Typhon: A Polar Stream from the Outer Halo Raining through the Solar Neighborhood

We report on the discovery in the Gaia DR3 astrometric and spectroscopic catalog of a new polar stream that is found as an overdensity in action space. This structure is unique as it has an extremely large apocenter distance, reaching beyond 100 kpc, and yet is detected as a coherent moving structure in the solar neighborhood with a width of ∼4 kpc. A subsample of these stars that was fortuitously observed by LAMOST has a mean spectroscopic metallicity of $\langle [\mathrm{Fe}/{\rm{H}}]\rangle =-{1.60}_{-0.16}^{+0.15}$ dex and possesses a resolved metallicity dispersion of $\sigma ([\mathrm{Fe}/{\rm{H}}])={0.32}_{-0.06}^{+0.17}$ dex. The physical width of the stream, the metallicity dispersion, and the vertical action spread indicate that the progenitor was a dwarf galaxy. The existence of such a coherent and highly radial structure at their pericenters in the vicinity of the Sun suggests that many other dwarf galaxy fragments may be lurking in the outer halo.