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(abridged)We present a comprehensive characterization of the generalproperties (luminosity functions, mass, size, ages, etc) of optically selectedcompact stellar objects (knots) in a representative sample of 32 low-z(U)LIRGs. We use ACS images from the Hubble Space Telescope in F435W and F814Wbands. The galaxies in the sample represent different interaction phases andcover a wide luminosity range (11.46 \leq log (L_IR (Lsun)) \leq 12.54). With amedian size of 32 pc, most of the nearly 3000 knots detected consists ofcomplexes of star clusters. Some of the knots (~15%) are so blue that theircolors indicate a young (i.e., < 30 Myr) and almost extinction-free population.There is a clear correlation of the mass of these blue knots with their radius,M \propto R^{1.91\pm0.14}, similar to that found in complexes of clusters inM51 and in Giant Molecular Clouds. This suggests that the star formation withinthe knots is proportional to the gas density at any given radius. The starformation of all the knots is characterized by luminosity functions (LFs) ofthe knots with slopes close to 2. Though, we see a marginally significantindication that the LF evolves with the interaction process, becoming steeperfrom early to advanced merger phases. Due to size-of-sample effects we areprobably sampling knots in ULIRGs intrinsically more luminous (x4) than in lessluminous systems. They also have sizes and are likely to have massescharacteristic of clumps in galaxies at z \gtrsim 1. Knots in post-mergers areon average larger (x 1.3-2), more luminous (2 mag) in the I-band, and 0.5 magredder than those in systems in earlier phases. Two scenarios are brieflydiscussed: (1) the likely presence of relatively high extinction in the mostadvanced mergers; (2) the dissolution of the less massive clusters and/or theircoalescence into more massive, evolved superclusters.

Characterization of optically-selected star forming knots in (U)LIRGs