CHARACTERIZATION OF OPTICALLY SELECTED STAR-FORMING KNOTS IN (U)LIRGs

We present a comprehensive characterization of the general properties (luminosity functions (LFs), mass, size, ages, etc.) of optically selected compact stellar objects (knots) in a representative sample of 32 low-z luminous and ultraluminous infrared galaxies ((U)LIRGs). It is important to understand the formation and evolution of these properties in such systems, which represent the most extreme cases of starbursts in the low-z universe. We have made use of high angular resolution Advanced Camera for Surveys images from the Hubble Space Telescope in F435W (similar to B) and F814W (similar to I) bands. The galaxies in the sample represent different interaction phases (first contact, pre-merger, merger, and post-merger) and cover a wide luminosity range (11.46 <= log (L-IR/L-circle dot) <= 12.54). With a median size of 32 pc, most of the nearly 3000 knots detected consist of complexes of star clusters. Some of the knots (similar to 15%) are so blue that their colors 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, where M proportional to R1.91+/-0.14, similar to that found in complexes of clusters in M51 and in giant molecular clouds. This suggests that the star formation within the knots is proportional to the gas density at any given radius. This relation does not depend significantly on either the infrared luminosity of the system or the interaction phase. The star formation of all the knots is characterized by LFs of the knots with slopes close to 2. Nevertheless, we see a marginally significant indication that the LF evolves with the interaction process, becoming steeper from early to advanced merger phases. Due to size-of-sample effects we are probably sampling knots in ULIRGs that intrinsically more luminous (by a factor of about four) than in less luminous systems. They also have sizes and are likely to have masses characteristic of clumps in galaxies at z greater than or similar to 1. Knots in post-mergers are on average (1.3-2 times) larger, more luminous (2 mag) in the I band, and 0.5 mag redder than those in systems in earlier phases. Two scenarios are briefly discussed: (1) the likely presence of relatively high extinction in the most advanced mergers and (2) the dissolution of the less massive clusters and/or their coalescence into more massive, evolved superclusters.