  We investigate the centrality dependence of the pT-correlations in the
event-by-event analysis of relativistic heavy-ion collisions at RHIC made
recently by the PHENIX and STAR Collaborations. We notice that sigma^2
dynamical scales to a very good accuracy with the inverse number of the
produced particles, n. This scaling can be naturally explained by formation of
clusters. We discuss the nature of clusters and provide numerical estimates of
correlations coming from resonance decays, which are tiny, and a model where
particle are emitted from local thermalized sources moving at the same
collective velocity. This "lumped cluster" model can explain the data when on
the average 6-15 particles are contained in a cluster. We also point out simple
relations of the popular correlation measures to the covariance, which arise
under the assumptions (well holding at RHIC) that the distributions are sharply
peaked in n and that the dynamical compared to statistical fluctuations are
small.
