Abstract
We examine subhaloes and galaxies residing in a simulated LCDM galaxy cluster
($M^crit_200=1.1\times10^15M_ødot/h$) produced by hydrodynamical
codes ranging from classic Smooth Particle Hydrodynamics (SPH), newer SPH
codes, an adaptive mesh code and a moving mesh scheme. These codes use subgrid
models to capture galaxy formation physics. We compare how well these codes
reproduce the same subhaloes/galaxies in gravity only, non-radiative
hydrodynamics and full radiative physics runs by looking at the overall
subhalo/galaxy distribution and on an individual objects basis. We find the
subhalo population is reproduced to within $łesssim10\%$ for both dark matter
only and non-radiative runs, with individual objects showing code-to-code
scatter of $łesssim0.1$ dex, although the gas in non-radiative simulations
shows significant scatter. Including radiative physics significantly increases
the diversity seen. The subhalo mass and $V_max$ distributions vary by
$\approx20\%$, a result of feedback moving significant baryonic mass around.
Galaxies also show striking code-to-code variations. Although the Tully-Fisher
relation is similar in almost all codes, the number of galaxies with
$10^9M_ødot/hM_*10^12M_ødot/h$ can differ by a factor
of 4. Individual galaxies show code-to-code scatter of $\sim0.5$ dex in stellar
mass. Moreover, strong systematic differences exist, with some codes producing
galaxies $70\%$ smaller than others. The diversity partially arises from the
inclusion/absence of AGN feedback. Our results combined with our companion
papers, Sembolini et al. (2015a,b), demonstrate that subgrid physics is not
just subject to fine-tuning, but the complexity of building galaxies in all
environments remains a challenge. We argue that even basic galaxy properties,
such as the stellar mass to halo mass, should be treated with errors bars of
$\sim0.2-0.5$ dex.
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