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, 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 chemo-dynamical 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: $⟨V+V_{\mathrm{LSR}}⟩\simeq 171{\mathrm{km}\mathrm{s}}^{-1}$, ${\sigma }_V\simeq 37{\mathrm{km}\mathrm{s}}^{-1}$, $⟨e⟩\simeq 0.36$, $⟨[Fe/H]⟩\simeq -1.35$, $⟨[Mg/Fe]⟩\simeq +0.27$, $⟨[Al/Fe]⟩\simeq -0.13$, and $⟨[Mn/Fe]⟩\simeq -0.39$. From the CMD 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' stars are consistent with the debris of a dwarf galaxy with a stellar mass of $\sim 10^9{M}_{\odot }$ accreted onto a primordial disk on an initial prograde low-inclination orbit.

18 pages, 8 figures, 3 Tables. Accepted for publication into The Astrophysical Journal