On properness of moduli stacks of $D^{\times }$-shtukas over ramified legs

Abstract

Given a maximal order $\mathcal{D}$ of a central division algebra $D$ over a global function field $F$, we prove an explicit sufficient condition for moduli stacks of ${\mathcal{D}}^{\times }$-shtukas to be proper over a finite field (modulo a suitable central action) in terms of the \emph{local invariants} of $D$ and \emph{bounds}. Our proof is a refinement of E.~Lau's result (Duke Math. J. \textbf{140} (2007)), which showed the properness of the \emph{leg morphism} (or \emph{characteristic morphism}) away from the ramification locus of $D$. %, by carefully measuring the contribution of ``ramified legs''. We also establish non-emptiness of Newton and Kottwitz--Rapoport strata for moduli stacks of ${\mathcal{B}}^{\times }$-shtukas, where $\mathcal{B}$ is a maximal order of a central simple algebra over $F$.