FOGGIE: Figuring Out Gas and Galaxies In Enzo. XII. The Formation and Evolution of Extended H <scp>i</scp> Galactic Disks and Warps with a Dynamic Circumgalactic Medium

Abstract Atomic hydrogen (H i ) is an important component of gas in and around galaxies and forms extended disklike structures well beyond the extent of starlight. Here we investigate the properties and evolution of extended H i disks that emerge in six Milky Way–mass galaxies using cosmological zoom-in simulations from the Figuring Out Gas and Galaxies in Enzo (FOGGIE) suite. We focus on the formation, evolution, and morphology of extended gaseous disks that emerge in all six systems. We find that the median H i column densities drop sharply at the disk edge, with the mean column densities outside the disk dominated by dense ( N HI ∼ 10 19 cm −2 ), clumpy structures. All systems have significant misaligned features (warps or polar rings) at some point in their evolution; however, their frequencies, lifetimes, and origins vary significantly. We find that the morphologies of the FOGGIE disks are correlated with properties of their circumgalactic medium (CGM). We place these systems along a continuum based on how populated their CGMs are with H i relative to their central disk. All systems kinematically settle similarly by z = 0. The less populated systems tend to form coherently rotating, thin, extended disks while the more populated systems do not. Location on this continuum is independent of disk and halo mass, implying a relation to local environmental factors. Our results indicate a connection between CGM content and disk formation that is not yet fully understood. A second paper investigates observational aspects of these structures.