Variation of Canonical Height for Fatou points on $\mathbb{P}^1$

Abstract

Let $f: \mathbb{P}^1\to \mathbb{P}^1$ be a map of degree $>1$ defined over a function field $k = K(X)$, where $K$ is a number field and $X$ is a projective curve over $K$. For each point $a \in \mathbb{P}^1(k)$ satisfying a dynamical stability condition, we prove that the Call-Silverman canonical height for specialization $f_t$ at point $a_t$, for $t \in X(\bar{\mathbb{Q}})$ outside a finite set, induces a Weil height on the curve $X$; i.e., we prove the existence of a $\mathbb{Q}$-divisor $D = D_{f,a}$ on $X$ so that the function $t\mapsto \hat{h}_{f_t}(a_t) - h_D(t)$ is bounded on $X(\bar{\mathbb{Q}})$ for any choice of Weil height associated to $D$. We also prove a local version, that the local canonical heights $t\mapsto \hat{\lambda}_{f_t, v}(a_t)$ differ from a Weil function for $D$ by a continuous function on $X(\mathbb{C}_v)$, at each place $v$ of the number field $K$. These results were known for polynomial maps $f$ and all points $a \in \mathbb{P}^1(k)$ without the stability hypothesis, and for maps $f$ that are quotients of endomorphisms of elliptic curves $E$ over $k$. Finally, we characterize our stability condition in terms of the geometry of the induced map $\tilde{f}: X\times \mathbb{P}^1 \rightarrow X\times \mathbb{P}^1$ over $K$; and we prove the existence of relative N\'eron models for the pair $(f,a)$, when $a$ is a Fatou point at a place $\gamma$ of $k$, where the local canonical height $\hat{\lambda}_{f,\gamma}(a)$ can be computed as an intersection number.