We study the statistics of peaks in a weak lensing reconstructed mass map of the first 450 square degrees of the Kilo Degree Survey. The map is computed with aperture masses directly applied to the shear field with an NFW-like compensated filter. We compare the peak statistics in the observations with that of simulations for various cosmologies to constrain the cosmological parameter S 8 = σ 8√ Ω m /0.3, which probes the (Ω m , σ 8 ) plane perpendicularly to its main degeneracy. We estimate S 8 = 0.750 ± 0.059, using peaks in the signalto-noise range 0 ≤ S/N ≤ 4, and accounting for various systematics, such as multiplicative shear bias, mean redshift bias, baryon feedback, intrinsic alignment, and shear-position coupling. These constraints are ∼ 25% tighter than the constraints from the high significance peaks alone (3 ≤ S/N ≤ 4) which typically trace single-massive halos. This demonstrates the gain of information from low-S/N peaks. However we find that including S/N < 0 peaks does not add further information. Our results are in good agreement with the tomographic shear two-point correlation function measurement in KiDS-450. Combining shear peaks with non-tomographic measurements of the shear two-point correlation functions yields a ∼ 20% improvement in the uncertainty on S 8 compared to the shear two-point correlation functions alone, highlighting the great potential of peaks as a cosmological probe.

Obtaining large samples of galaxy clusters is important for cosmology: cluster counts as a function of redshift and mass can constrain the parameters of our Universe. They are also useful in order to understand the formation and evolution of clusters. We develop an improved version of the Adami & MAzure Cluster FInder (AMACFI), now the Adami, MAzure & Sarron Cluster FInder (AMASCFI), and apply it to the 154 deg 2 of the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS) to obtain a large catalogue of 1371 cluster candidates with mass M 200 > 10 14 M and redshift z ≤ 0.7. We derive the selection function of the algorithm from the Millennium simulation, and cluster masses from a richness-mass scaling relation built from matching our candidates with X-ray detections. We study the evolution of these clusters with mass and redshift by computing the i -band galaxy luminosity functions (GLFs) for the early-type (ETGs) and late-type galaxies (LTGs). This sample is 90% pure and 70% complete, and therefore our results are representative of a large fraction of the cluster population in these redshift and mass ranges. We find an increase in both the ETG and LTG faint populations with decreasing redshift (with Schechter slopes α ETG = −0.65 ± 0.03 and α LTG = −0.95 ± 0.04 at z = 0.6, and α ETG = −0.79 ± 0.02 and α LTG = −1.26 ± 0.03 at z = 0.2) and also a decrease in the LTG (but not the ETG) bright end. Our large sample allows us to break the degeneracy between mass and redshift, finding that the redshift evolution is more pronounced in high-mass clusters, but that there is no significant dependence of the faint end on mass for a given redshift. These results show that the cluster red sequence is mainly formed at redshift z > 0.7, and that faint ETGs continue to enrich the red sequence through quenching of brighter LTGs at z ≤ 0.7. The efficiency of this quenching is higher in large-mass clusters, while the accretion rate of faint LTGs is lower as the more massive clusters have already emptied most of their environment at higher redshifts.

Aims. Based on XMM-Newton, Chandra, and SDSS data, we investigate the baryon distribution in groups and clusters and its use as a cosmological constraint. For this, we considered a sample of 123 systems with temperatures kT 500 = 1.0−9.0 keV, total masses in the mass range M 500 = (∼10 13 −4 × 1070 M , and redshifts 0.02 < z < 1.3. Methods. The gas masses and total masses are derived from X-ray data under the assumption of hydrostatic equilibrium and spherical symmetry. The stellar masses are based on SDSS-DR8 optical photometric data. For the 37 systems out of 123 that had both optical and X-ray data available, we investigated the gas, stellar, and total baryon mass fractions inside r 2500 and r 500 and the differential gas mass fraction within the spherical annulus between r 2500 and r 500 , as a function of total mass. For the other objects, we investigated the gas mass fraction only. Results. We find that the gas mass fraction inside r 2500 and r 500 depends on the total mass. However, the differential gas mass fraction does not show any dependence on total mass for systems with M 500 > 10 14 M . The stellar mass fraction inside r 2500 and r 500 increases towards low-mass systems more steeply than the f gas decrease with total mass. Adding the gas and stellar mass fractions to obtain the total baryonic content, we find it to increase with cluster mass, reaching the WMAP-7 value for clusters with M 500 ∼ 10 14 M . This led us to investigate the contribution of the intracluster light to the total baryon budget for lower mass systems, but we find that it cannot account for the difference observed. Conclusions. The gas mass fraction dependence on total mass observed for groups and clusters could be due to the difficulty of low-mass systems to retain gas inside the inner region (r < r 2500 ). Because of their shallower potential well, non-thermal processes are more effective in expelling the gas from their central regions outwards. Since the differential gas mass fraction is nearly constant, it provides better constraints for cosmology. Moreover, we find that the gas mass fraction does not depend on redshift at a 2σ level. Using our total f b estimates, our results imply Ω m < 0.55, and taking the highest significant estimates for f b , Ω m > 0.22.