On compactifications of ${\mathcal{M}}_{g,n}$ with colliding markings

Abstract

In this paper, we study all ways of constructing modular compactifications of the moduli space ${\mathcal{M}}_{g,n}$ of $n$-pointed smooth algebraic curves of genus $g$ by allowing markings to collide. We find that for any such compactification, collisions of markings are controlled by a simplicial complex which we call the collision complex. Conversely, we identify modular compactifications of ${\mathcal{M}}_{g,n}$ with essentially arbitrary collision complexes, including complexes not associated to any space of weighted pointed stable curves. These moduli spaces classify the modular compactifications of ${\mathcal{M}}_{g,n}$ by nodal curves with smooth markings as well as the modular compactifications of ${\mathcal{M}}_{1,n}$ with Gorenstein curves and smooth markings. These compactifications generalize previous constructions given by Hassett, Smyth, and Bozlee--Kuo--Neff.

41 pages; minor corrections since last revision, comments very welcome