DEEP<i>SPITZER</i>OBSERVATIONS OF INFRARED-FAINT RADIO SOURCES: HIGH-REDSHIFT RADIO-LOUD ACTIVE GALACTIC NUCLEI?

Abstract

Infrared-faint radio sources (IFRSs) are a rare class of object which are\nrelatively bright at radio wavelengths but very faint at infrared and optical\nwavelengths. Here we present sensitive near-infrared observations of a sample\nof these sources taken as part of the Spitzer Extragalactic Representative\nVolume Survey (SERVS). Nearly all the IFRSs are undetected at a level of ~ 1\n\\mu$Jy in these new deep observations, and even the detections are consistent\nwith confusion with unrelated galaxies. A stacked image implies that the median\nflux density is $S_{3.6\\mu m} ~ 0.2$ \\mu$Jy or less, giving extreme values of\nthe radio-infrared flux density ratio. Comparison of these objects with known\nclasses of object suggests that the majority are probably high-redshift\nradio-loud galaxies, possibly suffering from significant dust extinction.\n