along the entire line-of-sight dimension of the simulation box. The first row shows the profiles for DM in the context of CDM, and the second row displays the corresponding profiles for stars. The third and fourth rows illustrate the profiles for DM and stars in the context of $\psi$DM--“WDM”. In each row, the left panel represents the density profile at a redshift of $z=5.56$, while the right panel shows it at $z=2.23$. “WDM” and $\psi$DM exhibit remarkably similar evolutionary trends across different redshifts, suggesting their resemblance in terms of the development of large-scale structures, in contrast to CDM Mocz:2019Mocz:2020. We have added colored circles to identify each galaxy throughout this work: Galaxy 1 (G1) marked in red, Galaxy 2 (G2) in green, and Galaxy 3 (G3) in blue.
• Figure 2: Logarithm of the projected number density of stellar particles in each simulated Galaxy. The row headers indicate the halo number, the DM model, and the redshift of the data. Additionally, we zoom in on the central regions of the Galaxies for a closer view. Notably, the motion of the soliton is evident, resulting in an offset (compared to the centroid of the full 2D stellar distribution, marked by the white star) and asymmetry (exclusively addressing the observed comet tail in the core of the Galaxy) in the distribution of stellar particles, a contrast to the symmetry observed in CDM and “WDM” scenarios. The data is represented in comoving units.
• Figure 3: $\psi$DM vs. CDM extracted stellar profiles from Mocz:2019Mocz:2020 simulation data ($z=5.56$). This figure presents the extracted stellar profiles of the $\psi$DM and CDM Galaxies from the simulation data of Mocz:2019Mocz:2020 at $z=5.56$. We zoom in on the inner part of the galaxy using 0.01 kpc binning. We fitted $\psi$DM profiles with a pure soliton--isothermal (magenta solid line) and NFW (black solid line) profiles to highlight the differences between the core and the outer interference pattern. The combination of these profiles is crucial for detecting a potential core-halo structure. This is evident in the case of Galaxy one, where both profiles (soliton and NFW) intersect, while still providing a good fit for the entire profile in the $\psi$DM scenario. Meanwhile, for CDM, there is no such two-regime structure necessity, where the entire profile can be described with a single NFW profile or an isothermal profile used to describe cored CDM profiles (dashed black and magenta lines, except for Galaxy 3 where the same NFW profile can be used to describe both DM models). The vertical black line represents the comoving resolution limit of the data, indicating that values smaller than this limit should be treated with caution due to their potentially unreliable nature, and is why we did not take them into account for the fitting.
• Figure 4: “WDM” vs. CDM extracted stellar profiles from Mocz:2019Mocz:2020 simulation data ($z=2.23$). We zoom in on the interior part of the Galaxy. In this case, there is not a significant discrepancy between the two models in the inner region, indicating that CDM profiles need to be described by the presence of a core, as for “WDM”, even if it contradicts expectations for CDM--“WDM”. However, a noticeable difference becomes apparent near the theoretical $\psi$DM transition point in all three profiles, where the presence of a transition point similar to that expected for the $\psi$DM profiles is easily identifiable, particularly in the case of G3. This is particularly interesting as “WDM” does not have a dark matter core-halo structure as $\psi$DM does. Specifically the solitonic cored profile. The vertical black line represents the comoving resolution limit of the data, indicating that values smaller than this limit should be treated with caution due to their potentially unreliable nature. This is why we did not take them into account for the fitting.
• Figure 5: Representation of the stellar, dark matter, and gaseous profiles for three Galaxies from different DM scenarios: $\psi$DM, “WDM”, and CDM. The core-halo structure is visible for all three $\psi$DM dark matter profiles with a clear transition point dividing both regimes, while CDM and “WDM” exhibit the expected cuspy shapes. This difference is also visible in stellar profiles between different DM models, with bigger flat cores and more prominent falls in the $\psi$DM cases compared to the more smoothed shapes of “WDM”--CDM. For $\psi$DM G1, the stellar and gaseous profiles display a similar core radius as well as transition points compared to the DM profile. The vertical dashed black line represents the core radius of the respective solitons. The vertical solid black line represents the comoving resolution limit of the data, indicating that values smaller than this limit should be treated with caution due to their potentially unreliable nature. This is why we did not take them into account for the fitting