The AGORA High-resolution Galaxy Simulations Comparison Project. III:
  Cosmological zoom-in simulation of a Milky Way-mass halo
  We present a suite of high-resolution cosmological zoom-in simulations to
$z=4$ of a $10^{12}\,{\rm M}_{\odot}$ halo at $z=0$, obtained using seven
contemporary astrophysical simulation codes widely used in the numerical galaxy
formation community. Physics prescriptions for gas cooling, heating, and star
formation, are similar to the ones used in our previous {\it AGORA} disk
comparison but now account for the effects of cosmological processes. In this
work, we introduce the most careful comparison yet of galaxy formation
simulations run by different code groups, together with a series of four
calibration steps each of which is designed to reduce the number of tunable
simulation parameters adopted in the final run. After all the participating
code groups successfully completed the calibration steps, we reach a suite of
cosmological simulations with similar mass assembly histories down to $z=4$.
With numerical accuracy that resolves the internal structure of a target halo,
we find that the codes overall agree well with one another in e.g., gas and
stellar properties, but also show differences in e.g., circumgalactic medium
properties. We argue that, if adequately tested in accordance with our proposed
calibration steps and common parameters, the results of high-resolution
cosmological zoom-in simulations can be robust and reproducible. New code
groups are invited to join this comparison by generating equivalent models by
adopting the common initial conditions, the common easy-to-implement physics
package, and the proposed calibration steps. Further analyses of the
simulations presented here will be in forthcoming reports from our
Collaboration.
