HR-GO II: chemical abundances of low-$E$ retrograde dynamically-tagged-groups: Revealing Thamnos as a very metal-poor substructure

TL;DR

This study tests whether the low-energy, retrograde dynamical groups Rg8 and Rg9 are chemically connected to the Thamnos halo structure by obtaining high-resolution chemical abundances for 35 stars. Using LTE MOOG analysis with MARCS models and spectral synthesis, the authors find Rg8 and Rg9 are chemically indistinguishable and largely align with Thamnos in dynamical space, with a bimodal MDF peaking at $[Fe/H]\approx-2.1$ and $-1.5$, the former dominating the sample. The lack of an $\alpha$-knee and the abundance patterns across C to Eu, including flat $[Eu/Mg]$ and rising $[Ba/Eu]$, point to a short, early star-formation history consistent with a low-mass, accreted dwarf galaxy. The results strongly support Thamnos as a genuine ex-situ relic with a progenitor mass around $10^6\,M_\odot$ and early accretion, offering a parallel to Cetus and implications for MW assembly. Data and abundances are publicly available for reproducibility and broad use in Galactic archaeology.

Abstract

Milky Way halo substructures identified in dynamical space are known to suffer from contamination from the Milky Way in-situ stars, which makes their accreted origins uncertain. We present detailed chemical abundances of 35 stars belonging to two sets of dynamically tagged groups, Rg8 and Rg9, to investigate their accreted nature. Both groups are composed of stars with low orbital energy and very retrograde orbits. We find that Rg8 and Rg9 are chemically indistinguishable across all elements, from C to Eu, strongly indicating that they belong to the same structure. The iron-abundance distribution of this low-$E$ retrograde group has a prominent peak at [Fe/H] $\approx-2.1$, revealing that its main population is very metal-poor, and a secondary peak at [Fe/H] $\approx-1.5$, very likely due to contamination from Milky Way in-situ stars. These groups also heavily overlap with the Thamnos substructure in dynamical space, and we thus use them to investigate the chemical properties of Thamnos. The dominant, low-metallicity population provides strong evidence for the ex-situ origin of Thamnos, as well as its very metal-poor nature. We do not see any evidence of an $α$ knee in our sample, which is consistent with previous studies. Comparison with the Cetus-Palca stream in the chemical space shows similar abundance distributions, and thus it suggests that the Thamnos progenitor dwarf galaxy had a truncated star formation history due to its early merger with the Milky Way.

Figures (8)

• Figure 1: Distributions of the Rg8 and Rg9 stars in energy and action space, along with stars used for comparison. The gray dots represent the halo sample from the LAMOST DR3 VMP catalogue Li+2018ApJS..238...16L, updated with Gaia DR3 kinematics. Purple diamonds are Thamnos members from Dodd+2023AA...670L...2D, shown as a reference sample. The two groups studied in this paper are represented by orange filled circles (Rg8) and green filled circles (Rg9). Additionally, black open circles denote specific stars selected from the LAMOST VMP catalog, and the black open diamonds mark low-Mg outliers.
• Figure 2: Comparison of our EW measurements with those of Li+2022 for three stars in common (each row corresponds to one star, as labelled). The average logarithmic difference of equivalent widths and the standard deviation are presented in the right panels. The results show no significant offsets, confirming the consistency of our EW measurements.
• Figure 3: Example of the spectral fitting of the CH 4323 Å line for Rg9_17 with LTE models. The observed spectrum is represented with black dots and the best-fitting result, with $A$(C) = 6.14, is shown as a solid line with a shaded uncertainty region.
• Figure 4: Example of the spectral fitting of the Eu 4205 Å line for Rg9_19 with LTE models. Symbols are the same as in Figure \ref{['CH']}, with the best-fitting result having $A$(Eu) = $-0.84\pm0.12$.
• Figure 5: Chemical abundance ratios [X/Fe] as a function of metallicity [Fe/H] for our sample and comparison stars. Our sample stars are shown as orange (Rg8) and green (Rg9) circles, with outliers indicated by empty diamonds of the same colors. Literature comparison samples include blue triangles (Cetus) and gray dots (R14).