Lemma 101.25.6. Let f : \mathcal{X} \to \mathcal{Y} be a flat morphism of algebraic stacks. Let \mathop{\mathrm{Spec}}(A) \to \mathcal{Y} be a morphism where A is a valuation ring. If the closed point of \mathop{\mathrm{Spec}}(A) maps to a point of |\mathcal{Y}| in the image of |\mathcal{X|} \to |\mathcal{Y}|, then there exists a commutative diagram
\xymatrix{ \mathop{\mathrm{Spec}}(A') \ar[r] \ar[d] & \mathcal{X} \ar[d] \\ \mathop{\mathrm{Spec}}(A) \ar[r] & \mathcal{Y} }
where A \to A' is an extension of valuation rings (More on Algebra, Definition 15.123.1).
Proof.
The base change \mathcal{X}_ A \to \mathop{\mathrm{Spec}}(A) is flat (Lemma 101.25.3) and the closed point of \mathop{\mathrm{Spec}}(A) is in the image of |\mathcal{X}_ A| \to |\mathop{\mathrm{Spec}}(A)| (Properties of Stacks, Lemma 100.4.3). Thus we may assume \mathcal{Y} = \mathop{\mathrm{Spec}}(A). Let U \to \mathcal{X} be a surjective smooth morphism where U is a scheme. Then we can apply Morphisms of Spaces, Lemma 67.42.4 to the morphism U \to \mathop{\mathrm{Spec}}(A) to conclude.
\square
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