Hessenberg character

Abstract

Let $G$ be a connected reductive group with Borel subgroup $B$, flag variety $\mathcal B=G/B$, and Weyl group $W$. We define the \emph{Hessenberg functor} on the geometric Hecke category by microlocalizing along the diagonal in $\mathcal B\times\mathcal B$ and pushing forward along the Springer map. It induces the \emph{Hessenberg character} of the Iwahori--Hecke algebra, with values in graded $W$-representations. On Kazhdan--Lusztig basis elements indexed by smooth elements of $W$, this character recovers the cohomology of the associated regular semisimple Hessenberg varieties. We identify the Hessenberg functor with the microlocalization of Lusztig's character functor and compute the Hessenberg character using Lusztig's nonabelian Fourier pairing. More generally, for families of Hessenberg schemes defined by pure-dimensional $B$-stable closed cones in the tangent space $T_{eB}\mathcal B$, we show that monodromy on the intersection cohomology of regular semisimple fibers factors through $W$. When the cone is the tangent cone of a Schubert variety, we give a local condition at $eB$ under which the same character formula computes the graded $W$-character of this intersection cohomology. Combined with the methods of Precup and Sommers, our character formula can be used to compute the graded $W$-characters of all regular semisimple Hessenberg varieties in all Lie types of rank at most ten, including all exceptional types. These computations also motivate positivity and unimodality conjectures for Hessenberg varieties and Lusztig varieties in types $B$, $C$, and $D$, together with positivity conjectures for their partial flag versions.