The odd primordial halo of the Milky Way implied by Gaia. A shallow core, but a steep decline

TL;DR

This work uses Gaia DR3 rotation data to infer the Milky Way's primordial dark matter halo by reversing adiabatic contraction with an action-based distribution function, parameterized by an Einasto profile with shape parameter $\alpha$ and characterized by $(V_{200}, C_{200}, \alpha)$. An $MCMC$ framework ($\text{emcee}$) samples the primordial halo parameters across 12 baryonic models while iteratively contracting the halo via baryonic infall, implemented in the compress code. The results show a primordial Milky Way halo with $M_{200}$ in the range $(1.09-1.42)\times 10^{11} M_\odot$ and unusually low $C_{200}$, with an inner core too shallow and an outer density decline too steep to be compatible with standard CDM halos; neither warm nor fuzzy dark matter alone resolves this tension. The findings imply that either the Milky Way is atypical or that fundamental aspects of dark matter physics or galaxy formation must be revised, highlighting the necessity of incorporating baryonic compression when linking galaxy dynamics to primordial dark matter distributions. The methodology demonstrates a robust path to connect detailed rotation-curve data with primordial halo structure, offering a stringent test for DM models and galaxy formation theories.

Abstract

Primordial dark matter halos are well understood from cold dark matter-only simulations. Since they can contract significantly as baryons settle into their centers, direct comparisons with observed galaxies are complicated. We present an approach to reversing the halo contraction by numerically calculating the halo response to baryonic infall and iterating the initial condition. This allowed us to derive spherically averaged primordial dark matter halos for observed galaxies. We applied this approach to the Milky Way and found that the latest Gaia measurements for the rotation velocities imply an odd primordial Galactic halo: Its concentration and total mass differ by more than 3$σ$ from the predictions, and the density profile presents an inner core that is too shallow and an outer decline that is too steep to be compatible with the cold dark matter paradigm.

Figures (6)

• Figure 1: Example of a circular velocity fit using the McGaugh19 model for baryonic mass distributions. The purple, blue, and green lines represent the contributions of the bar, disk, and gas components, respectively. The solid and dashed black lines show the current and primordial dark matter halos, respectively. The solid red line indicates the total velocity profile. The black points show the latest Gaia measurements Jiao2023, and the gray upward triangles and squares show the terminal velocities from McClure-Griffiths2007McClure-Griffiths2016, and Portail2017, respectively. The data marked with open symbols were not fit because they do not consider the systematic uncertainties. The fits with 12 baryonic models are shown in Fig. \ref{['fig:fits']}.
• Figure 2: Halo masses and concentrations of the primordial Galactic halos derived from the Gaia circular velocity fits using 12 baryonic models. The red and blue stars with errors represent the halos with and without adiabatic contraction, respectively. The predicted halo mass-concentration relation within 1 $\sigma$ from simulations DuttonMaccio2014 is shown as the declining band. The vertical band shows the expected range of the MW halo mass according to the abundance-matching relation Moster2013. The upper and lower limits are set by the highest stellar mass (model A&S) plus 1 $\sigma$ and the lowest stellar mass (model I) minus 1 $\sigma$, respectively.
• Figure 3: Structure of the inferred primordial and current Galactic halos, along with predictions for the cold and warm dark matter. The density profiles are scaled so that there is no need to assume or consider the masses or concentrations for these halos. The gray band indicates the range of the current halos derived from the Gaia velocity fits using the 12 baryonic models, and the red band shows their corresponding primordial halos within 1$\sigma$. The blue band presents the simulated halos with cold dark matter only DuttonMaccio2014. The purple band shows the warm dark matter halos (normalized to match the primordial Galactic halo) with a core size spanning from 4.56 kpc (WDM5 in Maccio2012) to 7.0 kpc, corresponding to a particle mass of 0.05 keV and lower.
• Figure 4: Fits of Galactic circular velocities using the Einasto model and the posterior distributions of fitting parameters without implementing adiabatic halo contraction using 12 baryonic models.
• Figure 5: Fits of Galactic circular velocities using the Einasto model and the posterior distributions of fitting parameters implementing adiabatic halo contraction using 12 baryonic models.