Quasi-homogeneity of the moduli space of stable maps to homogeneous spaces

Abstract

Let $G$ be a connected, simply connected, simple, complex, linear algebraic group. Let $P$ be an arbitrary parabolic subgroup of $G$. Let $X=G/P$ be the $G$-homogeneous projective space attached to this situation. Let $d\in H_2(X)$ be a degree. Let $\overline{M}_{0,3}(X,d)$ be the (coarse) moduli space of three pointed genus zero stable maps to $X$ of degree $d$. We prove under reasonable assumptions on $d$ that $\overline{M}_{0,3}(X,d)$ is quasi-homogeneous under the action of $G$. The essential assumption on $d$ is that $d$ is a minimal degree, i.e. that $d$ is a degree which is minimal with the property that $q^d$ occurs with non-zero coefficient in the quantum product $\sigma_u\star\sigma_v$ of two Schubert cycles $\sigma_u$ and $\sigma_v$, where $\star$ denotes the product in the (small) quantum cohomology ring $QH^*(X)$ attached to $X$. We prove our main result on quasi-homogeneity by constructing an explicit morphism which has a dense open $G$-orbit in $\overline{M}_{0,3}(X,d)$. To carry out the construction of this morphism, we develop a combinatorial theory of generalized cascades of orthogonal roots which is interesting in its own right.