A moduli space of stable quotients of the rank
trivial sheaf on stable curves is introduced. Over nonsingular curves, the
moduli space is Grothendieck’s Quot scheme. Over nodal curves, a relative
construction is made to keep the torsion of the quotient away from the singularities.
New compactifications of classical spaces arise naturally: a nonsingular and
irreducible compactification of the moduli of maps from genus 1 curves to
projective space is obtained. Localization on the moduli of stable quotients
leads to new relations in the tautological ring generalizing Brill–Noether
constructions.
The moduli space of stable quotients is proven to carry a canonical
–term
obstruction theory and thus a virtual class. The resulting system of descendent
invariants is proven to equal the Gromov–Witten theory of the Grassmannian in all
genera. Stable quotients can also be used to study Calabi–Yau geometries. The
conifold is calculated to agree with stable maps. Several questions about the behavior
of stable quotients for arbitrary targets are raised.
Dedicated to William Fulton on the
occasion of his 70th birthday