Abstract：The first two-and-a-half-year observations of JWST have unveiled an unexpectedly large number of accreting black holes with low-to-intermediate luminosities at 2 < z < 11. These JWST-selected AGN exhibit peculiar properties different from the local AGN as well as bright QSOs at high redshift. They appear over-massive compared to the stellar content of their host galaxies, generally show non-detection in the hard X-ray band, and show weak variability in the rest-frame optical. Among the broad-line AGN host galaxies selected by JWST, there is an even more puzzling population of red compact galaxies known as the “Little Red Dots” (LRDs), which show red optical colors, flat UV slopes, and strong Balmer breaks connecting the optical and UV regimes. The strong Balmer breaks, if originate from stellar populations, imply extremely high stellar masses and stellar mass surface densities potentially in tension with standard cosmology. Interestingly, recent discoveries of non-stellar Balmer absorption lines in these AGN imply obscuration of the accretion disks by large amount of high-density neutral gas, which potentially originate in the proximity of the broad-line region. We propose that the same neutral gas can produce the Balmer breaks via bound-free absorption without invoking massive stellar populations. The physical origin of the obscuring gas is currently unclear. On the galaxy scale, it might be related to the dense gas environments at early times, where star clusters are efficiently forming in the nuclear region and might have left peculiar chemical imprints, which are also revealed by JWST. Finally, the gas enshrouded AGN model can potentially explain the lack of hard X-ray emission, although an intrinsically steep X-ray slope from highly accreting black holes might also be needed.
Bio：Xihan Ji is currently a postdoc at Cavendish Lab and Kavli Institute for Cosmology at the University of Cambridge. He received his PhD in 2023 from the University of Kentucky. His research interests include chemical evolution of galaxies throughout the cosmic time and observations and photoionization modeling of AGN and their host galaxies. Specifically, his recent works focus on chemical abundances of galaxies/AGN and nuclear environments of AGN at high redshift with JWST observations. He is actively involved in the JWST programs JADES, GA-NIFS, and BlackTHUNDER, and the upcoming local-volume survey AMASE.