Comparison of Nuclear Starburst Luminosities between Seyfert 1 and 2 Galaxies Based on Near-infrared Spectroscopy

Abstract

We report on infrared K- (2-2.5 micron) and L-band (2.8-4.1 micron) slit spectroscopy of 23 Seyfert 1 galaxies in the CfA and 12 micron samples. A polycyclic aromatic hydrocarbon (PAH) emission feature at 3.3 micron in the L band is primarily used to investigate nuclear star-forming activity in these galaxies. The 3.3 micron PAH emission is detected in 10 sources (=43%), demonstrating that detection of nuclear star-formation in a significant fraction of Seyfert 1 galaxies is now feasible. For the PAH-detected nuclei, the surface brightness values of the PAH emission are as high as those of typical starbursts, suggesting that the PAH emission probes the putative nuclear starbursts in the dusty tori around the central active galactic nuclei (AGNs). The magnitudes of the nuclear starbursts are quantitatively estimated from the observed 3.3 micron PAH emission luminosities. The estimated starburst luminosities relative to some indicators of AGN powers in these Seyfert 1s are compared with 32 Seyfert 2s in the same samples that we have previously observed. We find that there is no significant difference in nuclear starburst to AGN luminosity ratios between Seyfert 1 and 2 galaxies, and that nuclear starburst luminosity positively correlates with AGN power in both types of Seyferts. Our results favor a slightly-modified AGN unification model, which predicts that nuclear starbursts occurring in the dusty tori of Seyferts are physically connected to the central AGNs, rather than the classical unification paradigm, in which the dusty tori simply hide the central AGNs of Seyfert 2s and reprocess AGN radiation as infrared dust emission in Seyferts. No significant differences in nuclear star formation properties are recognizable between Seyfert 1s in the CfA and 12 micron samples.