Instability of Black Holes in AdS$_3 \times S^3$

Extremal and near extremal black holes are unstable, unless they are protected by supersymmetry. For black holes in AdS$_3 \times S^3$, we study the range of parameters that bound the unstable region, the fate of extremal black holes that are unstable, and the true ground state in super-selection sectors where no isolated black hole solution exists. Thermodynamics generally favors a central black hole surrounded by chiral primaries that carry a significant fraction of the conserved charges. We give a dynamical interpretation of decay, by computing the linear response functions in the dual CFT$_2$ and showing that the onset of instability agrees with thermodynamics. To understand the nature of the decay products, we construct several new families of classical string solutions. They are ``giants" that move on both the AdS$_3$ and the $S^3$, and may have interesting applications beyond those we develop.