Recent JWST insights into Little Red Dots and unresolved mysteries
Speaker: Kohei Ichikawa (Waseda University)
Abstract:
The James Webb Space Telescope (JWST) has unveiled numerous massive black holes (BHs) in faint, broad-line active galactic nuclei (AGN), even though their survey areas are only ~10^2 arcmin^2, which were previously considered too small for significant number of AGN discoveries before the JWST launch. This discovery highlights the presence of a dust-reddened AGN population, referred to as little red dots (LRDs), which are more abundant than X-ray selected AGNs that are less influenced by obscuration. In this talk, we summarize the recent observational properties of LRDs and discuss what we can learn from them about the growth of supermassive black holes (SMBHs) in the early Universe at z>=5. The large number density of LRDs indicates that the cosmic growth rate of BHs within this population does not decrease but rather increases at higher redshifts beyond z~6. The BH accretion rate density deduced from their luminosity function is significantly higher than that from other AGN surveys in X-ray and infrared bands. To align the cumulative mass density accreted by BHs with the observed BH mass density at z~4-5, as derived from the integration of the BH mass function, the radiative efficiency must be doubled from the canonical 10% value, achieving significance beyond the >2sigma confidence level.
This suggests the presence of rapid spins, with 96% of the maximum limit among these BHs, maintained by prolonged mass accretion instead of chaotic accretion with randomly oriented inflows. Accordingly, we propose a hypothesis that the dense, dust-rch environments within LRDs facilitate the emergence of rapidly spinning and overmassive BH populations during the epoch of reionization. This scenario predicts a potential association between relativistic jets and other high-energy phenomena with overmassive BHs in the early universe. If time allows, we will explore the future prospects of wide-area surveys such as UNIONS and Euclid as well as VLA 3 GHz radio surveys, which could further uncover high-luminosity and/or low-z analogs of LRDs.