The Milky Way Halo in Action Space

Abstract: We analyse the structure of the local stellar halo of the Milky Way using $\sim$ 60000 stars with full phase space coordinates extracted from the SDSS--{\it Gaia} catalogue. We display stars in action space as a function of metallicity in a realistic axisymmetric potential for the Milky Way Galaxy. The metal-rich population is more distended towards high radial action $J_R$ as compared to azimuthal or vertical action, $J_\phi$ or $J_z$. It has a mild prograde rotation $(\langle v_\phi \rangle \approx 25$ km s$^{-1}$), is radially anisotropic and highly flattened with axis ratio $q \approx 0.6 - 0.7$. The metal-poor population is more evenly distributed in all three actions. It has larger prograde rotation $(\langle v_\phi \rangle \approx 50$ km s$^{-1}$), a mild radial anisotropy and a roundish morphology ($q\approx 0.9$). We identify two further components of the halo in action space. There is a high energy, retrograde component that is only present in the metal-rich stars. This is suggestive of an origin in a retrograde encounter, possibly the one that created the stripped dwarf galaxy nucleus, $\omega$Centauri. Also visible as a distinct entity in action space is a resonant component, which is flattened and prograde. It extends over a range of metallicities down to [Fe/H] $\approx -3$. It has a net outward radial velocity $\langle v_R \rangle \approx 12$ km s$^{-1}$ within the Solar circle at $|z| <3.5$ kpc. The existence of resonant stars at such extremely low metallicities has not been seen before.

The Milky Way Halo in Action Space: An Analytical Perspective

This study by Myeong et al. provides a thorough investigation into the structure of the local stellar halo of the Milky Way, primarily utilizing a dataset of approximately 60,000 stars derived from the SDSS--Gaia catalogue. Using a realistic axisymmetric potential for the galaxy, the researchers plotted these celestial objects in action space, distinguishing them by metallicity. Their findings show multiple components within the stellar halo, offering both nuanced insights into its architecture and potential implications for understanding galactic evolution.

The halo is stratified primarily into two populations based on metallicity. The metal-rich stars are characterized by a significant dispersion toward high radial action ($J_R$), mild prograde rotation ($\langle v_\phi \rangle \approx 25$ km/s), and a flattened morphology with axis ratios ($q \approx 0.6 - 0.7$). Conversely, the metal-poor stars exhibit broader distribution across radial, azimuthal, and vertical actions, suggestive of a spherical morphology ($q\approx 0.9$) and larger prograde rotation ($\langle v_\phi \rangle \approx 50$ km/s).

Perhaps most intriguingly, two additional components of the halo are discerned. First is the high-energy, retrograde component, limited to metal-rich stars, hypothesized to result from a retrograde merger, potentially related to the disruption of a dwarf galaxy wherein $\omega$Centauri originated. Secondly, the paper identifies a resonant component, observable across a wide range of metallicities, down to extremely low [Fe/H] values. These resonant stars exhibit both flattened structure and outward radial velocity ($\langle v_R \rangle \approx 12$ km/s) within the Solar circle, hinting at dynamical resonances akin to those found in disk stars forming the Hercules stream.

Implications and Future Prospects

The findings contribute significant implications for both theoretical and practical astrophysical research. The separation into distinct metallic populations offers a clearer understanding of the Milky Way's assembly history and halo dynamics. The identification of the retrograde component connected to $\omega$Centauri underlines the importance of galactic mergers in shaping halo structures, aligning with traditional models of galaxy formation. Meanwhile, the resonant component, especially within metal-poor stars, advances existing theories on galactic resonance impacts beyond disk star populations.

Future research directions are abundant; forthcoming Gaia data releases, alongside advanced spectroscopic surveys, will further refine our understanding of halo substructures, particularly regarding their dynamic origins and distributions. The precision and expansion of datasets will facilitate deeper exploration into the interactions between galactic dynamics and stellar populations, highlighting potential synergies with cosmological models and simulations.

Overall, Myeong et al.'s study enriches our comprehension of the Milky Way halo through a robust framework in action space, promising valuable pathways for continued exploration and theoretical advancement in galactic dynamics.