Abstract

Numerical simulations have become an indispensable tool in astrophysics. To interpret their results, it is critical to understand their intrinsic variability, i.e. how much the results change with numerical noise or inherent stochasticity of the physics model. We present a set of seven realizations of high-resolution cosmological zoom-in simulations of a Milky Way-like galaxy with the Auriga galaxy formation model. All realizations share the same initial conditions and code parameters, but draw different random numbers for the inherently stochastic parts of the model. We show that global galaxy properties at z=0, including stellar mass, star formation history, masses of stellar bulge and stellar disc, the radius and height of the stellar disc change by less than 10per cent between the different realizations, and that magnetic field structures in the disc and the halo are very similar. In contrast, the star formation rate today can vary by a factor of 2, and the internal morphological structure of the stellar disc can change. The time and orbit of satellite galaxies and their galaxy properties when falling into the main halo are again very similar, but their orbits start to deviate after the first pericentre passage. Finally, we show that changing the mass resolution of all matter components by a factor of 8 in the Auriga model changes galaxy properties significantly more than the intrinsic variability of the model, and that these changes are systematic. This limits detailed comparisons between simulations at different numerical resolutions. © The Author(s) 2025. Published by Oxford University Press on behalf of Royal Astronomical Society.