Abstract

The standard techniques for detecting galaxy clusters (optical/near-IR “red sequence”, X-ray emission, SZ effect) preferentially or exclusively select evolved objects, with mature galaxy populations and a hot intra-cluster medium. As a result, most known clusters are at redshifts

The Planck Collaboration has recently published a catalogue of more than two thousands sources with cold sub-mm colours, suggestive of z>2, over about 26% of the sky, at high Galactic latitudes. Follow-up Herschel observations of a sub-sample of these sources have shown that almost all of them are highly significant overdensities of star-forming galaxies, supporting their classification as galaxy (proto)-clusters. However, we have found that the number densities (or the flux-densities) of these objects are far in excess of the expectations from the standard scenario for the evolution of large-scale structure. Our simulations, based on a physically motivated galaxy evolution model, show that essentially all cold peaks brighter than S(545GHz) = 500 mJy found in Planck maps after having removed the Galactic dust emission can be interpreted not as individual (proto)-clusters but as positive Poisson fluctuations of the number of high-z overdensities of dusty galaxies within the Planck beam. The simulations accurately reproduce the statistic of the Planck detections. Also the distributions of sizes and ellipticities are in qualitative agreement with observations. The redshift distribution of the brightest proto-clusters contributing to the cold peaks has a broad maximum at 1.5 < z < 3. Perspectives for sub-mm (proto)-cluster surveys are discussed.