Broadband Optical and Infrared Observations of Seyfert Galaxies

Abstract

Broadband observations between 0.3 and 3.4 μ are reported for II bright Seyfert galaxies. The ultraviolet and two micron fluxes of NGC 4151 have both increased by ∼ 0.3 mag between 1970 and 1971. 3C 120 was constant to within about ± 0.1 mag in the infrared during a period when it varied in the optical by ∼ 0.8 mag. Comparison with published data suggests NGC 4051 is an optical variable. The spatial distribution of the emitted flux shortward of 2.2 μ suggests that the emission from Seyfert galaxies can be decomposed into a nuclear source showing ultraviolet and infrared excesses and an extended source with normal starlike colours. The U–B and B–V colours fall on the two-colour diagram in positions consistent with a mixture of light from sources with galaxy-like and quasar-like colours and can be used to derive the ratio of the light from the nuclear source to that from the surrounding galaxy. This ratio is used to separate the contributions of nuclear source and galaxy in the infrared. The infrared spectrum of the nuclear source in NGC 1068 is steeper than that in the other Seyfert galaxies and more closely resembles that of non-Seyfert infrared galaxies such as M82 and NGC 253 rather than that of quasars. A wide range exists in the proportion of blue light contributed by the nuclear sources but the (B–[2.2 μ])₀ colours of the combined emission range only by ± 0.3 mag implying large differences exist in the (B–[2.2 μ])₀ of the nuclear source alone, in contrast to the homogeneity of the purely optical colours.