The anticipated transition from a liquid to a magnetically-induced Wigner solid (MIWS) ground state in a two-dimensional electron system is investigated by bandgap photoluminescence (electron-valence band hole recombination) in two GaAs single heterojunctions of different densities and intersubband spacings. For Landau level filling factors ν< 1 5 the photoluminescence spectra are dominated by a new line, the temperature dependence of which is used to map a phase boundary which is situated close to the boundary associated with a pinned MIWS phase mapped by a variety of other techniques. A weak peak, split off to lower energy from the main PL emission, may also be of relevance to the MIWS. Comparison is made with optical data that probes this transition by electron-neutral acceptor recombination in Be δ-doped samples. In our higher density sample (∼ 1011cm-2), a temperature-dependent laser-induced 2D electron density depletion effect observed at high magnetic fields and low temperatures is investigated systematically. It has also been possible to study 2D electron to acceptor-bound hole recombination in this sample associated with residual, low concentration carbon impurity levels as a further probe of the QHE regime. Additionally, a definitive optical signature of the ν= 1 3 FQHE hierarchy is reported.