Half-BPS giants, free fermions and microstates of superstars

We consider 1/2-BPS states in AdS/CFT. Using the matrix model description ofchiral primaries explicit mappings among configurations of fermions, giantgravitons and the dual-giant gravitons are obtained. These maps lead to a`duality' between the giant and the dual-giant configurations which is thereflection of particle-hole duality of the fermion picture. These dualitiesgive rise to some interesting consequences which we study. We then calculatethe degeneracy of 1/2-BPS states both from the CFT and string theory and showthat they match. The asymptotic degeneracy grows exponentially with thecomformal dimension. We propose that the five-dimensional single charge`superstar' geometry should carry this density of states. An appropriatestretched horizon can be placed in this geometry and the entropy predicted bythe CFT and the string theory microstate counting can be reproduced by theBekenstein-Hawking formula up to a numerical coefficient. Similar M-theoryexamples are also considered.Comment: 21 pages, v2:typos corrected and references adde