  We identify universal properties of the low-energy subspace of a wide class
of quantum optical models in the ultrastrong coupling limit, where the coupling
strength dominates over all other energy scales in the system. We show that the
symmetry of the light-matter interaction is at the origin of a two-fold
degeneracy in the spectrum. We prove analytically this result for bounded
Hamiltonians and extend it to a class of models with unbounded operators. As a
consequence, we show that the emergence of superradiant phases previously
investigated in the context of critical phenomena, is a general property of the
ultrastrong coupling limit. The set of models we consider encompasses different
scenarios of possible interplay between critical behaviour and superradiance.
