Near-infrared and star-forming properties of local luminous infrared galaxies

We use Hubble Space Telescope (HST) NICMOS continuum and Pa alpha observations to study the near-infrared and star formation properties of a representative sample of 30 local (d similar to 35-75 Mpc) luminous infrared galaxies (LIRGs, infrared [8 - 1000 mu m] luminosities of log L-IR 11-11.9 L-circle dot). The data provide spatial resolutions of 25 - 50 pc and cover the central similar to 3.3 - 7.1 kpc regions of these galaxies. About half of the LIRGs show compact (similar to 1 - 2 kpc) Pa alpha emission with a high surface brightness in the form of nuclear emission, rings, and minispirals. The rest of the sample show Pa alpha emission along the disk and the spiral arms extending over scales of 3 - 7 kpc and larger. About half of the sample contains H (II) regions with H alpha luminosities significantly higher than those observed in normal galaxies. There is a linear empirical relationship between the mid-IR 24 mu m and hydrogen recombination (extinction-corrected Pa alpha) luminosity for these LIRGs, and the H (II) regions in the central part of M51. This relation holds over more than four decades in luminosity, suggesting that the mid-IR emission is a good tracer of the star formation rate (SFR). Analogous to the widely used relation between the SFR and total IR luminosity of R. Kennicutt, we derive an empirical calibration of the SFR in terms of the monochromatic 24 mu m luminosity that can be used for luminous, dusty galaxies.