For any surface $\Sigma$ of infinite topological type, we study the Torelli subgroup $\mathcal{I}(\Sigma)$ of the mapping class group ${\rm Mod}(\Sigma)$, whose elements are those mapping classes that act trivially on the homology of $\Sigma$. Our first result asserts that $\mathcal{I}(\Sigma)$ is topologically generated by the subgroup of ${\rm Mod}(\Sigma)$ consisting of those elements which have compact support. In particular, using results of Birman and Powell, we deduce that $\mathcal{I}(\Sigma)$ is topologically generated by {\em separating twists} and {\em bounding pair maps}. Next, we prove that the abstract commensurator of $\mathcal{I}(\Sigma)$ coincides with ${\rm Mod}(\Sigma)$. This extends results of Farb-Ivanov and Kida to the setting of infinite--type surfaces. 查看全文>>