A remark on the virtual homotopical dimension of some moduli spaces of stable Riemann surfaces

Abstract

Inspired by his vanishing results of tautological classes and by Harer's computation of the virtual cohomological dimension of the mapping class group, Looijenga conjectured that the moduli space of smooth Riemann surfaces admits a stratification by affine subsets with a certain number of layers. Similarly, Roth and Vakil extended the conjecture to the moduli spaces of Riemann surfaces of compact type, of Riemann surfaces with rational tails and of Riemann surfaces with at most k rational components. As a consequence of Lefschetz's theorem, Roth-Vakil's conjecture would also imply that the previous (coarse) moduli spaces are homotopy equivalent to cellular complexes of a certain dimension. Using Harer's computation for the moduli spaces of smooth Riemann surfaces, we prove this last statement.