Typhon: a polar stream from the outer halo raining through the Solar neighborhood

TL;DR

This paper reports the discovery of a unique polar stellar stream in Gaia DR3 data, originating from a dwarf galaxy, with a large apocenter and detectable in the Solar neighborhood, revealing insights into the outer halo's structure.

Contribution

The discovery of a new polar stream with an extremely large apocenter and its characterization as a dwarf galaxy remnant is a novel addition to halo substructure studies.

Findings

Stream has an apocenter beyond 100 kpc.

Metallicity of the stream's stars is around -1.6 dex.

The progenitor was a dwarf galaxy.

Abstract

We report on the discovery in the Gaia DR3 astrometric and spectroscopic catalog of a new polar stream that is found as an over-density 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 $\sim 4$ kpc. A sub-sample of these stars that was fortuitously observed by LAMOST has a mean spectroscopic metallicity of $\langle {\rm [Fe/H]}\rangle = -1.60^{+0.15}_{-0.16}$ dex and possesses a resolved metallicity dispersion of $\sigma({\rm [Fe/H]}) = 0.32^{+0.17}_{-0.06}$ 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.