Letter to the Editor

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

¹ School of Astronomy and Space Science, Nanjing University, Nanjing, Jiangsu 210023, China
² Department of Astronomy, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106, USA
³ Leibniz-Institute for Astrophysics, An der Sternwarte 16, 14482 Potsdam, Germany
⁴ Observatoire de Paris, Université PSL, CNRS, Place Jules Janssen, Meudon 92195, France
⁵ Steward Observatory, University of Arizona, 933 N Cherry Ave, Tucson, AZ 85722, USA

^⋆ Corresponding author: pli@nju.edu.cn

Received: 28 August 2025
Accepted: 3 November 2025

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 3σ 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.