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

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 ($ρ\propto\,r^{-4}$), low velocity dispersions ($σ_r / σ_{\rm max} \lesssim 0.6$), and radially biased orbits ($β\gtrsim 0.5$ 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.

Figures (14)

• Figure 1: Isolated $\textsc{Dorcha}$ haloes along with the parent Dorcha_00: The subplots show the total projection along the $z$-direction for our simulations. The top-left larger subplot shows $100~h^{-1}{\rm Mpc}\times100~h^{-1}{\rm Mpc}$ projection of our parent Dorcha_00 simulation, with the locations of the isolated and paired Milky Way Analogues (MWAs) - our $\textsc{Dorcha}$ haloes - indicated by the blue and pink circles respectively. The smaller subplots show $200~h^{-1}{\rm kpc}\times200~h^{-1}{\rm kpc}$ cutouts around the $\textsc{Dorcha}$ haloes from the Dorcha_01-15 simulations. These figures illustrate the diverse nature, in number as well as spatial distribution, of the subhaloes within the $\textsc{Dorcha}$ suite.
• Figure 2: Paired $\textsc{Dorcha}$ haloes: Similar to Figure \ref{['fig:parent1']}, but for the paired $\textsc{Dorcha}$ simulations (Dorcha_15-20). The cutouts show $1~h^{-1}{\rm Mpc}\times1~h^{-1}{\rm Mpc}$ centred at the centre-of-mass of the two haloes in the pair.
• Figure 3: Halo growth histories: We show the mean(black), median(brown), 10-and 90-percentile(red), and individual(light-grey) growth histories of all 25 $\textsc{Dorcha}$ haloes. The inset on the top-left zooms in onto $z\leq6$. Takeaway: Almost all of the haloes have progenitors that are resolved by $z\sim25$ and have formation histories that are relatively smooth.
• Figure 4: Formation redshift, $z_{\rm form}\equiv z_{50}$: Distribution of the redshifts at which 50 per cent of the present day mass of the $\textsc{Dorcha}$ haloes were assembled. See also the last column of Table \ref{['tab:sim_info']}. The box-plot encompasses the 1-$\sigma$ values. Indicated also are the mean (=1.325) in solid and median (=1.33) in dashed vertical lines. Takeaway: The $\textsc{Dorcha}$ haloes exhibit a variety of formation redshifts, enabling us to cleanly separate out the impact of the surroundings (early- and late-forming kinds) and mergers.
• Figure 5: Radial profiles for all $\textsc{Dorcha}$ s: The rows from top to bottoms shows, respectively, the density profile $(\rho)$, mean radial velocity $(\overline{v}_{\rm r})$, radial velocity dispersion $(\sigma_{\rm r})$, velocity anisotropy $(\beta)$, and mean rotational velocity $(\overline{v}_{\rm rot})$. From left to right the columns are for, respectively, all the particles that comprise the halo, the particles that were tagged at high redshifts in different colours, and the particle list for old $(z\geq10)$ and new $(z\leq5)$ particles. Mean(median) trend is shown with solid(dashed) curves. The shaded region for the $\beta$-profiles show the 1-$\sigma$ variation. The particles are binned into 50 radial bins evenly spaced in logarithm between $10^{-3}\leq r/R_{200}\leq 3$. Takeaway: The radial profiles in the 2$^{\rm nd}$-column shows that most of the high-$z$ progenitors are now part of the main subhalo at $z=0$. The earlier a particular set of particles were tagged, the more centrally concentrated they are at $z=0$.