Pickles on <scp>fire</scp> : the 3D shape evolution of simulated Milky Way-mass galaxies

ABSTRACT We use reduced-mass eigentensors to quantify the 3D ellipsoidal shape evolution of 13 Milky Way-mass galaxies simulated using zoom simulations with fire-2 physics; all but one form discs at $z=0$. We find that all of our Milky Way progenitors go through phases when they are elongated. They often oscillate between spheroidal and elongated shapes in the early Universe over billion-year time-scales, with $\sim 25-45~{{\ \rm per\ cent}}$ of the population having elongated luminosity-weighted shapes at any given time at $z = 0.5-8.5$. In contrast, all stellar populations in our $z=0$ Milky Way analogues are symmetric about their minor axes at $z=0$, even though the old and intermediate-age stellar populations were often arranged in the shape of elongated pickles or triaxial spheroids at the time they formed meaning these populations changed shape significantly over time. During their transient elongated phases, our galaxies have anisotropic velocity dispersion ellipsoids directed along their spatial major axis; however, their shapes do not correlate with their dark matter fraction nor with the shapes and orientations of their underlying dark matter haloes. We find that when treated as a population, the fraction of our galaxy progenitors that are elongated at $z\gt 0.5$ is roughly consistent with what is observed for systems of the same mass and redshift. Our results suggest that observed elongated galaxies seen in the early Universe with James Webb Space Telescope and Hubble Space Telescope are not stable structures, but rather transitory phases that are nevertheless statistically common. Some of these observed objects may evolve into Milky Way-like galaxies at $z=0$.