Sunyaev-Zel'dovich clusters in Millennium Gas simulations
  We have exploited the large-volume Millennium Gas cosmological N-body
hydrodynamics simulations to study the SZ cluster population at low and high
redshift, for three models with varying gas physics. We confirm previous
results using smaller samples that the intrinsic (spherical) Y_{500}-M_{500}
relation has very little scatter (sigma_{log_{10}Y}~0.04), is insensitive to
cluster gas physics and evolves to redshift one in accord with self-similar
expectations. Our pre-heating and feedback models predict scaling relations
that are in excellent agreement with the recent analysis from combined Planck
and XMM-Newton data by the Planck Collaboration. This agreement is largely
preserved when r_{500} and M_{500} are derived using the hydrostatic mass
proxy, Y_{X,500}, albeit with significantly reduced scatter
(sigma_{log_{10}Y}~0.02), a result that is due to the tight correlation between
Y_{500} and Y_{X,500}. Interestingly, this assumption also hides any bias in
the relation due to dynamical activity. We also assess the importance of
projection effects from large-scale structure along the line-of-sight, by
extracting cluster Y_{500} values from fifty simulated 5x5 square degree sky
maps. Once the (model-dependent) mean signal is subtracted from the maps we
find that the integrated SZ signal is unbiased with respect to the underlying
clusters, although the scatter in the (cylindrical) Y_{500}-M_{500} relation
increases in the pre-heating case, where a significant amount of energy was
injected into the intergalactic medium at high redshift. Finally, we study the
hot gas pressure profiles to investigate the origin of the SZ signal and find
that the largest contribution comes from radii close to r_{500} in all cases.
The profiles themselves are well described by generalised Navarro, Frenk &
White profiles but there is significant cluster-to-cluster scatter.
