Flat-Spectrum Radio Sources with Faint Optical Counterparts

For the last 15 years, We have monitored the infrared variability of 14 flat-spectrum radio sources with very faint optical-infrared counterparts and of the steep spectrum source 3C 422. We find that variability is a salient property of most of these objects, demonstrating that the near-infrared is sampling directly the output of the active galactic nuclei (AGNs). However, the infrared-to-optical continua tend to be so steep that these sources are frequently classified as flat-spectrum radio galaxies, and many of them have narrow, moderate-ionization emission-line spectra in support Of this classification. We conclude that many radio galaxy identifications of flat-spectrum radio sources would have been classified as quasars with infrared observations. The red colors that cause these AGNs to drop from sight in the visible appear in many cases to arise from reddening within the QSO host galaxy or a foreground galaxy along the line of sight. However, the faint identifications are not all of this type, but they include traditional high-redshift galaxies, sources with intrinsically red continua, and objects that have anomalously faint optical outputs for their radio flux densities. For example, the steepest intrinsic nonthermal continua appear to have spectral indices >= -2.5 between the infrared and optical. One object in our sample, 0742+103, and a closely related object, 1413+349, have ratios of infrared-optical to radio luminosity far below the average for these sources and may represent a rare class of truly optically quiet quasar.