2024A&A...681A.105N

Aims. We combined the LOw-Frequency ARray (LOFAR) Two-metre Sky Survey (LoTSS) second data release (DR2) catalogue with gravitational lensing maps from the cosmic microwave background (CMB) to place constraints on the bias evolution of LoTSS-detected radio galaxies, and on the amplitude of matter perturbations.
Methods. We constructed a flux-limited catalogue from LoTSS DR2, and analysed its harmonic-space cross-correlation with CMB lensing maps from Planck, C_ℓ^gk, as well as its auto-correlation, C_ℓ^gg. We explored the models describing the redshift evolution of the large-scale radio galaxy bias, discriminating between them through the combination of both C_ℓ^gk and C_ℓ^gg. Fixing the bias evolution, we then used these data to place constraints on the amplitude of large-scale density fluctuations, parametrised by σ₈.
Results. We report the significance of the C_ℓ^gk signal at a level of 26.6σ. We determined that a linear bias evolution of the form b_g(z) = b_g,D/D(z), where D(z) is the growth rate, is able to provide a good description of the data, and we measured b_g,D = 1.41 ± 0.06 for a sample that is flux limited at 1.5 mJy, for scales ℓ < 250 for C_ℓ^gg, and ℓ < 500 for C_ℓ^gk. At the sample's median redshift, we obtained b(z = 0.82) = 2.34 ± 0.10. Using σ₈ as a free parameter, while keeping other cosmological parameters fixed to the Planck values, we found fluctuations of σ₈ = 0.75_–0.04^+0.05. The result is in agreement with weak lensing surveys, and at 1σ difference with Planck CMB constraints. We also attempted to detect the late-time-integrated Sachs-Wolfe effect with LOFAR data; however, with the current sky coverage, the cross-correlation with CMB temperature maps is consistent with zero. Our results are an important step towards constraining cosmology with radio continuum surveys from LOFAR and other future large radio surveys.