Quantifying the intrinsic variability due to randomness of the Auriga galaxy formation model

TL;DR

This study assesses the intrinsic variability of galaxy formation simulations using the Auriga model, revealing that stochastic effects cause some properties to vary significantly while others remain stable, and that resolution impacts results more than stochasticity.

Contribution

It provides the first systematic quantification of the intrinsic stochastic variability in high-resolution galaxy formation simulations.

Findings

Global properties vary less than 10% across realizations.

Star formation rate can differ by a factor of two.

Resolution changes have a larger systematic impact than stochastic variability.

Abstract

Numerical simulations have become an indispensable tool in astrophysics. To interpret their results, it is critical to understand their intrinsic variability, that is, how much the results change with numerical noise or inherent stochasticity of the physics model. We present a set of seven realisations of high-resolution cosmological zoom-in simulations of a Milky Way-like galaxy with the Auriga galaxy formation model. All realisations 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 disk change by less than $10\%$ between the different realisations, 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 two 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 first pericenter 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.