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 (< Z(max)> less than or similar to 0.5 kpc, < V + V-LSR > similar or equal to 231 km s(-1)). Its peculiar chemical ([Fe/H] -1.45, [Mg/Fe] -0.02) and dynamical (mean eccentricity similar or equal to 0.11) properties are consistent with the accretion of debris from a dwarf galaxy progenitor with a stellar mass of similar to 10(9)M(circle dot;) on an initial prograde low-inclination orbit, similar to 10 degrees. 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.