Kavli Affiliate: Andrey Kravtsov
| First 5 Authors: Adam M. Dillamore, Vasily Belokurov, Andrey Kravtsov, Andreea S. Font,
| Summary:
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 ($M_mathrm{200c}
approx2times10^{11}M_odot$ at $t_L=12$ Gyr). We find that discs form when
the halo’s virial mass reaches a threshold of
$M_mathrm{200c}approx(6pm3)times10^{11}M_odot$, 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.
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