Minimal Lagrangian submanifolds via the geodesic Gauss map

Abstract

For an oriented isometric immersion $f:M\to S^n$ the spherical Gauss map is the Legendrian immersion of its unit normal bundle $UM^\perp$ into the unit sphere subbundle of $TS^n$, and the geodesic Gauss map $\gamma$ projects this into the manifold of oriented geodesics in $S^n$ (the Grassmannian of oriented 2-planes in $\mathbb{R}^{n+1}$), giving a Lagrangian immersion of $UM^\perp$ into a Kaehler-Einstein manifold. We give expressions for the mean curvature vectors for both the spherical and geodesic Gauss maps in terms of the second fundamental form of $f$, and show that when $f$ has conformal shape form this depends only on the mean curvature of $f$. In particular we deduce that the geodesic Gauss map of every minimal surface in $S^n$ is minimal Lagrangian. We also give simple proofs that: deformations of $f$ always correspond to Hamiltonian deformations of $\gamma$; the mean curvature vector of $\gamma$ is always a Hamiltonian vector field. This extends work of Palmer on the case when $M$ is a hypersurface.