The DORCHA suite: nature, nurture, and the phase space distribution of the Milky Way's high redshift progenitors today

TL;DR

This study uses high-resolution simulations to predict that the oldest stars from the Milky Way's early universe are concentrated in the inner regions, with distinctive kinematic signatures useful for Galactic Archaeology.

Contribution

The paper introduces the DORCHA suite of simulations and demonstrates that ancient stellar remnants are centrally concentrated with specific dynamical properties, robust across different galaxy environments.

Findings

Oldest stars are mostly within 15 kpc of the Galactic center.

These stars exhibit steep density profiles and radially biased orbits.

Their dynamical signatures are consistent across diverse galaxy formation histories.

Abstract

Where in the present-day Milky Way should we search for the remnants of its earliest stars? We address this question using the DORCHA (Gaelic for Dark; DUR-uh-khuh) suite: a set of 25 high-resolution, dark-matter-only cosmological zoom-in simulations of Milky Way analogue (MWA) haloes evolved to $z=0$. Of these, 15 are isolated and the rest are in pairs, similar to the MW and M31. By identifying and tagging the most bound material in high-redshift ($z\geq5$) progenitor haloes -- those likely to host early star formation -- we track the present-day phase-space distribution of this ancient component. We find that this material is highly centrally concentrated at $z=0$, with 90 -- 100 per cent residing within $r \lesssim 15\,h^{-1}\,\mathrm{kpc}$. It exhibits steep density profiles ($\rho\propto\,r^{-4}$), low velocity dispersions ($\sigma_r / \sigma_{\rm max} \lesssim 0.6$), and radially biased orbits ($\beta \gtrsim 0.5$ for $r \gtrsim 0.1\,R_{200}$), consistent with a relaxed, centrally embedded population. These results hold across haloes with diverse formation histories and environments, suggesting that the dynamical signature of early progenitors is robust to later mergers and interactions. Our findings imply that the fossil record of the first generations of stars -- including Population III and extremely metal-poor stars -- should be sought in the innermost regions of the Milky Way, where they retain distinctive kinematic imprints. While these stellar populations may overlap, we caution that low metallicity does not uniquely identify ancient stars, nor vice versa. The DORCHA suite thus provides a physically motivated baseline for interpreting observations from Galactic Archaeology surveys targeting the bulge and inner halo.