Icarus Revisited: An Ancient, Metal-poor Accreted Stellar Stream in the Disk of the Milky Way

Abstract

The search for accreted satellites in the Galactic disk is a challenging task, to which Gaia plays a crucial role in synergy with ground-based spectroscopic surveys. In 2021, P. Re Fiorentin et al. discovered five substructures with disk kinematics including Icarus. To gain more insight into the origin of Icarus as a remnant of a dwarf galaxy rather than a signature of secular processes of disk formation, we complement astrometric Gaia DR3 data with spectroscopy from APOGEE DR17 and GALAH DR3, and explore the chemodynamical distributions within 3 kpc of the Sun. We select 622 stars in the accreted/unevolved regions of [Mg/Mn]–[Al/Fe] and [Mg/Fe]–[Fe/H], where we identify 81 and 376 stars with ‑2 < [Fe/H] < ‑0.7 belonging to Icarus and Gaia-Sausage-Enceladus (GSE), respectively. The revised properties of Icarus are: <V + V _LSR> ≃ 171 km s^‑1, σ _V ≃ 37 km s^‑1, <e> ≃ 0.36, <[Fe/H]> ≃ ‑1.35, <[Mg/Fe]> ≃ +0.27, <[Al/Fe]> ≃ ‑0.13, and <[Mn/Fe]> ≃ ‑0.39. From the color–magnitude diagram of its members, Icarus appears older than 12 Gyr. Such age and dynamical properties are reminiscent of the metal-weak thick disk. However, detailed chemical analysis in the diagnostic spaces [Ni/Fe]–[(C+N)/O], [Y/Eu]–[Fe/H], [Eu/Mg]–[Fe/H], [Ba/Y]–[Fe/H], and [Ba/Mg]–[Mg/H] evidences that Icarus and GSE occupy the accreted region, well separated from the bulk of in situ disk stars. Updated comparisons with N-body simulations confirm that Icarus's stars are consistent with the debris of a dwarf galaxy with a stellar mass of ∼10⁹ M _☉ accreted onto a primordial disk on an initial prograde low-inclination orbit.