Morita equivalence for convolution categories: Appendix to arXiv:0805.0157

In this brief postscript to our paper "Integral transforms and Drinfeld centers in derived algebraic geometry", we describe a Morita equivalence for derived, categorified matrix algebras implied by theory developed since its appearance. We work in the setting of perfect stacks X and their stable infinity-categories Q(X) of quasicoherent sheaves. Perfect stacks include all varieties and common stacks in characteristic zero, and their stable infinity-categories of sheaves are well behaved refinements of their quasicoherent derived categories, satisfying natural analogues of common properties of function spaces. To a morphism of perfect stacks pi:X-->Y, we associate the categorified matrix algebra Q(X x_Y X) of sheaves on the derived fiber product equipped with its monoidal convolution product. We show that for pi faithfully flat (as a corollary of the 1-affineness theorem of Gaisgory) or for pi proper and surjective and X,Y smooth (as an application of proper descent, cf. Gaitsgory and Preygel), there is a Morita equivalence between Q(X x_Y X) and Q(Y), that is, an equivalence of their infinity-categories of stable module infinity-categories. In particular, this immediately implies an identification of their Drinfeld centers (as previously established in 0805.0157), and more generally, an identification of their associated topological field theories. Another consequence is that for an affine algebraic group G in characteristic zero and an algebraic subgroup K, passing to K-invariants induces an equivalence from stable infinity-categories with algebraic G-action to modules for the Hecke category Q(K\G/K).