Probing Demographics of Supermassive Black Holes and Host Galaxies in the Early Universe
Speaker: Masafusa Onoue (Waseda University)
Abstract:
Quasars in the distant universe are a key population for exploring the early growth history of supermassive black holes. In this talk, I will present recent JWST observations of moderate-luminosity quasars at z > 6. These quasars, originally discovered by the Hyper Suprime-Cam Subaru Strategic Program, occupy the luminosity range between the most luminous quasars and the much fainter AGN detected by JWST, thus bridging the luminosity gap. NIRCam imaging of a subset of this sample has detected stellar emission from quasar host galaxies — a feat that was not possible in the pre-JWST era due to the difficulty of detecting underlying galaxy light. Most host galaxies have stellar masses as high as ~10^10.5 solar masses. Among 12 observed, we found two galaxies that exhibit Balmer absorption lines in their NIRSpec rest-frame optical spectra — a direct signature that these accreting black holes are, to our surprise, hosted by quenching galaxies. Follow-up observations confirmed that one of these galaxies shows a clear photometric Balmer break and little excess in an H-alpha filter, confirming its quiescence. We measured the black hole–to–stellar mass ratios in this sample and found that they are not as extreme as those of more luminous quasars, which are overmassive in black hole mass by more than 1 dex. I will also present our ongoing JWST projects based on the HSC quasars, including the identification of dust-obscured broad-line quasars.