Evolution in the bias of faint radio sources to z ~ 2.2

Quantifying how the baryonic matter traces the underlying dark matter distribution is key to both understanding galaxy formation and our ability to constrain the cosmological model. Using the cross-correlation function of radio and near-infrared galaxies, we present a large-scale clustering analysis of radio galaxies to z ~ 2.2. We measure the angular auto-correlation function of K_s<23.5 galaxies in the VISTA Deep Extragalactic Observations (VIDEO-XMM3) field with photometric redshifts out to z = 4 using VIDEO and CFHTLS (Canada-France-Hawaii Telescope Legacy Survey) photometry in the near-infrared and optical. We then use the cross-correlation function of these sources with 766 radio sources at S_1.4>90μJy to infer linear bias of radio galaxies in four redshift bins. We find that the bias evolves from b = 0.57 ± 0.06 at z ~ 0.3 to 8.55 ± 3.11 at z ~ 2.2. Furthermore, we separate the radio sources into subsamples to determine how the bias is dependent on the radio luminosity, and find a bias which is significantly higher than predicted by the simulations of Wilman et al., and consistent with the lower luminosity but more abundant FR-I population having a similar bias to the highly luminous but rare FR-IIs. Our results are suggestive of a higher mass, particularly for FR-I sources than assumed in simulations, especially towards higher redshift.