WISE data as a photometric redshift indicator for radio AGN
  We show that mid-infrared data from the all-sky WISE survey can be used as a
robust photometric redshift indicator for powerful radio AGN, in the absence of
other spectroscopic or multi-band photometric information. Our work is
motivated by a desire to extend the well-known K-z relation for radio galaxies
to the wavelength range covered by the all-sky WISE mid-infrared survey. Using
the LARGESS radio spectroscopic sample as a training set, and the mid-infrared
colour information to classify radio sources, we generate a set of redshift
probability distributions for the hosts of high-excitation and low-excitation
radio AGN. We test the method using spectroscopic data from several other radio
AGN studies, and find good agreement between our WISE-based redshift estimates
and published spectroscopic redshifts out to z ~ 1 for galaxies and z ~ 3-4 for
radio-loud QSOs. Our chosen method is also compared against other
classification methods and found to perform reliably. This technique is likely
to be particularly useful in the analysis of upcoming large-area radio surveys
with SKA pathfinder telescopes, and our code is publicly available. As a
consistency check, we show that our WISE-based redshift estimates for sources
in the 843 MHz SUMSS survey reproduce the redshift distribution seen in the
CENSORS study up to z ~ 2. We also discuss two specific applications of our
technique for current and upcoming radio surveys; an interpretation of large
scale HI absorption surveys, and a determination of whether low-frequency
peaked spectrum sources lie at high redshift.
