Uchuu-²GC galaxies and AGN: cosmic variance forecasts of high-redshift AGN for JWST, Euclid, and LSST

Measurements of the luminosity function of active galactic nuclei (AGN) at high redshift (z greater than or similar to 6) are expected to suffer from field-to-field variance, including cosmic and Poisson variances. Future surv e ys, such as those from the Euclid telescope and JWST, will also be affected by field variance. We use the Uchuu simulation, a state-of-the-art cosmological N-body simulation with 2.1 trillion particles in a volume of 25.7 Gpc(3), combined with a semi-analytic galaxy and AGN formation model, to generate the Uchuu-nu(2)GC catalogue, publicly available, that allows us to investigate the field-to-field variance of the luminosity function of AGN. With this Uchuu-nu(2)GC model, we quantify the cosmic variance as a function of surv e y area, AGN luminosity, and redshift. In general, cosmic variance decreases with increasing surv e y area and decreasing redshift. We find that at z similar to 6 - 7, the cosmic variance depends weakly on AGN luminosity. This is because the typical mass of dark matter haloes in which AGN reside does not significantly depend on luminosity. Due to the rarity of AGN, Poisson variance dominates the total field-to-field variance, especially for bright AGN. We also examine the effect of parameters related to galaxy formation physics on the field variance. We discuss uncertainties present in the estimation of the faint-end of the AGN luminosity function from recent observations, and extend this to make predictions for the expected number of AGN and their variance for upcoming observations with Euclid , JWST, and the Le gac y Surv e y of Space and Time (LSST).