Infrared spectrophotometry of three Seyfert galaxies and 3C 273

Spectrophotometry in the range 2.1-4.0 microns is presented for the Seyfert galaxies NGC 1068, NGC 4151 and Mrk 231 and the quasar 3C 273, together with broadband and narrowband observations of the Seyfert galaxies in the range 8-13 microns. The spectra of NGC 1068 and NGC 4151 are found to contain a significant component due to starlight, especially at shorter wavelengths. The nonstellar component in NGC 1068 is observed to fall off rapidly at wavelengths shorter than 4 microns, consistent with the interpretation of the excess beyond 5 microns as thermal reradiation by dust. Observations confirm the variability of NGC 4151, and indicate the presence of two components of the flux other than starlight: a nonthermal variable component predominant at shorter wavelengths and a constant, probably thermal component at wavelengths greater than 3 microns. Mrk 231 and 3C 273 exhibit no discernable stellar component and were not observed to vary by more than 10%. Evidence is obtained for a broad minimum in the 8 to 13 micron spectrum of Mrk 231, as well as possible structure between rest wavelengths of 2.8 and 2.9 microns, and the spectrum is not a power law. The spectrum of 3C 273 is consistent with a power law from 1.2 to 10 microns, with small but significant deviations.