Description
The James Webb Space Telescope has revealed a large number of galaxies with broad hydrogen/helium lines and bright infrared continua in increasing numbers at $z>4$. Unlike most known Active Galactic Nuclei (AGN), these systems, dubbed "Little Red Dots", lack characteristic X-ray or radio emission, as well as optical variability. Together with frequent Balmer absorption features in them, the evidence suggests that they have large column densities of neutral absorbing gas with a high covering fraction. In this work, for a sample of high-quality spectra we show that line broadening is dominated by electron scattering in dense ionized gas with only narrow line cores being explained by Doppler motion. As the electon-scattered lines yield system sizes less than tens of light days, their luminosities can only be explained by the SMBH accretion. Therefore, these high-redshift SMBHs have low masses ($10^5-10^7\,M_{\odot}$) and accrete close to the Eddington limit. They are embedded into Compton-thick gas, which likely absorbs some of the low-energy X-rays and suppresses the radio emission. This gas is likely accreted in a quasi-spherical structure that has not yet flattened into a torus-like system. Together, this evidence suggests that we are seeing an early stage of SMBH growth that has a unique set of observed characteristics.
