A theory of shake-up processes in photoabsorption of an interactinglow-density two-dimensional electron gas (2DEG) in strong magnetic fields ispresented. In these processes, an incident photon creates an electron-hole pairand, because of Coulomb interactions, simultaneously excites one particle tohigher Landau levels (LL's). In this work, the spectra of correlated chargedspin-singlet and spin-triplet electron-hole states in the first hole LL andoptical transitions to these states (i.e., shake-ups to the first hole LL) arestudied. Our results indicate, in particular, the presence of optically-activethree-particle quasi-discrete states in the exciton continuum that may giverise to surprisingly sharp Fano resonances in strong magnetic fields. Therelation between shake-ups in photoabsorption of the 2DEG and in the 2D holegas (2DHG), and shake-ups of isolated negative X^- and positive X^+ trions arediscussed.
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