  Using an isolated Milky Way-mass galaxy simulation, we compare results from 9
state-of-the-art gravito-hydrodynamics codes widely used in the numerical
community. We utilize the infrastructure we have built for the AGORA
High-resolution Galaxy Simulations Comparison Project. This includes the common
disk initial conditions, common physics models (e.g., radiative cooling and UV
background by the standardized package Grackle) and common analysis toolkit yt,
all of which are publicly available. Subgrid physics models such as Jeans
pressure floor, star formation, supernova feedback energy, and metal production
are carefully constrained across code platforms. With numerical accuracy that
resolves the disk scale height, we find that the codes overall agree well with
one another in many dimensions including: gas and stellar surface densities,
rotation curves, velocity dispersions, density and temperature distribution
functions, disk vertical heights, stellar clumps, star formation rates, and
Kennicutt-Schmidt relations. Quantities such as velocity dispersions are very
robust (agreement within a few tens of percent at all radii) while measures
like newly-formed stellar clump mass functions show more significant variation
(difference by up to a factor of ~3). Systematic differences exist, for
example, between mesh-based and particle-based codes in the low density region,
and between more diffusive and less diffusive schemes in the high density tail
of the density distribution. Yet intrinsic code differences are generally small
compared to the variations in numerical implementations of the common subgrid
physics such as supernova feedback. Our experiment reassures that, if
adequately designed in accordance with our proposed common parameters, results
of a modern high-resolution galaxy formation simulation are more sensitive to
input physics than to intrinsic differences in numerical schemes.
