Where are the Population III star relics in the simulated Milky Way analogues?

TL;DR

This study investigates the present-day distribution and kinematics of Population III relics (PSR) in six Milky Way–like halos from the Auriga simulations, using metal-free star particles as PSR tracers and a four-way orbital decomposition. PSR form over $z \in [22,4]$ with a mean around $z\sim10$, comprising about $2\times10^{-5}$ of the stellar population, and they are largely halo-dominated, with the outer halo ($r>30$ kpc) hosting about $46$–$49\%$ of PSR. About $18\%$ of PSR reside in satellites, with the fraction in a given satellite correlating with its stellar mass; some low-mass satellites can be entirely PSR-rich while others lack PSR due to environmental effects. The results show good numerical convergence between the two resolutions, providing robust guidance for observationally searching for PSR, particularly in the outer halo and in low-mass, early-formed satellites, with implications for interpreting upcoming stellar archaeology surveys.

Abstract

Using 6 Milky Way analogues with two different numerical resolutions from the Auriga simulation, we investigate the total mass, spatial distribution and kinematics of the Population III star relics in the Milky Way analogues at $z=0$. These relics (primarily second generation stars) formed over a wide redshift range, from about $z=22$ to $z=4$, with an average formation redshift of $z \sim 10.0$, and comprise about $2\times10^{-5}$ of the entire galactic stellar population. The disk and bulge components host only a small fraction of these relics, contributing less than $12$ percent in total. The stellar halo, in particular the outer stellar halo of which galactic radius $r>30$ kpc, hosts the largest fraction (about 46 percent on average), with an average of one relic star for per $4,000$ to $10,000$ stars, making it a promising region for observational searches. Additionally, around $18$ percent of the Population III star relics are found in satellite galaxies, with smaller and older satellite galaxies tending to contain a higher proportion of these stars. Thus, low-mass and early-formed satellite galaxies are also ideal targets for finding such relics, although some satellite galaxies may lack them entirely. The spatial distribution and kinematics of these stars show good numerical convergence across different simulation resolutions. Our results provide valuable guidance for searches of the Population III star relics and offer insights for interpreting findings from ongoing and future stellar archaeology surveys.

Figures (6)

• Figure 1: The total mass of formed PSR as a function of formation redshift for six Milky Way analogues in the Auriga simulation suites. Results are presented for two different numerical resolutions. The red (blue) lines represent the median values of all six samples, while the shaded regions indicate the 16th–84th percentiles. The vertical lines represent the half-mass formation redshift of the PSR.
• Figure 2: The projected distribution of PSR (red dots) over underlying dark matter density fields of the Au6 (left) and Au16 (right) halos. The results are shown for the level 3 resolution runs. Each panel spans a physical scale of 300 kpc per side.
• Figure 3: The radial distribution of the PSR and all stars in Milky Way analogues at z=0. The bold solid lines represent the average radial distribution of the PSR, while the dashed lines indicate the average distribution of all stars, as specified in the legend. The light-colored lines correspond to results from individual simulated samples.
• Figure 4: The grey 2D histograms depict the orbital distribution of all stars in the central galaxy of the Auriga-L3, while the red dots represent the orbital properties of PSR. The regions outlined by blue lines correspond to different orbital types. The cyan numbers in each region indicate the mass ratio (logarithmic to base 10) of PSR with the corresponding orbital type to all stars, while the red numbers show the mass ratio of PSR with the corresponding orbital type to the total PSR population.
• Figure 5: Same as the content of Fig \ref{['fig:k3']}, but it is the results for Auriga-L4.