Taking the Milky Way for a spin: disc formation in the ARTEMIS simulations

TL;DR

This study uses the ARTEMIS simulations to explore how Milky Way-like galaxies form their discs, revealing that early halo mass assembly and merger history are key factors influencing disc formation timing and properties.

Contribution

The paper provides new insights into the conditions and timing of disc formation in Milky Way-mass galaxies, highlighting the role of early halo mass and merger events.

Findings

Discs form when halo virial mass reaches about 6×10^{11} M_sun.

Galaxies with early disc formation have smaller accreted stellar fractions.

Milky Way assembled mass and formed its disc earlier than typical galaxies of similar mass.

Abstract

We investigate the formation (spin-up) of galactic discs in the ARTEMIS simulations of Milky Way-mass galaxies. In almost all galaxies discs spin up at higher [Fe/H] than the Milky Way (MW). Those that contain an analogue of the Gaia Sausage-Enceladus (GSE) spin up at a lower average metallicity than those without. We identify six galaxies with spin-up metallicity similar to that of the MW, which form their discs $\sim 8-11$ Gyr ago. Five of these experience a merger similar to the GSE. The spin-up times correlate with the halo masses at early times: galaxies with early spin-up have larger virial masses at a lookback time $t_L=12$ Gyr. The fraction of stars accreted from outside the host galaxy is smaller in galaxies with earlier spin-ups. Accreted fractions small enough to be comparable to the MW are only found in galaxies with the earliest disc formation and large initial virial masses ($M_\mathrm{200c} \approx2\times10^{11}M_\odot$ at $t_L=12$ Gyr). We find that discs form when the halo's virial mass reaches a threshold of $M_\mathrm{200c}\approx(6\pm3)\times10^{11}M_\odot$, independent of the spin-up time. However, the failure to form a disc in other galaxies appears to be instead related to mergers at early times. We also find that discs form when the central potential is not particularly steep. Our results indicate that the MW assembled its mass and formed its disc earlier than the average galaxy of a similar mass.