Near‐infrared K ‐Band Spectroscopic Investigation of Seyfert 2 Nuclei in the CfA and 12 Micron Samples

Abstract

We present near-infrared K-band slit spectra of the nuclei of 25 Seyfert2 galaxies in the CfA and 12 {μ}m samples. The strength of the COabsorption features at 2.3-2.4 {μ}m produced by stars is measured interms of a spectroscopic CO index. A clear anticorrelation between theobserved CO index and the nuclear K-L color is present, suggesting thata featureless hot dust continuum heated by an active galactic nucleus(AGN) contributes significantly to the observed K-band fluxes in thenuclei of Seyfert 2 galaxies. After correction for this AGNcontribution, we estimate nuclear stellar K-band luminosities for allsources and CO indices for sources with modestly large observed COindices. The corrected CO indices for 10 (=40%) Seyfert 2 nuclei arefound to be as high as those observed in star-forming or elliptical(=spheroidal) galaxies. We combine the K-band data with measurements ofthe L-band 3.3 {μ}m polycyclic aromatic hydrocarbon (PAH) emissionfeature, another powerful indicator for star formation, and find thatthe 3.3 {μ}m PAH to K-band stellar luminosity ratios are substantiallysmaller than those of starburst galaxies. Our results suggest that the3.3 {μ}m PAH emission originates in the putative nuclear starbursts inthe dusty tori surrounding the AGNs, because of its high surfacebrightness, whereas the K-band CO absorption features detected at thenuclei are dominated by old bulge (=spheroid) stars and thus may not bea powerful indicator for the nuclear starbursts. We see no cleardifference in the strength of the CO absorption and PAH emissionfeatures between the CfA and 12 {μ}m Seyfert 2 galaxies.