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.
