Affine Springer fibers and depth zero L-packets

Let $G$ be a connected reductive group over a field $F=\mathbb F_q((t))$ splitting over $\overline{\mathbb F}_q((t))$. Following [KV,DR], a tamely unramified Langlands parameter $\lambda:W_F\to{}^L G(\overline{\mathbb Q}_{\ell})$ in general position gives rise to a finite set $\Pi_{\lambda}$ of irreducible admissible representations of $G(F)$, called the $L$-packet. The main goal of this work is to provide a geometric description of characters $\chi_{\pi}$ of $\pi\in\Pi_{\lambda}$ and of their endoscopic linear combinations $\chi_{\lambda}^{\kappa}$ in terms of homology of affine Springer fibers. As an application, we prove that the sum $\chi_{\lambda}^{st}:=\sum_{\pi\in\Pi_{\lambda}}\chi_{\pi}$ is stable and show that the $\chi_{\lambda}^{st}$'s are compatible with inner twistings. More generally, we prove that each $\chi_{\lambda}^{\kappa}$ is ${\mathcal E}_{\lambda,\kappa}$-stable.
Comment: v.2, 94 pages, seriously revised version: sign in the statement of a theorem of Yun is corrected, proof of endoscopic property of $\kappa$-linear combinations is included, treatment of generalized traces was made much more conceptual