We develop a clustering-based redshift estimation approach for CMB lensing tomography, focusing on the kernel function of the lensing galaxies. Within a linear galaxy bias framework, we derive estimators for this kernel from two-point cross-correlations between lens mass and reference samples. The reconstructed kernel then enables a theoretical prediction for the angular cross-power spectrum \(C_{ g\kappa}\) between CMB lensing convergence and lens galaxies. As a proof of concept, we measure \(C_{ g\kappa}\) by correlating the \emph{ Planck} PR4 convergence map with NVSS+SUMSS radio galaxies (\(0\lesssim z\lesssim 3\)). We estimate the radio-galaxy kernel by collectively cross-correlating their distribution with spectroscopic and photometric surveys (2MPZ, LOWZ-CMASS, eBOSS DR16 LRGs, and Gaia-unWISE QSOs). From the measured \(C_{ g\kappa}\), we obtain \(\sigma_8 = 0.86^{ +0.12}_{ -0.09}\) when the density parameter is set to the { \it Planck} value of $\Omega_m = 0.315$; this is in good agreement with the \emph{ Planck} normalisation of $\sigma_8 = 0.812$.
Categories: astro-ph.CO astro-ph.GA