Antaeus: A Retrograde Group of Tidal Debris in the Milky Way’s Disk Plane

Abstract

We present the discovery of a wide retrograde moving group in the disk plane of the Milky Way using action-angle coordinates derived from the Gaia DR3 catalog. The structure is identified from a sample of its members that are currently almost at the pericenter of their orbit and are passing through the solar neighborhood. The motions of the stars in this group are highly correlated, indicating that the system is probably not phase mixed. With a width of at least 1.5 kpc and with a probable intrinsic spread in metallicity, this structure is most likely the wide remnant of a tidal stream of a disrupted ancient dwarf galaxy (age ∼12 Gyr, 〈[Fe/H]〉 ∼ −1.74). The structure presents many similarities (e.g., in energy, angular momentum, metallicity, and eccentricity) with the Sequoia merging event. However, it possesses extremely low vertical action J z , which makes it unique even among Sequoia dynamical groups. As the low J z may be attributable to dynamical friction, we speculate that these stars may be the remnants of the dense core of the Sequoia progenitor.

Gastro Library. I. The Simulated Chemodynamical Properties of Several Gaia–Sausage–Enceladus-like Stellar Halos

“…We also note that the large spread in the chemical space is expected for relatively large dwarfs (see, e.g., Tolstoy et al 2009, for a review). Thus, our results suggest that Sequoia, Arjuna, I'itoi, and other retrograde structures with these characteristics (e.g., Dodd et al 2022;Oria et al 2022) are naturally explained as stars originating from the outskirts of the GSE satellite. This was shown in Koppelman et al (2020) and Naidu et al (2021) with N-body-only models; here we have demonstrated the same result with models where the satellite (and the disk) has a self-consistent metal enrichment.…”

Gaia DR3 view of dynamical substructure in the stellar halo near the Sun

“…This cluster has been identified before by Ruiz-Lara et al (2022, as cluster 64, see their Fig. 11, and re-discovered by Oria et al 2022), where it was argued to be independent given Sequoia's estimated mass (from its mean metallicity), which would be inconsistent with such a broad extent in the IoM space (Koppelman et al 2019). The three stars with APOGEE abundances have lower [Mg/Fe] than Gaia-Enceladus.…”

A Ghost in Boötes: The Least-Luminous Disrupted Dwarf Galaxy

“…Our simple Monte Carlo trial of the H3 selection function implies that dozens more diffuse systems like Specter could reside at similar distances, and hundreds more might lie undiscovered throughout the Galaxy. This is further supported by recent discoveries of extremely diffuse debris using 6D kinematics (Oria et al 2022;Tenachi et al 2022). Although most streams discovered by kinematic techniques do not have formal luminosity estimates, several are likely as ephemeral as Specter (e.g., Ibata et al 2019;Ji et al 2022;.…”

“…Recent works, armed with proper motions from Gaia (Brown et al 2021;Lindegren et al 2021), have pushed the SB frontier by employing kinematic filtering to peer through the Galactic foreground and detect more diffuse intact dwarfs (Torrealba et al 2019). Incorporating kinematic information has likewise pushed the frontier to detect fainter and more diffuse stellar streams (e.g., Malhan & Ibata 2018;Ibata et al 2019;Yuan et al 2020;Oria et al 2022;Tenachi et al 2022). The bestcharacterized streams with dwarf progenitors have typical luminosities M V ∼ −5.5, whereas those with globular cluster (GC) progenitors tend to be narrower and have M V ∼ −3.5 (Shipp et al 2018;.…”