3.3 MICRON EMISSION FEATURE IN INFRARED GALAXIES

We present 3 mu m spectra of infrared-bright galaxies M82, Arp 299 (NGC 3690 + IC 694), NGC 1614, NGC 2623, NGC 2992, NGC 3227, NGC 5033, NGC 5256, and NGC 5506. The 3.3 mu m feature was detected in M82, Arp 299, NGC 1614, and even in Seyfert galaxies NGC 3227 and NGC 5256. The intensities of the 3.3 mu m feature are examined in the relation to star formation activity. Band luminosities of the 3.3 mu m feature are roughly at the level of 0.1% of the far-infrared luminosity in galaxies where active star formation is proceeding. The ratio of the 3.3 mu m feature to H-2 v = 1-0 S(1) emission in starburst galaxies is in a range of 50-100. It is shown that these correlations can be explained by the physical processes in photodissociation regions associated with relatively massive young stars. The enhancement of S(1) emission to the 3.3 mu m feature in active galactic nuclei and some peculiar galaxies is noticed. The 3 mu m spectrum of NGC 5506 seems to exhibit an absorption feature at 3.4 mu m similar to that seen in Galactic center sources and other highly obscured objects.