Maximum likelihood degree, complete quadrics and ${\mathbb C}^*$-action

We study the maximum likelihood (ML) degree of linear concentration models in algebraic statistics. We relate it to an intersection problem on a smooth compact moduli space of orbits of a ${\mathbb C}^*$ action on the Lagrangian Grassmannian which we call Gaussian moduli. This allows us to provide an explicit, basic, albeit of high computational complexity, formula for the ML-degree. The Gaussian moduli is an exact analog for symmetric matrices of the permutohedron variety for the diagonal matrices.