2025ApJ...991..141C

Understanding the origin and evolution of supermassive black holes (SMBHs) stands as one of the most important challenges in astrophysics and cosmology, with little current theoretical consensus. Improved observational constraints on the cosmological evolution of SMBH demographics are needed. Here we report results of a search via photometric variability for SMBHs appearing as active galactic nuclei (AGN) in the cosmological volume defined by the Hubble Ultra Deep Field. This work includes particular focus on a new observation carried out in 2023 with the Hubble Space Telescope using the WFC3/IR/F140W, which is compared directly to equivalent data taken 11 yr earlier in 2012. Two earlier pairs of observations from 2009 to 2012 with WFC3/IR/F105W and WFC3/IR/F160W are also analyzed. We identify 521, 188, and 109 AGN candidates as nuclear sources that exhibit photometric variability at a level of 2σ, 2.5σ, and 3σ, respectively, in at least one filter. This sample includes 13, 3, and 2 AGN candidates at redshifts z > 6, when the Universe was ≲900 Myr old. After variability and luminosity function (down to M_UV = -17 mag) completeness corrections, we estimate the comoving number density of SMBHs, n_SMBH(z). At z ≳ 6, n_SMBH ≳ 6 × 10^–3 cMpc^–3. At low z our observations are sensitive to AGN fainter than M_UV = -17 mag, and we estimate n_SMBH ≳ 10^–2 cMpc^–3. We discuss how these results place strong constraints on a variety of SMBH seeding theories.