Discovery of a Partially Obscured Supermassive Binary Black Hole System

Galaxy mergers are a crucial pathway for the growth and evolution of galaxies at the cosmic center. Supermassive black holes (SMBHs) are commonly present at the centers of galaxies. During galaxy mergers, the central SMBHs may form binary black hole systems and, in some cases, merge. Detecting and studying supermassive binary black hole systems within galaxies is a significant aspect of galaxy evolution research. Active black holes produce prominent broad emission lines, which serve as effective probes of such activity. The presence of two distinct sets of broad emission lines with velocity differences has traditionally been considered a key indicator for identifying binary black holes. However, the probability of both black holes in a binary system being active is extremely low. More commonly, binary systems consist of one active and one quiescent black hole. The spectral signature of such systems is characterized by a significant velocity offset between the broad emission lines and the system's narrow emission lines. This velocity difference has also led to the discovery of "recoiling" black holes, driven by gravitational wave radiation from closely bound binary systems. Recoiling black holes are crucial observational targets for studying black hole mergers, binary orbital evolution, and galaxy mergers.We have identified a class of recoiling black holes with partially obscured nuclear regions. Dust in the nuclear region attenuates the intense radiation from the active black hole, allowing the host galaxy's emission to become visible alongside the nuclear radiation and broad-line features. This discovery provides a unique perspective for exploring the physical connection between the evolution of binary black hole systems and galaxy evolution. Further studies using multi-wavelength photometry, high-resolution spectroscopic analysis, and long-term spectral monitoring of partially obscured supermassive binary black hole systems are expected to reveal the physical processes underlying black hole mergers and galaxy mergers, as well as their significant role in galaxy evolution.