Semi-infinite orbits in affine flag varieties and homology of affine Springer fibers

Abstract: Let $G$ be a connected reductive group over an algebraically closed field $k$, and let $Fl$ be the affine flag variety of $G$. For every regular semisimple element $\gamma$ of $G(k((t)))$, the affine Springer fiber $Fl_{\gamma}$ can be presented as a union of closed subvarieties $Fl^{\leq w}{\gamma}$, defined as the intersection of $Fl{\gamma}$ with an affine Schubert variety $Fl^{\leq w}$. The main result of this paper asserts that if elements $w_1,\ldots,w_n$ are sufficiently regular, then the natural map $H_i(\bigcup_{j=1}^n Fl^{\leq w_j}{\gamma})\to H_i(Fl{\gamma})$ is injective for every $i\in{\mathbb Z}$. It plays an important role in our work [BV]. One can view this statement as providing a categorification of the notion of a weighted orbital integral. Along the way we also show that every affine Schubert variety can be written as an intersection of closures of semi-infinite orbits.