Antaeus: a retrograde group of tidal debris in the Milky Way's disk plane

TL;DR

This paper reports the discovery of a wide, retrograde stellar group in the Milky Way's disk, likely a remnant of a disrupted dwarf galaxy, with unique dynamical properties suggesting a connection to the Sequoia merger event.

Contribution

The study identifies a new retrograde stellar group using Gaia DR3 data and characterizes its properties, proposing it as a remnant of a dwarf galaxy related to the Sequoia event.

Findings

The group is wide, at least 1.5 kpc, with a metallicity spread indicating an ancient origin.

It exhibits highly correlated motions, suggesting it is not phase mixed.

It has extremely low vertical action, possibly representing the dense core of its progenitor.

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 \textit{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 $\sim 12$ Gyr, $\langle {\rm [Fe/H]} \rangle \sim -1.74$). 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 amongst Sequoia dynamical groups. As the low $J_z$ may be attributable to dynamical friction, we speculate that the these stars may be the remnants of the dense core of the Sequoia progenitor.