Identification of a dwarf galaxy stream in Gaia, and its possible association with the vast polar structure

Low-surface-density streams are important tracers to study the formation and evolution of the Milky Way. Using the accurate astrometric measurements from Gaia mission, we discover a low-surface-density stream in the northern hemisphere, with a length of ∼110 degree and width of 1.23 kpc. The vertical velocity dispersion perpendicular to the stream is around 22.4 km s −1 . The spectral data of member candidate stars from LAMOST and DESI shows a large metallicity range from −1.8 to −0.7. Based on those properties, we claim that the stream originated from a dwarf galaxy. The median metallicity of [Fe/H] = −1.3 indicates a massive dwarf galaxy origination with a stellar mass of around 2.0 × 10 7 M ⊙ , which is comparable with the Fornax dwarf galaxy and smaller than Large and Small Magellanic Clouds (LMC and SMC) and Sagittarius. We also find the globular cluster Pyxis is highly associated with the stream in the phase space E − L Z and in its metallicity. The massive progenitor also suggests that many dwarf galaxies, including massive ones, have been disrupted during their evolution orbiting the Milky Way and left with very low-surface-density structures. This is important for our understanding of the ‘missing satellites’ problem. The orbit information of the stream shows a tight association between its progenitor and the Vast POlar Structure (VPOS), which indicates that the satellites fell into the Milky Way in groups, bringing many globular clusters into the Milky Way.