Deep Spitzer observations of infrared-faint radio sources: high-redshift   radio-loud AGN?
  Infrared-faint radio sources (IFRSs) are a rare class of object which are relatively bright at radio wavelengths but very faint at infrared and optical wavelengths. Here we present sensitive near-infrared observations of a sample of these sources taken as part of the Spitzer Extragalactic Representative Volume Survey (SERVS). Nearly all the IFRSs are undetected at a level of ~ 1 \mu$Jy in these new deep observations, and even the detections are consistent with confusion with unrelated galaxies. A stacked image implies that the median flux density is $S_{3.6\mu m} ~ 0.2$ \mu$Jy or less, giving extreme values of the radio-infrared flux density ratio. Comparison of these objects with known classes of object suggests that the majority are probably high-redshift radio-loud galaxies, possibly suffering from significant dust extinction. 
Ray P. Norris, Jose Afonso, Antonio Cava, Duncan Farrah, Minh T.   Huynh, R. J. Ivison, Matt Jarvis, Mark Lacy, Minnie Mao, ...
astro-ph.CO