Koszul divisors on moduli spaces of curves

Abstract

Given a moduli problem with a nice coarse moduli space, which is the most intrinsic and natural divisor on this moduli space? We describe a general method of constructing effective divisors on a large class of moduli spaces using the syzygies of the parametrized objects. In this paper we treat the case of the moduli space of curves and we compute the slope of a doubly infinite sequence of Koszul divisors on M_g. We present a single formula which apart from giving myriads of new counterexamples to the Harris-Morrison Slope Conjecture it also encodes virtually all interesting divisor class calculations on M_g. In particular our formula specializes to earlier results due to Harris-Mumford (the case of the Brill-Noether divisors), Eisenbud-Harris (Giseker-Petri divisors), Khosla as well as to our earlier counterexamples to the Slope Conjecture obtained using K3 surfaces and Hurwitz schemes. We also describe five ways of constructing Koszul divisors on moduli spaces of pointed curves. As an application, we improve most of Logan's results on the Kodaira type of M_{g,n}.