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We study the origin and properties of 'extra' or 'excess' central light inthe surface brightness profiles of gas-rich merger remnants. Combining a largeset of hydrodynamical simulations with data on observed mergers (spanning abroad range of profiles at various masses and degrees of relaxation), we showhow to robustly separate the physically meaningful extra light -- stellarpopulations formed in a compact central starburst during a gas-rich merger --from the outer profile established by violent relaxation acting on starsalready present in the progenitors prior to the final merger. This separationis sensitive to the profile treatment, and we demonstrate that certain fittingprocedures can yield physically misleading results. We show that our methodreliably recovers the younger starburst population, and examine how theproperties of this component scale with mass, gas content, and other aspects ofthe progenitors. We consider the time evolution of profiles in different bands,and estimate biases introduced by observational studies at different times andwavelengths. We show that extra light is ubiquitous in observed and simulatedgas-rich merger remnants, with sufficient mass (~3-30% of the stellar mass) toexplain the discrepancy in the maximum phase-space densities of ellipticals andtheir progenitor spirals. The nature of this central component providespowerful new constraints on the formation histories of observed systems.

Dissipation and Extra Light in Galactic Nuclei: I. Gas-Rich Merger Remnants