Geometric and homological properties of affine Deligne-Lusztig varieties

Abstract

This paper studies affine Deligne-Lusztig varieties $X_{\tilde w}(b)$ in the affine flag variety of a quasi-split tamely ramified group. We describe the geometric structure of $X_{\tilde w}(b)$ for a minimal length element $\tilde w$ in the conjugacy class of an extended affine Weyl group. We then provide a reduction method that relates the structure of $X_{\tilde w}(b)$ for arbitrary elements $\tilde w$ in the extended affine Weyl group to those associated with minimal length elements. Based on this reduction, we establish a connection between the dimension of affine Deligne-Lusztig varieties and the degree of the class polynomial of affine Hecke algebras. As a consequence, we prove a conjecture of Görtz, Haines, Kottwitz and Reuman.