Galaxy bias of radio AGN and star-forming galaxies from MIGHTEE and CMB lensing

Abstract

Radio galaxies play a significant role in galaxy evolution through feedback mechanisms, making it important to characterize the environments in which they reside. We measure the cross-correlation between the positions of radio sources in the MIGHTEE survey and cosmic microwave background lensing measurements from the Atacama Cosmology Telescope over $20\ {\rm deg}^2$ to flux densities $S_{\rm 1.4\,GHz}>40\ {\rm μJy}$, detecting the signal at $\sim4σ$ significance. By combining this with auto-correlation measurements of low- and high-redshift radio samples, we constrain the redshift evolution of the galaxy bias parameter, $b(z)$. Fitting different models for the redshift evolution of the bias, we find that both a model inversely proportional to the linear growth factor, $b(z) = (1.63^{+0.19}_{-0.22})/D(z)$, and a model where bias grows linearly with redshift, $b(z) = (0.49^{+0.44}_{-0.32}) + (1.62^{+0.37}_{-0.51})z$, provide comparably good fits to the data, with the current measurements unable to distinguish between them. Using a radio luminosity function, we estimate the fraction of sources that are active galactic nuclei (AGNs) or star-forming galaxies (SFGs) as a function of redshift, and model the individual contributions of AGNs and SFGs to the total bias. This approach allows us to measure the bias of AGNs and SFGs without individual source classifications. Though no single model is favoured significantly, the best-fitting model gives $b_{\rm AGN}(z) = (2.60^{+0.65}_{-0.91})/D(z)$ compared to $b_{\rm SFG}(z) = (0.52^{+0.47}_{-0.34})/D(z)$. Incorporating SFG bias measurements from previous work as a prior tightens these constraints to $b_{\rm AGN}(z) = (1.96^{+0.37}_{-0.40})/D(z)$ and $b_{\rm SFG}(z) = (0.90^{+0.06}_{-0.06})/D(z)$.