nIFTY galaxy cluster simulations III: The Similarity & Diversity of
  Galaxies & Subhaloes
  We examine subhaloes and galaxies residing in a simulated LCDM galaxy cluster
($M^{\rm crit}_{200}=1.1\times10^{15}M_\odot/h$) produced by hydrodynamical
codes ranging from classic Smooth Particle Hydrodynamics (SPH), newer SPH
codes, adaptive and moving mesh codes. 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
feedback 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 $\lesssim10\%$ for both dark matter only and non-radiative runs, with
individual objects showing code-to-code scatter of $\lesssim0.1$ dex, although
the gas in non-radiative simulations shows significant scatter. Including
feedback physics significantly increases the diversity. Subhalo mass and
$V_{max}$ distributions vary by $\approx20\%$. The galaxy populations also show
striking code-to-code variations. Although the Tully-Fisher relation is similar
in almost all codes, the number of galaxies with $10^{9}M_\odot/h\lesssim
M_*\lesssim 10^{12}M_\odot/h$ 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 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 even basic
galaxy properties, such as the stellar mass to halo mass, should be treated
with errors bars of $\sim0.2-0.4$ dex.
