  We study algebraic (Artin) stacks over $\bar{\mathcal M}_g$ giving a
functorial way of compactifying the relative degree $d$ Picard variety for
families of stable curves. We also describe for every $d$ the locus of genus
$g$ stable curves over which we get Deligne-Mumford stacks strongly
representable over $\bar{\mathcal M}_g$.
