Tracing Thamnos: Chemical Clues to a Very Metal-Poor Galactic Immigrant

The authors set out to clarify the origins of two dynamically tagged groups of stars, called Rg8 and Rg9, that travel on highly retrograde, low-energy orbits around the Milky Way. Such stars are prime candidates for being relics of ancient dwarf galaxies swallowed by the Milky Way. However, past discoveries of similar halo substructures have been complicated by contamination from stars formed inside the Milky Way. In this study, Xie et al. use high-resolution spectroscopy to measure detailed chemical abundances for 35 stars in Rg8 and Rg9, aiming to determine whether these groups represent genuine debris from an accreted system and to evaluate their relationship to a known retrograde structure called Thamnos.

Data Collection and Methods

To carry out this investigation, the team first selected bright candidate stars from previous catalogs and then added new members by searching Gaia’s kinematic data with a clustering algorithm called StarGO. High-resolution spectra were obtained using telescopes in both hemispheres, including Subaru, Magellan, CFHT, and the VLT. From these spectra, the authors derived precise stellar parameters, such as temperature, surface gravity, iron content, and microturbulent velocity, using established photometric and spectroscopic techniques. With these parameters in hand, they measured abundances for elements ranging from carbon to europium, using both equivalent-width measurements and detailed spectral synthesis where necessary.

Chemical Abundances and Group Similarities

Across all elements studied, Rg8 and Rg9 turn out to be chemically indistinguishable. Their carbon abundances, α-element ratios (such as Mg, Si, Ca, and Ti), and iron-peak and neutron-capture elements all follow the same trends, with no meaningful offsets. Both groups show [α/Fe] ≈ 0.4 without evidence for an “α-knee”, a downturn that typically appears when Type Ia supernovae begin contributing iron to a galaxy’s chemical evolution. This plateau suggests that the progenitor of these stars experienced truncated or inefficient star formation, similar to what is observed in low-mass dwarf galaxies such as the Cetus stream, which the authors use for comparison throughout the study.

Metallicity Distribution and Implications

A major result emerges from the metallicity distribution of the combined sample. The stars display a strong peak at [Fe/H] ≈ −2.1, indicating that the primary population is very metal-poor. A secondary peak near [Fe/H] ≈ −1.5 likely reflects contamination from Milky Way “in-situ” stars, perhaps those that were kicked into halo-like orbits during ancient merger events. The dominance of the extremely metal-poor component strengthens the argument that Rg8 and Rg9 trace a true accreted structure rather than a population formed within the Milky Way.

Connection to the Thamnos Substructure

By comparing the groups’ orbital properties with those of the known Thamnos substructure, the authors find that the low-energy retrograde stars strongly overlap with Thamnos. Because the chemical signatures of Rg8 and Rg9 match each other so closely, and because both show a chemical evolution pattern consistent with dwarf-galaxy debris, the authors conclude that these groups represent the metal-poor core of the Thamnos system. Their analysis also suggests that previous samples of Thamnos were heavily contaminated by Milky Way stars, explaining why earlier studies showed a more metal-rich distribution.

Conclusions and Broader Significance

Taken together, the chemical homogeneity, the metallicity distribution, and the orbital characteristics all point to Rg8, Rg9, and Thamnos being different observational glimpses of the same ancient dwarf galaxy that merged with the Milky Way. Its very low metallicity and lack of an α-knee imply that this galaxy was low-mass and experienced limited star formation before being accreted early in the Milky Way’s history. Through high-resolution spectroscopy, this study reinforces the power of chemical tagging in uncovering the fossil record of our Galaxy’s growth.

Source: Xie