The kernel of the Gysin homomorphism for smooth projective curves

TL;DR

This paper investigates the structure of the kernel of the Gysin homomorphism for smooth projective curves on surfaces, revealing it as a union of shifts of specific abelian subvarieties related to the vanishing cohomology, with a detailed stratification approach.

Contribution

It establishes a precise description of the Gysin kernel as a union of shifts of abelian subvarieties, connecting it to vanishing cohomology and using algebraic stacks for the stratification.

Findings

The Gysin kernel is a union of shifts of a certain abelian subvariety.

Either the abelian subvariety coincides with the vanishing cohomology subvariety or is zero.

The structure is analyzed via a filtration of algebraic stacks.

Abstract

Let $S$ be a smooth projective connected surface over an algebraically closed field $k$ and $\Sigma$ the linear system of a very ample divisor $D$ on $S$. Let $d:=\dim(\Sigma)$ be the dimension of $\Sigma$ and $\phi_{\Sigma}: S \hookrightarrow \mathbb{P}^{d}$ the closed embedding of $S$ into $\mathbb{P}^{d}$, induced by $\Sigma$. For any closed point $t\in\Sigma \cong\mathbb{P}^{d^*}$, let $C_t$ be the corresponding hyperplane section on $S$, and let $ r_t:C_t\hookrightarrow S$ be the closed embedding of the curve $C_t$ into $S$. Let $\Delta:= \{t \in \Sigma: C_t \text{ is singular}\}$ be the discriminant locus of $\Sigma$ and let $U :=\Sigma\setminus \Delta$. For $t \in U$, the kernel of the Gysin homomorphism of the Chow groups of $0$-cycles of degree zero, from $CH_0(C_t)_{deg=0}$ to $CH_0(S)_{deg=0}$ is the countable union of shifts of a certain abelian subvariety $A_t$ inside $J(C_t)$, the Jacobian of the curve $C_t$ (\cite{PS24} for $k \cong \mathbb{C}$, \cite{SW25} for $k \cong \overline{\mathbb{F}_q((t))}$). We prove that for every closed point $t \in U$ either $A_t$ coincides with the abelian variety $B_t$ inside $J(C_t)$ corresponding to the vanishing cohomology $H^1(C_t, k')_{van}$, where $k'$ is the minimal field of definition of $k$, and then the Gysin kernel is a countable union of shifts of $B_t$, or $A_t = 0$, in which case the Gysin kernel is countable. Using the language of algebraic stacks as a generalisation of algebraic varieties this is done by constructing an increasing filtration of Zariski countable open substacks $U_i, i \in I,$ of $U$, where $I$ is a countable set and by applying a convergence argument.