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.

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