Near-infrared K-band spectroscopic investigation of Seyfert 2 nuclei in the CfA and 12 micron samples.
IMANISHI M. and ALONSO-HERRERO A.
Abstract (from CDS):
We present near-infrared K-band slit spectra of the nuclei of 25 Seyfert 2 galaxies in the CfA and 12 µm samples. The strength of the CO absorption features at 2.3-2.4 µm produced by stars is measured in terms of a spectroscopic CO index. A clear anticorrelation between the observed CO index and the nuclear K-L color is present, suggesting that a featureless hot dust continuum heated by an active galactic nucleus (AGN) contributes significantly to the observed K-band fluxes in the nuclei of Seyfert 2 galaxies. After correction for this AGN contribution, we estimate nuclear stellar K-band luminosities for all sources and CO indices for sources with modestly large observed CO indices. The corrected CO indices for 10 (=40%) Seyfert 2 nuclei are found to be as high as those observed in star-forming or elliptical (=spheroidal) galaxies. We combine the K-band data with measurements of the L-band 3.3 µm polycyclic aromatic hydrocarbon (PAH) emission feature, another powerful indicator for star formation, and find that the 3.3 µm PAH to K-band stellar luminosity ratios are substantially smaller than those of starburst galaxies. Our results suggest that the 3.3 µm PAH emission originates in the putative nuclear starbursts in the dusty tori surrounding the AGNs, because of its high surface brightness, whereas the K-band CO absorption features detected at the nuclei are dominated by old bulge (=spheroid) stars and thus may not be a powerful indicator for the nuclear starbursts. We see no clear difference in the strength of the CO absorption and PAH emission features between the CfA and 12 µm Seyfert 2 galaxies.