Hodge integrals and degenerate contributions

Degenerate contributions to higher genus Gromov-Witten invariants of Calabi-Yau 3-folds are computed via Hodge integrals. The vanishing of contributions of covers of elliptic curves conjectured by Gopakumar and Vafa is proven. A formula for degree 1 covers for all genus pairs is computed in agreement with M-theoretic calculations of Gopakumar and Vafa. Finally, these results lead to a proof of a formula in the tautological ring of the moduli space of curves previously conjectured by Faber.Comment: 21 pages, LaTeX2e. Expanded discussion of relationships with the M-theoretic calculations of Gopakumar-Vafa. New results for arbitrary 3-folds following a suggestion of Jinzenji and Xion