Icarus: A Flat and Fast Prograde Stellar Stream in the Milky Way Disk

We explore the local volume of the Milky Way via chemical and kinematical measurements from high-quality astrometric and spectroscopic data recently released by the Gaia, APOGEE, and GALAH programs. We chemically select 1137 stars up to 2.5 kpc of the Sun and [Fe/H] ≤ -1.0 dex, and find evidence of statistically significant substructures. Clustering analysis in velocity space classifies 163 objects into eight kinematical groups, whose origin is further investigated with high-resolution N-body numerical simulations of single merging events. The two retrograde groups appear associated with Gaia-Sausage-Enceladus (GSE), while the slightly prograde group could be connected to GSE or possibly Wukong. We find evidence of a new 44-member-strong prograde stream that we name Icarus; to our knowledge, Icarus is the fast-rotating stream closest to the Galactic disk to date ( $\langle {Z}_{\max }\rangle \lesssim 0.5\,\mathrm{kpc}$ , ⟨V + V_LSR⟩ ≃ 231 km s^-1). Its peculiar chemical (⟨[Fe/H]⟩ ≃ -1.45, ⟨[Mg/Fe]⟩ ≃ -0.02) and dynamical (mean eccentricity ≃ 0.11) properties are consistent with the accretion of debris from a dwarf galaxy progenitor with a stellar mass of ∼10⁹M_⊙ on an initial prograde low-inclination orbit, ∼10°. The remaining prograde groups are either streams previously released by the same progenitor of Icarus (or Nyx), or remnants from different satellites accreted on initial orbits at higher inclination.