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

TL;DR

This paper reconstructs the primordial dark matter halo of the Milky Way using Gaia data, revealing an unusual core and steep outer decline inconsistent with standard cold dark matter models.

Contribution

It introduces a novel method to reverse halo contraction effects, deriving primordial halos from observed galaxy data, applied here to the Milky Way.

Findings

Primordial Milky Way halo has a shallow core and steep outer decline.

Observed halo parameters differ significantly from cold dark matter predictions.

Gaia data suggests the Milky Way's halo is atypical compared to standard models.

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$\sigma$ 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.