We present a comprehensive characterization of the general properties ( luminosity functions , mass , size , ages , etc ) of optically selected compact stellar objects ( knots ) in a representative sample of 32 low-z Luminous and Ultraluminouos 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 ACS images from the Hubble Space Telescope in F435W ( \sim B ) and F814W ( \sim 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 \leq log ( L _ { IR } / L _ { \odot } ) \leq 12.54 ) .
With a median size of 32 pc , most of the nearly 3000 knots detected consists of complexes of star clusters .
Some of the knots ( \sim 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 \propto R ^ { 1.91 \pm 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 on the interaction phase .
The star formation of all the knots is characterized by luminosity functions ( 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 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 \gtrsim 1 .
Knots in post-mergers are on average larger ( \times 1.3-2 ) , 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 ; ( 2 ) the dissolution of the less massive clusters and/or their coalescence into more massive , evolved superclusters .