Comparison of AGN and Nuclear Starburst Activity in Seyfert 1 and 2 Galaxies over a Wide Luminosity Range Based on Near-Infrared 2–4μm Spectroscopy

Abstract

Abstract We present near-infrared $K$- (1.9–2.5$\ \mu$m) and $L$- (2.8–4.2$\ \mu$m) band spectroscopy of 22 Seyfert nuclei. We used two methods to investigate the presence of nuclear starbursts: (1) the Polycyclic Aromatic Hydrocarbon (PAH) emission feature at $\lambda_{\rm rest}$$=$ 3.3$\ \mu$m in the rest frame of the $L$-band spectrum (a starburst indicator) and (2) the CO absorption feature at $\lambda_{\rm rest}$$=$ 2.3–2.4$\ \mu$m in the rest frame of the $K$-band spectrum, originating in the CO molecule. We clearly detected the 3.3$\ \mu$m PAH emission features in five objects and the CO absorption features in 17 objects. Seyfert 2 galaxies tend to show bluer $K$$-$$L$ colors compared with Seyfert 1 galaxies. We interpret the discrepancy as resulting from the relative strength of stellar emission because AGN emission is affected by dust extinction. The 3.3$\ \mu$m PAH emission luminosity ($L_{\rm 3.3PAH}$) distributions for the Seyfert 1s and Seyfert 2s are very similar when normalized to the AGN power. The star-formation rates estimated from $L_{\rm 3.3PAH}$ could be large enough to inflate the dusty torus by a supernova explosion. We find that $L_{\rm 3.3PAH}$ positively correlates with the $N$-band luminosity with small aperture over a wide AGN luminosity range, and is independent of the physical area we probed. The results suggest that the nuclear region has a concentration of star formation and it would control the AGN activity.