Energy diagnoses of nine infrared luminous galaxies based on 3-4 micron spectra

The energy sources of nine infrared luminous galaxies (IRLGs) are diagnosed based on their ground-based 3-4 mum spectra. Both the equivalent width of the 3.3 mum polycyclic aromatic hydrocarbon (PAH) emission feature and the 3.3 mum PAH to far-infrared luminosity ratio (L-3.3/L-FIR) are analyzed. Assuming that nuclear compact starburst in these sources produces the 3.3 mum PAH emission as strongly as that in starburst galaxies with lower far-infrared luminosities, the following results are found. For six IRLGs, both the observed equivalent widths and the L-3.3/L-FIR ratios are too small to explain the bulk of their far-infrared luminosities by compact starburst activity, indicating that active galactic nucleus (AGN) activity is a dominant energy source. For the other three IRLGs, while the 3.3 mum PAH equivalent widths are within the range of starburst galaxies, the L-3.3/L-FIR ratios after correction for screen dust extinction are a factor of similar to3 smaller. The uncertainty in the dust extinction correction factor and in the scatter of the intrinsic L-3.3/L-FIR ratios for starburst galaxies does not allow a determination of the ultimate energy sources for these three IRLGs.