  We study the quantum Hall liquid and the metal-insulator transition in a high
mobility two dimensional electron gas, by means of photoluminescence and
magneto-transport. In the integer and fractional regime at nu > 1/3, analyzing
the emission energy dispersion we probe the magneto-Coulomb screening and the
hidden symmetry of the electron liquid. In the fractional regime above above nu
=1/3 the system undergoes the metal-to-insulator transition, and in the
insulating phase the dispersion becomes linear with evidence of an increased
renormalized mass.
