On the Cluster Physics of Sunyaev-Zel'dovich and X-ray Surveys IV:
  Characterizing Density and Pressure Clumping due to Infalling Substructures
  Understanding the outskirts of galaxy clusters at the virial radius (R200)
and beyond is critical for an accurate determination of cluster masses and to
ensure unbiased cosmological parameter estimates from cluster surveys. This
problem has drawn renewed interest due to recent determinations of gas mass
fractions beyond R200, which appear to be considerably larger than the cosmic
mean, and because the clusters' total Sunyaev-Zel'dovich flux receives a
significant contribution from these regions. Here, we use a large suite of
cosmological hydrodynamical simulations to study the clumpiness of density and
pressure and employ different variants of simulated physics, including
radiative gas physics and thermal feedback by active galactic nuclei. We find
that density and pressure clumping closely trace each other as a function of
radius, but the bias on density remains on average < 20% within the virial
radius R200. At larger radius, clumping increases steeply due to the continuous
infall of coherent structures that have not yet passed the accretion shock.
Density and pressure clumping increase with cluster mass and redshift, which
probes on average dynamically younger objects that are still in the process of
assembling. The angular power spectra of gas density and pressure show that the
clumping signal is dominated by comparably large substructures with scales
>R200/5, signaling the presence of gravitationally-driven "super-clumping". In
contrast, the angular power spectrum of the dark matter (DM) shows an almost
uniform size distribution due to unimpeded subhalos. The quadrupolar anisotropy
dominates the signal and correlates well across different radii as a result of
the prolateness of the DM potential. We provide a synopsis of the radial
dependence of the clusters' non-equilibrium measures (kinetic pressure support,
ellipticity, and clumping) that all increase sharply beyond R200.
