Divisors in the moduli space of Debarre-Voisin varieties

Abstract: Let $V_{10}$ be a 10-dimensional complex vector space and let $\sigma\in\bigwedge^3V_{10}^\vee$ be a non-zero alternating 3-form. One can define several associated degeneracy loci: the Debarre-Voisin variety $X_6^\sigma\subset\mathrm{Gr}(6,V_{10})$, the Peskine variety $X_1^\sigma\subset\mathbf{P}(V_{10})$, and the hyperplane section $X_3^\sigma\subset \mathrm{Gr}(3,V_{10})$. Their interest stems from the fact that the Debarre-Voisin varieties form a locally complete family of projective hyperk\"ahler fourfolds of $\mathrm{K3}^{[2]}$-type. We prove that when smooth, the varieties $X_6^\sigma$, $X_1^\sigma$, and $X_3^\sigma$ share one same integral Hodge structure, and that $X_1^\sigma$ and $X_3^\sigma$ both satisfy the integral Hodge conjecture in all degrees. This is obtained as a consequence of a detailed analysis of the geometry of these varieties along three divisors in the moduli space. On one of the divisors, an associated K3 surface $S$ of degree 6 can be constructed geometrically and the Debarre-Voisin fourfold is shown to be isomorphic to a moduli space of twisted sheaves on $S$, in analogy with the case of cubic fourfolds containing a plane.