The Stacks project

Proof. Choose a representable stack in groupoids $\mathcal{U}$ over $S$ and a $1$-morphism $f : \mathcal{U} \to \mathcal{H}_ d(\mathcal{X})$ which is representable by algebraic spaces, smooth, and surjective. This is possible because $\mathcal{H}_ d(\mathcal{X})$ is an algebraic stack by Lemma 97.14.1. Consider the $2$-fibre product

Since $\mathcal{U}$ is representable (in particular a stack in setoids) it follows from Examples of Stacks, Lemma 95.18.3 and Stacks, Lemma 8.6.7 that $\mathcal{W}$ is a stack in setoids. The $1$-morphism $\mathcal{W} \to \mathcal{H}_ d(\mathcal{X}/\mathcal{Y})$ is representable by algebraic spaces, smooth, and surjective as a base change of the morphism $f$ (see Algebraic Stacks, Lemmas 94.9.7 and 94.10.6). Thus, if we can show that $\mathcal{W}$ is representable by an algebraic space, then the lemma follows from Algebraic Stacks, Lemma 94.15.3.

The diagonal of $\mathcal{Y}$ is representable by algebraic spaces according to Lemma 97.4.3. We may apply Lemma 97.12.5 to see that the $1$-morphism

is representable by algebraic spaces. Consider the $2$-fibre product

The projection morphism $\mathcal{V} \to \mathcal{U} \times \mathcal{X}$ is representable by algebraic spaces as a base change of the last displayed morphism. Hence $\mathcal{V}$ is an algebraic space (see Bootstrap, Lemma 80.3.6 or Algebraic Stacks, Lemma 94.9.8). The $1$-morphism $\mathcal{V} \to \mathcal{U}$ fits into the following $2$-cartesian diagram

because

Hence $\mathcal{V} \to \mathcal{W}$ is representable by algebraic spaces, surjective, flat, and locally of finite presentation as a base change of $F$. It follows that the same thing is true for the corresponding sheaves of sets associated to $\mathcal{V}$ and $\mathcal{W}$, see Algebraic Stacks, Lemma 94.10.4. Thus we conclude that the sheaf associated to $\mathcal{W}$ is an algebraic space by Bootstrap, Theorem 80.10.1. $\square$