3.3 micron emission feature in infrared galaxies

Abstract

We present 3 micron 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 micron 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 micron feature are examined in the relation to star formation activity. Band luminosities of the 3.3 micron 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 micron feature to H2 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 photodissocation regions associated with relatively massive young stars. The enhancement of S(1) emission to the 3.3 micron feature in active galactic nuclei and some peculiar galaxies is noticed. The 3 micron spectrum of NGC 5506 seems to exhibit an absorption feature at 3.4 micron similar to that seen in Galactic center sources and other highly obscured objects.