Jormungandr: A Large Substructure Circling the Milky Way Identified from Gaia DR3 Data

Abstract We report the discovery of a large substructure in the Milky Way passing through the solar neighborhood, detected in actions <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mfenced close=")" open="("> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>J</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>ϕ</mml:mi> </mml:mrow> </mml:msub> <mml:mo>,</mml:mo> <mml:msub> <mml:mrow> <mml:mi>J</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">R</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:mfenced> </mml:math> calculated from Gaia DR3 data. We perform extensive tests to evaluate the selection effects present in the sample and conclude that they cannot produce an artifact with the same characteristics. This substructure, which we call Jormungandr, is orbiting the Milky Way with high L Z , higher eccentricity (median ∼0.50) and larger rotation velocity V ϕ compared to disk stars. It leaves obvious patterns in the integrals of motion and kinematics. The chemical abundances from the Apache Point Observatory Galactic Evolution Experiment DR17 for members of Jormungandr resemble those of the Galactic disk high- α sequence. It shows a significant spread in metallicity, as well as age, according to its color–magnitude diagram. Understanding its formation and nature requires further analysis and simulations.