Understanding the early stages of galaxy formation using very metal-poor stars from the Hamburg/ESO survey

TL;DR

This study investigates very metal-poor stars in the Milky Way, revealing their origins through chemo-dynamical analysis and simulations, highlighting early galaxy assembly processes and star formation mechanisms.

Contribution

It provides new insights into the formation of very metal-poor stars, distinguishing between accretion and in-situ formation, and links their properties to early galaxy assembly.

Findings

Disk and halo VMP stars have distinct origins.

Accretion from early satellites is the main formation mechanism.

Most old, metal-poor stars are alpha-enriched and very ancient.

Abstract

We explore the chemo-dynamical properties of a sample of very metal-poor (VMP) stars selected from the Hamburg/ESO survey, matched with Gaia EDR3, in the phase-space identified by the three integrals of motion ($L_z$, $E$, $I_3$). Disk and halo orbits are separated by using the criteria defined in Carollo et al. (2021). We found 26 stars with $[Fe/H] \leq -2.5$ possessing disk kinematics, of which 14 are extremely metal-poor. At these metallicities, the number of stars with disk kinematics is three times its retrograde counterpart. In the same range of metallicity we also identified 37 halo stars most tightly bound to the gravitational potential of the progenitor halo. The origin of these stars are investigated by comparing the observational results with simulated galaxies from the Aquarius Project and the IllustrisTNG simulations. We found two mechanisms of formation of VMP stars with disk kinematics: accretion from early satellites (which is dominant), and {\it in-situ} formation. These stars are very old, with ages > 12.5 Gyr ($z$ > 5), and they are $\alpha$-enriched. Accretion and {\it in-situ} formation are also found for the retrograde counterparts with being accretion also the dominant mode. Contributing accreted satellites have stellar masses in the range $[10^{6}-10^9]$ M_sun, and are very gas-rich. The most bound halo stars are the oldest detected with a median age of ~ 13.3 Gyr ($z$ ~ 11), and $\alpha$-enriched. Our finding clearly show that very old, very metal-poor stars store important information on the first stages of assembly of our Galaxy and its halo.