The 3D Shape Evolution of Milky Way-Mass Galaxies with FIRE Simulations

We study the evolution of the intrinsic 3D shape of Milky Way-mass disk galaxies through FIRE-2 zoom simulations. Taking advantage of FIRE's ability to resolve down to individual star particles, we employ an iterative reduced mass eigentensor to measure the 3D shapes of galaxies as ellipsoids. We measure each galaxy of the eleven galaxies in our sample from a set of lookback times corresponding to z=7 to z=0, restricting the set of star particles to a radius of 0.1r_vir and only including stars born within 500Myr of each selected lookback time. Measurements of galaxies in their bursty, chaotic high-redshift phase show that their shape transitions back and forth between spheroidal and elongated structures. Also seen observationally, the elongated phase is consistent with the theory that these galaxies form along large-scale dark matter filaments. As the galaxies evolve into their bursty disk and settled disk phases, their geometry evolves correspondingly by a spheroid collapsing into a thick disk and then settling down into a thin disk. We also take an archeological approach where we measure the same stellar populations at z=0 to understand how these shapes relate to back when they were just formed. We find that each stellar population back in time evolves to become circular in the plane of the intermediate and major axes, with older populations forming spheroids from elongated shapes.