Hunting Supermassive Black Holes in the Early Universe

Abstract

Supermassive black holes (SMBHs) hold the essential ingredient into understanding the galaxy formation and evolution across cosmic time. The physical properties of SMBH are found to tightly correlated with host galaxies in local universe. Whether such relationship apply in the early universe remains debated, observation shows that active galactic nuclei, or quasars, already exist when the universe is less than 1 Gyr years old. This thesis focuses on using state-of-the-art instrument to highlight novel approaches and anticipated findings in the quest to characterize the earliest and faintest AGNs. We developed quasar survey by joining the next generation Rubin Legacy Survey of Space and Time (LSST) and Roman High-Latitude Wide Area Survey (HLWAS). We extend current empirical simulation of quasars and contaminants, low-redshift red galaxies and galactic cool dwarfs, to the surveys limit. We studied the optimal selection, which include colors, surface densities, and proper motion, to maximize quasar yields.Our result showed that Roman and Rubin surveys are able to reach 90% completeness while maintaining a minimum 10% selection efficiency. We are able to triple current z>6.5 quasar sample, with potential discovery of z>9 quasars existed at J~25. We investigated the potential of using variability to discover a faint high-redshift AGN. By comparing two-epoch photometry with long temporal separation, we use flux variability to identify outlier variables in extragalactic deep fields with archival Hubble Space Telescope (HST) and James Webb Space Telescope (JWST) data. We identify 44 variable AGNs at 4<z<7, and derive the AGN luminosity function which show elevated abundance compared with other conventional AGN/quasar selection. Assuming AGN variability is a ubiquitous properties of accreting SMBH, we estimate the variability of red, compact, high-redshift galaxies with V-shaped spectral energy distribution (SED), or known as "little red dots" (LRDs). We found that LRDs show little-to-none variability in rest-frame UV, and AGN contribute atmost 30% to the rest-frame UV light. In order to build a statistical large sample of high-redshift variables, we conducted systematic search of high-redshift transients and variables in COSMOS field with JWST data. We identify 13 z>4 transients and variables, and one z=7.03 variable OIII emitter, which could potentially be a low-luminosity AGN. The discovery of such system are crucial into understanding the AGN fraction and the BH-galaxy co-evolution.