Towards a theta correspondence in families for type II dual pairs

Abstract: Let $R$ be a commutative $\mathbb{Z}[1/p]$-algebra, let $m \leq n$ be positive integers, and let $G_n=\text{GL}n(F)$ and $G_m=\text{GL}_m(F)$ where $F$ is a $p$-adic field. The Weil representation is the smooth $R[G_n\times G_m]$-module $C_c^{\infty}(\text{Mat}{n\times m}(F),R)$ with the action induced by matrix multiplication. When $R=\mathbb{C}$ or is any algebraically closed field of banal characteristic compared to $G_n$ and $G_m$, the local theta correspondence holds by the work of Howe and M\'inguez. At the level of supercuspidal support, we interpret the theta correspondence as a morphism of varieties $\theta_R$, which we describe as an explicit closed immersion. For arbitrary $R$, we construct a canonical ring homomorphism $\theta^#_{R} : \mathfrak{Z}{R}(G_n)\to \mathfrak{Z}{R}(G_m)$ that controls the action of the center $\mathfrak{Z}{R}(G_n)$ of the category of smooth $R[G_n]$-modules on the Weil representation. We use the rank filtration of the Weil representation to first obtain $\theta{\mathbb{Z}[1/p]}^#$, then obtain $\theta^#_R$ for arbitrary $R$ by proving $\mathfrak{Z}_R(G_n)$ is compatible with scalar extension. In particular, the map $\text{Spec}(\mathfrak{Z}_R(G_m))\to \text{Spec}(\mathfrak{Z}_R(G_n))$ induced by $\theta_R^#$ recovers $\theta_R$ in the $R=\mathbb{C}$ case and in the banal case. We use gamma factors to prove $\theta_R^#$ is surjective for any $R$. Finally, we describe $\theta^#_R$ in terms of the moduli space of Langlands parameters and use this description to give an alternative proof of surjectivity in the tamely ramified case.