Steinberg quotients and Smith-Treumann localization

Abstract

Smith-Treumann localization for sheaves on the affine Grassmannian of a reductive group has previously been studied by Leslie-Lonergan (for spherical sheaves) and by Riche-Williamson (for Iwahori-Whittaker sheaves). In this paper, we show that the two versions are related by a commutative diagram that involves "convolution with the Steinberg module." As an application, we "categorify" certain formal characters of a reductive group called Steinberg quotients, previously introduced and studied by the second author. Specifically, we show that Steinberg quotients describe the stalks of spherical parity sheaves on the $\mathbb{Z}/p\mathbb{Z}$-fixed-locus of the affine Grassmannian.