We have used HST /NICMOS broad-band ( at 1.6 \mu m ) and narrow-band Pa \alpha ( \lambda _ { rest } = 1.87 \mu m ) images to identify star clusters and H ii regions respectively in a sample of 8 luminous infrared galaxies ( LIRGs ) .
These observations have revealed the presence of a large population of super star clusters and bright H ii regions .
A significant fraction of the H ii regions shows H \alpha luminosities above that of 30 Doradus , the prototypical giant H ii region .
The excess of extremely luminous H ii regions in LIRGs has been confirmed by comparison with normal galaxies observed at similar spatial resolutions .
Despite the large numbers of identified star clusters and H ii regions in LIRGs , we only find a small fraction of coincidences , between 4 % and 30 % of the total number of detected sources .
Using evolutionary synthesis models we have reproduced the relative fractions of young H ii regions , intermediate and old star clusters observed in Arp 299 and the central region NGC 3256 using a Salpeter IMF and instantaneous star formation .
H ii regions with no detected near-infrared cluster counterpart ( 25 - 39 % of the detected sources ) represent the youngest sites of star formation , with ages of up to approximately 5 Myr and mostly intermediate mass ( \simeq 10 ^ { 5 } { M } _ { \odot } ) ionizing clusters .
For these two galaxies , and within the present detection threshold we can only detect coincidences ( 4 - 10 % of the detected sources ) between an H ii region and a near-infrared star cluster for the most massive star clusters ( \simeq 10 ^ { 6 } { M } _ { \odot } ) during the first 7 Myr of their evolution .
If there is significant extinction during the first million years , we may not detect the youngest star forming regions , and hence the observed fractions of H ii regions and coincidences will be lower limits .
The identified near-infrared SSCs with no detectable Pa \alpha emission represent the “ old ” population ( 53 - 66 % of the detected sources ) , with ages of between 7 and 20 - 40 Myr .
Older clusters possibly created in this or previous episodes of star formation are likely to exist in these systems but can not be identified with the present detection threshold .
Our study demonstrates that Pa \alpha narrow-band imaging of LIRGs and interacting galaxies identifies the youngest sites of star formation that could be otherwise missed by near-infrared broad-band continuum surveys .