  The dependence of the size of the thermal source on the centrality in
ultrarelativistic heavy-ion collisions is studied. The interaction region
consists of a well defined thermalized core, and of an outer mantle where the
production scales with the number of participants. The thermal source builds up
in the region with the largest density of participants in the transverse plane.
Particle production in the thermalized core is enhanced in comparison to the
wounded nucleon model. The change of the degree of strangeness saturation with
centrality is also discussed. We perform an estimate of high pT jet absorption
finding that an increase of the absorption in the thermal core is compatible
with the data.
