We consider a massless quantum scalar field on a two-dimensional space-time describing a thin shell of matter collapsing to form a Schwarzschildanti-de Sitter black hole. At early times, before the shell starts to collapse, the quantum field is in the vacuum state, corresponding to the Boulware vacuum on an eternal black hole space-time. The scalar field satisfies reflecting boundary conditions on the anti-de Sitter boundary. Using the Davies-Fulling-Unruh prescription for computing the renormalized expectation value of the stress-energy tensor, we find that at late times the black hole is in thermal equilibrium with a heat bath at the Hawking temperature, so the quantum field is in a state analogous to the Hartle-Hawking vacuum on an eternal black hole space-time. the fully dynamical space-time consisting of matter collapsing to form a black hole. However, quantum field theory on static space-times is technically much easier than on dynamical space-times. For an eternal Schwarzschild black hole, the Unruh state [2] contains an outgoing thermal flux of radiation at future null infinity, and therefore is the state analogous to that at late times for a black hole formed by gravitational collapse.Since all two-dimensional space-times are locally conformally flat, quantum field theory on two-dimensional space-times is rather simpler than in four space-time dimensions, particularly if one is considering dynamical spacetimes. In particular, two-dimensional black hole space-times have proven to be useful toy models for understanding various aspects of quantum field theory on four-dimensional black holes, in particular Hawking radiation and black hole evaporation [3]. One advantage of working in two space-time dimensions is that there is an exact prescription [4] for computing the renormalized expectation value of the stress-energy tensor T µν for a massless scalar field. Applying this prescription to a two-dimensional asymptotically flat space-time describing a black hole formed by gravitational collapse, it is found that at late times after the gravitational collapse, far from the black hole T µν contains an outgoing flux of energy compared to that at early times before the collapse starts [4]. This outgoing flux is the Hawking radiation and is also found in the expectation value T µν for the Unruh vacuum state on an eternal two-dimensional Schwarzschild black hole [5].A natural question is how the above picture is modified in space-times with different asymptotic structure. The case of a Schwarzschild-de Sitter black hole was studied many years ago. Working in two space-time dimensions, any static quantum state defined in the region between the event and cosmological horizons must have a stress-energy tensor T µν which diverges at either the event or the cosmological horizon [6,7], in agreement with the four-dimensional Kay-Wald theorem that there is no stationary, nonsingular state on Schwarzschild-de Sitter space-time [8]. However, a nonstatic quantum state can be defined which is regular on both the event and cosmo...