The near-infrared properties of galactic nuclei

Infrared J, H, and K observations are given for the nuclei of 107 galaxies, including 44 nuclei with blue colors or strong emission lines but not classified as Seyferts, 33 nuclei classified morphologically as being bright and compact, and 30 Seyfert galaxies. Comparison of (U - B) and (J - K) colors shows that most galaxies with (J - K) greater than 1.1 are Seyferts regardless of the (U - B). Such near-infrared excesses are attributed primarily to nonthermal continua rather than dust reradiation, because galaxy nuclei with bright ultraviolet continua not classified spectroscopically as Seyferts do not show infrared colors different from those expected for composite starlight. Virtually all nuclei with emission lines less than 250 km/s in width have the near-infrared colors of starlight, which quantifies suspicions that both broad emission lines and near-infrared color excesses are consequences of nonthermal processes.