Icarus: a Flat and Fast Prograde Stellar Stream in the Milky Way disk

TL;DR

This paper identifies and characterizes a new prograde stellar stream named Icarus in the Milky Way, revealing its origin from a dwarf galaxy and its relation to other known structures through chemical and kinematic analysis.

Contribution

The discovery and detailed analysis of the Icarus stream, including its chemical and dynamical properties, and its association with dwarf galaxy accretion events.

Findings

Icarus is a fast-rotating, low-inclination prograde stream close to the Galactic disk.

Icarus likely originated from a dwarf galaxy with a stellar mass of about 10^9 solar masses.

Icarus's properties suggest an accretion event from a low-inclination orbit.

Abstract

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 $\rm{[Fe/H]} \le -1.0$~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, while the slightly prograde group could be connected to GSE or possibly Wukong. We find evidence of a new 44-member-strong prograde stream we name Icarus; to our knowledge, Icarus is the fast-rotating stream closest to the Galactic disk to date ($\langle Z_{\rm max} \rangle \lesssim 0.5$~kpc, $\langle V+V_{\rm{LSR}}\rangle \simeq 231~\rm{km~s^{-1}}$). Its peculiar chemical ($\langle \rm{[Fe/H]}\rangle \simeq -1.45$, $\langle \rm{[Mg/Fe]}\rangle \simeq -0.02$) and dynamical (mean eccentricity $\simeq 0.11$) properties are consistent with the accretion of debris from a dwarf galaxy progenitor with a stellar mass of $\sim 10^9 M_\sun$ on an initial prograde low-inclination orbit, $\sim 10^\circ$. 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.