Let $G$ be a reductive algebraic group with Lie algebra $\mathfrak{ g}$ and $V$ a finite-dimensional representation of $G$. Costello-Gaiotto studied a graded Lie algebra $\mathfrak{ d}_{ \mathfrak{ g}, V}$ and the associated affine Kac-Moody algebra. In this paper, we show that this Lie algebra can be made into a sheaf of Lie algebras over $T^*[V/G]=[\mu^{ -1}(0)/G]$, where $\mu: T^*V\to \mathfrak{ g}^*$ is the moment map. We identify this sheaf of Lie algebras with the tangent Lie algebra of the stack $T^*[V/G]$. Moreover, we show that there is an equivalence of braided tensor categories between the bounded derived category of graded modules of $\mathfrak{ d}_{ \mathfrak{ g}, V}$ and graded perfect complexes of $[\mu^{ -1}(0)/G]$.
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