The Milky Way’s Peculiar Primordial Halo: A Shallow Core with a Steep Decline
In this study, Li et al. (2025) investigate the “primordial” dark matter halo of the Milky Way, the version of the halo that existed before the Galaxy’s visible matter, or baryons, settled into its center. Using data from the Gaia mission, the team reverse-engineered how baryonic matter compresses dark matter over time. By numerically modeling this process and iterating backward, they uncovered the structure of the Milky Way’s original halo. Their surprising finding: the Galaxy’s primordial halo appears to have both a shallow inner core and a steep outer decline, a combination that does not fit with standard cold dark matter (CDM) predictions.
Numerical Modeling and Data
Li and collaborators used an algorithm first proposed by Young (1980) to simulate adiabatic contraction, how a dark matter halo contracts as baryons collect at its center. They integrated this process into a Markov Chain Monte Carlo (MCMC) framework, implemented in their new open-source code compress. This allowed them to infer the primordial halo from the observed rotation curve of the Milky Way. Using Gaia DR3’s highly accurate stellar motions, they derived circular velocities and tested their models against twelve different “baryonic mass models,” representing various possible distributions of stars, gas, and the Galactic bar.
Modeling Results
The authors first attempted to fit the data using the well-known Navarro–Frenk–White (NFW) profile, which describes halos predicted by CDM-only simulations. However, the NFW model could not reproduce the Gaia-derived velocity data, which show a sharp Keplerian-like decline at large distances from the Galactic center. Li et al. therefore adopted the more flexible Einasto profile, which includes a shape parameter controlling how sharply density falls off. As shown in Figure 1 (page 2), this model successfully reproduced the Milky Way’s observed rotation curve when baryonic compression was taken into account.
Primordial Halo Properties
Their results (Figure 2, page 2) show that the Milky Way’s primordial halo mass ranges between 1.1 and 1.4 × 10¹¹ solar masses, significantly lower than CDM-based expectations from abundance matching, by more than three standard deviations. The inferred halos also have low concentrations, contradicting predictions from N-body simulations. In other words, when baryonic effects are removed, the Milky Way’s dark matter halo appears less dense and more diffuse than the CDM model suggests.
Structure and Implications
The structural comparison between current and primordial halos (Figure 3, page 3) reveals striking differences. The primordial halo shows a flattened inner region, a shallow core, while the outer parts drop off sharply in density. This combination intensifies the long-standing core-cusp problem, which arises when observations reveal flattened dark matter cores where CDM predicts steep central cusps. Li et al. note that, unlike small dwarf galaxies where stellar feedback can reshape halos, the Milky Way’s mass would require far stronger processes, such as energy injection from active galactic nuclei (AGN), to achieve such flattening. Yet even AGN feedback in large simulations (like IllustrisTNG) cannot produce both the shallow inner core and steep outer falloff seen in the Gaia data.
Broader Context and Conclusions
Alternative dark matter models, such as warm dark matter (WDM) or fuzzy dark matter (FDM), offer partial explanations. WDM can create larger cores, but doing so at the scale observed here would require particle masses so small that dwarf galaxies could not form. FDM, meanwhile, can generate soliton-like cores but fails to account for the steep outer density decline. Thus, neither model fully resolves the puzzle. Li and colleagues conclude that the Milky Way’s halo structure may require a combination of new dark matter physics and stronger baryonic feedback mechanisms to explain its odd shape.
Source: Li
