Unraveling the mysteries of supermassive black holes: formation, growth mechanisms, and their role in galaxy evolution

Abstract

Supermassive black holes (SMBHs) with masses of millions to billions of solar masses are central to our understanding of galaxy evolution and the cosmos. This review synthesizes current research on the formation, fueling, and growth mechanisms of SMBHs across cosmic time. We discuss theoretical frameworks regarding SMBH formation, including models such as direct collapse from primordial gas clouds and the hierarchical merging of stellar-mass black holes through accretion and mergers. The direct collapse model suggests that massive primordial gas clouds can collapse directly into SMBHs without forming stars, while the merger model posits that SMBHs grow through the merging of smaller black holes during galaxy collisions.We analyze the impacts of both major and minor galactic mergers on SMBH growth, particularly their role in triggering gas inflows and sustained accretion episodes. The review elaborates on specific fueling processes, such as cold gas inflows, which provide continuous material for accretion, and feedback mechanisms that regulate SMBH growth by stimulating or inhibiting gas inflow. For example, radiative feedback from active galactic nuclei can heat surrounding gas, preventing it from falling into the black hole, highlighting the balance between these processes in SMBH evolution.We aim to provide a comprehensive overview of the pathways through which SMBHs have evolved from the early Universe to their present state. This review highlights open questions in the field, particularly regarding the rapid growth of SMBHs in the early universe, and suggests future research directions.