Radio Galaxy populations and the multi-tracer technique: pushing the
  limits on primordial non-Gaussianity
  We explore the use of different radio galaxy populations as tracers of
different mass halos and therefore, with different bias properties, to
constrain primordial non-Gaussianity of the local type. We perform a Fisher
matrix analysis based on the predicted auto and cross angular power spectra of
these populations, using simulated redshift distributions as a function of
detection flux and the evolution of the bias for the different galaxy types
(Star forming galaxies, Starburst galaxies, Radio-Quiet Quasars, FRI and FRII
AGN galaxies). We show that such a multi-tracer analysis greatly improves the
information on non-Gaussianity by drastically reducing the cosmic variance
contribution to the overall error budget. By using this method applied to
future surveys, we predict a constraint of sigma_fnl=3.6 on the local
non-Gaussian parameter for a galaxy detection flux limit of 10 \muJy and
sigma_fnl=2.2 for 1 \muJy. We show that this significantly improves on the
constraints obtained when using the whole undifferentiated populations
(sigma_fnl=48 for 10 \muJy and sigma_fnl=12 for 1 \muJy). We conclude that
continuum radio surveys alone have the potential to constrain primordial
non-Gaussianity to an accuracy at least a factor of two better than the present
constraints obtained with Planck data on the CMB bispectrum, opening a window
to obtain sigma_fnl~1 with the Square Kilometer Array.
