A Néron model of the universal jacobian

Jacobians of degenerating families curves are well-understood over 1-dimensional bases due to work Néron and Raynaud; the fundamental tool is model its description via Picard functor. Over higher-dimensional models typically do not exist, but in this paper we construct a universal base change ? ˜ g,n ?? ¯ after which N /? jacobian does exist. This yields new partial compactification moduli space curves, it. The map separated relatively representable. has group law extending that on jacobian. We show Caporaso’s balanced stack acquires torsor structure pullback certain open substack .