Planck 2013 results. XX. Cosmology from Sunyaev-Zeldovich cluster counts. (arXiv:1303.5080v1 [astro-ph.CO])

Planck 2013 results. XX. Cosmology from Sunyaev-Zeldovich cluster counts. (arXiv:1303.5080v1 [astro-ph.CO]):
We present constraints on cosmological parameters using number counts as a
function of redshift for a sub-sample of 189 galaxy clusters from the Planck SZ
(PSZ) catalogue. The PSZ is selected through the signature of the
Sunyaev--Zeldovich (SZ) effect, and the sub-sample used here has a
signal-to-noise threshold of seven, with each object confirmed as a cluster and
all but one with a redshift estimate. We discuss the calculation of the
expected cluster counts as a function of cosmological parameters, the
completeness of the sample, and the likelihood construction method. Using a
relation between mass M and SZ signal Y based on comparison to X-ray
measurements, we derive constraints on the power spectrum amplitude sigma_8 and
matter density parameter \Omega_m in a flat LCDM model. We test the robustness
of our estimates and find that possible biases in the Y-M relation and the halo
mass function appear larger than the statistical uncertainties from the cluster
sample. Assuming a bias between the X-ray determined mass and the true mass of
20%, motivated by comparison of the observed mass scaling relations to those
from a set of numerical simulations, we find that
sigma_8(Omega_m/0.27)^0.3=0.78+-0.01, with one-dimensional ranges
sigma_8=0.77+-0.02 and Omega_m=0.29+-0.02. The values of the cosmological
parameters are degenerate with the mass bias, and it is found that the larger
values of sigma_8 and Omega_m preferred by the Planck's measurements of the
primary CMB anisotropies can be accommodated by a mass bias of about 45%.
Alternatively, consistency with the primary CMB constraints can be achieved by
inclusion of processes that suppress power on small scales, such as a component
of massive neutrinos. We place our results in the context of other
determinations of cosmological parameters, and discuss issues that need to be
resolved in order to make progress in this field.