The radio properties of infrared-faint radio sources
  Infrared-faint radio sources (IFRS) are objects that have flux densities of
several mJy at 1.4GHz, but that are invisible at 3.6um when using sensitive
Spitzer observations with uJy sensitivities. Their nature is unclear and
difficult to investigate since they are only visible in the radio.
High-resolution radio images and comprehensive spectral coverage can yield
constraints on the emission mechanisms of IFRS and can give hints to
similarities with known objects. We imaged a sample of 17 IFRS at 4.8GHz and
8.6GHz with the Australia Telescope Compact Array to determine the structures
on arcsecond scales. We added radio data from other observing projects and from
the literature to obtain broad-band radio spectra. We find that the sources in
our sample are either resolved out at the higher frequencies or are compact at
resolutions of a few arcsec, which implies that they are smaller than a typical
galaxy. The spectra of IFRS are remarkably steep, with a median spectral index
of -1.4 and a prominent lack of spectral indices larger than -0.7. We also find
that, given the IR non-detections, the ratio of 1.4GHz flux density to 3.6um
flux density is very high, and this puts them into the same regime as
high-redshift radio galaxies. The evidence that IFRS are predominantly
high-redshift sources driven by active galactic nuclei (AGN) is strong, even
though not all IFRS may be caused by the same phenomenon. Compared to the rare
and painstakingly collected high-redshift radio galaxies, IFRS appear to be
much more abundant, but less luminous, AGN-driven galaxies at similar
cosmological distances.
