The origin of the diverse morphologies and kinematics of Milky Way-mass galaxies in the FIRE-2 simulations

TL;DR

This study uses FIRE-2 simulations to investigate the diverse shapes and motions of Milky Way-mass galaxies, revealing that gas history and halo spin at star formation are key factors influencing galaxy morphology and kinematics.

Contribution

It identifies the primary role of halo gas spin at star formation in determining galaxy morphology, highlighting the importance of gaseous history over dark matter halo properties.

Findings

Gas always forms a thin, rotation-supported disk at z=0.

Stellar morphology correlates more with gaseous history than dark matter halo properties.

Most stars in MW-mass galaxies form in a disk, even in bulge-dominated systems.

Abstract

We use hydrodynamic cosmological zoom-in simulations from the FIRE project to explore the morphologies and kinematics of fifteen Milky Way (MW)-mass galaxies. Our sample ranges from compact, bulge-dominated systems with 90% of their stellar mass within 2.5 kpc to well-ordered disks that reach $\gtrsim15$ kpc. The gas in our galaxies always forms a thin, rotation-supported disk at $z=0$, with sizes primarily determined by the gas mass. For stars, we quantify kinematics and morphology both via the fraction of stars on disk-like orbits and with the radial extent of the stellar disk. In this mass range, stellar morphology and kinematics are poorly correlated with the properties of the halo available from dark matter-only simulations (halo merger history, spin, or formation time). They more strongly correlate with the gaseous histories of the galaxies: those that maintain a high gas mass in the disk after $z\sim1$ develop well-ordered stellar disks. The best predictor of morphology we identify is the spin of the gas in the halo at the time the galaxy formed 1/2 of its stars (i.e. the gas that builds the galaxy). High-$z$ mergers, before a hot halo emerges, produce some of the most massive bulges in the sample (from compact disks in gas-rich mergers), while later-forming bulges typically originate from internal processes, as satellites are stripped of gas before the galaxies merge. Moreover, most stars in $z=0$ MW-mass galaxies (even $z=0$ bulge stars) form in a disk: $\gtrsim$60-90% of stars begin their lives rotationally supported.