The Atacama Cosmology Telescope: measuring radio galaxy bias through cross-correlation with lensing

We correlate the positions of radio galaxies in the FIRST survey with the cosmic microwave background lensing convergence estimated from the Atacama Cosmology Telescope over 470 deg(2) to determine the bias of these galaxies. We remove optically cross-matched sources below redshift z = 0.2 to preferentially select active galactic nuclei (AGN). We measure the angular cross-power spectrum C-t(kg) at 4.4 sigma significance in the multipole range 100 < 1 < 3000, corresponding to physical scales within approximate to 2-60 Mpc at an effective redshift z(eff) = 1.5. Modelling the AGN population with a redshift-dependent bias, the cross-spectrum is well fitted by the Planck best-fitting A cold dark matter cosmological model. Fixing the cosmology and assumed redshift distribution of sources, we fit for the overall bias model normalization, finding b(z(eff)) = 3.5 +/- 0.8 for the full galaxy sample and b(z(eff)) = 4.0 +/- 1.1(3.0 +/- 1.1) for sources brighter (fainter) than 2.5 mJy. This measurement characterizes the typical halo mass of radio-loud AGN: we find log (M-halo/M-circle dot) = 13.6(-0.4)(+0.3).