Mainly on the Plane: Observing the Extended, Ionized Disks of Milky Way Analogs in IllustrisTNG

Abstract This paper explores the extent to which the circumgalactic medium (CGM) of Milky Way (MW)-like galaxies is located in an extended, ionized, disklike structure. To test this hypothesis, we analyze the spatial and kinematic distributions of different ion species within a sample of MW-like systems in IllustrisTNG. We model commonly observed ions (H I , Mg II , Si IV , C IV , and O VI ) and calculate (1) their angular momentum misalignment from the star-forming disk ( θ ) and (2) the fraction of absorption consistent with galaxy rotation ( f EWcorot ). We find that 63% of Mg ii , 45% of Si iv , 38% of C iv , and 35% of O vi mass along the major axis have kinematics aligned with the galaxy angular momentum axis. We extend this to a mock absorption line survey and quantify f EWcorot . We find that f EWcorot (Mg ii ) ∼ 80% and f EWcorot (O vi ) ∼ 60% at ∼0.5 R 200c , in agreement with recent observational work. We find that in the typical MW analog, there is evidence of cool–warm material in an extended, corotating structure, regardless of whether the angular momentum or observational definition is used. Hence, we expect that the typical MW CGM, especially in the low ions, should be mainly on the plane.