Impact of Resonance, Raman, and Thomson Scattering on Hydrogen Line Formation in Little Red Dots

Understand

Little Red Dots (LRDs) are compact sources at $z>5$ discovered through JWST spectroscopy.

• Their spectra exhibit broad Balmer emission lines ($\gtrsim1000\rm~km~s^{-1}$), alongside absorption features and a pronounced Balmer break -- evidence for a dense, neutral hydrogen medium with the $n=2$ state.
• When interpreted as arising from AGN broad-line regions, inferred black hole masses from local scaling relations exceed expectations given their stellar masses, challenging models of early black hole-galaxy co-evolution.
• However, radiative transfer effects in dense media may also impact the formation of hydrogen emission lines.