Early r-process Enrichment and Hierarchical Assembly Across the Sagittarius Dwarf Galaxy

TL;DR

This study analyzes the chemical compositions of 37 stars in the Sagittarius dwarf galaxy, revealing early heavy element formation and hierarchical assembly, with a focus on metal-poor stars and r-process enrichment.

Contribution

It provides the first high-resolution spectroscopic analysis of a significantly increased sample of extremely metal-poor stars in Sgr, highlighting early nucleosynthesis and galaxy assembly.

Findings

Identification of 10 EMP stars in Sgr, increasing known samples fivefold.

No significant difference in element patterns between core and stream stars.

Evidence of early r-process enrichment in Sgr progenitor.

Abstract

Dwarf galaxies like Sagittarius (Sgr) provide a unique window into the early stages of galactic chemical evolution, particularly through their metal-poor stars. By studying the chemical abundances of stars in the Sgr core and tidal streams, we can gain insights into the assembly history of this galaxy and its early heavy element nucleosynthesis processes. We efficiently selected extremely metal-poor candidates in the core and streams for high-resolution spectroscopic analysis using metallicity-sensitive photometry from SkyMapper DR2, and Gaia DR3 XP spectra and proper motions. This allowed us to obtain a high-purity selection of Sgr members based on stellar kinematics while reducing the chances of potential contamination from the Milky Way halo. We present a sample of 37 Sgr stars with detailed chemical abundances, of which we identify 10 extremely metal-poor (EMP; $\rm{[Fe/H]} \le -3.0$) stars, 25 very metal-poor (VMP; $\rm{[Fe/H]} \le -2.0$) stars, and 2 metal-poor (MP; $\rm{[Fe/H]} \le -1.0$) stars. This sample increases the number of extremely metal-poor Sgr stars analyzed with high-resolution spectroscopy by a factor of five. Of these stars, 15 are identified as members of the Sgr tidal stream, while the remaining 22 are associated with the core. We derive abundances for up to 20 elements and identify no statistically significant differences between the element abundance patterns across the core and stream samples. Intriguingly, we identify stars that may have formed in ultra-faint dwarf galaxies that accreted onto Sgr, in addition to patterns of C and r-process elements distinct from the Milky Way halo. Over half of the sample shows a neutron-capture element abundance pattern consistent with the scaled solar pure r-process pattern, indicating early r-process enrichment in the Sgr progenitor.