A near-infrared study of a homogeneous sample of optically selected active galactic nuclei. II. Analysis of the results and interpretation

New imaging and photometric IR data have been used to discuss the properties of nuclei and hosting galaxies of a well-defined and large sample of optically selected Seyfert 1 galaxies. The K-band luminosity distribution of galaxies hosting the active nuclei has been derived, and the comparison with the local luminosity function of the spirals confirms that the probability of a spiral galaxy hosting bright AGNs increases with the luminosity. On average, the colors of the galaxies are slightly redder than those of spirals, suggesting that hosting galaxies usually have moderately enhanced star formation rates. The analysis of the correlation of the K band with IRAS fluxes demonstrates that the nuclei largely dominate the 12 mum emission, whereas the galaxies are the major contributors at lambda greater-than-or-equal-to 60 mum. It is also shown that galaxies with spectral index between 12 and 60 mum flatter than -1.5 are likely hosting an active nucleus. The IR spectral data of the nuclei as well as the correlation of the X-ray with IR emissions are briefly discussed. The conclusion is that the IR emission of the nuclei is probably dominated at lambda greater-than-or-equal-to 2.2 mum by thermal dust radiation.

New imaging and photometric IR data have been used to discuss the properties of nuclei and hosting galaxies of a well-defined and large sample of optically selected Seyfert 1 galaxies. The K-band luminosity distribution of galaxies hosting the active nuclei has been derived, and the comparison with the local luminosity function of the spirals confirms that the probability of a spiral galaxy hosting bright AGNs increases with the luminosity. On average, the colors of the galaxies are slightly redder than those of spirals, suggesting that hosting galaxies usually have moderately enhanced star formation rates. The analysis of the correlation of the K band with IRAS fluxes demonstrates that the nuclei largely dominate the 12 μm emission, whereas the galaxies are the major contributors at λ ≥ 60 μm. It is also shown that galaxies with spectral index between 12 and 60 μm flatter than -1.5 are likely hosting an active nucleus. The IR spectral data of the nuclei as well as the correlation of the X-ray with IR emissions are briefly discussed. The conclusion is that the IR emission of the nuclei is probably dominated at λ ≥ 2.2 μn by thermal dust radiation.