Impact of resonance, Raman, and Thomson scattering on hydrogen line formation in Little Red Dots

ABSTRACT Little Red Dots (LRDs) are compact sources at $z>5$ discovered through James Webb Space Telescope spectroscopy. Their spectra exhibit broad Balmer emission lines ($\gtrsim 1000\rm ~km~s^{-1}$), alongside absorption features and a pronounced Balmer break – evidence for a dense, neutral hydrogen medium, in which the $n = 2$ state is significantly populated. When interpreted as arising from active galactic nucleus 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. We model three scattering processes shaping hydrogen line profiles: resonance scattering by hydrogen in the $n=2$ state, Raman scattering of ultraviolet (UV) radiation by ground-state hydrogen, and Thomson scattering by free electrons. Using 3D Monte Carlo radiative transfer simulations, we examine their imprint on line shapes and ratios. Resonance scattering produces strong deviations from Case B flux ratios, clear differences between H$\alpha$ and H$\beta$, and encodes gas kinematics in line profiles but cannot broaden H$\beta$ due to conversion to Pa$\alpha$. While Raman scattering can yield broad wings, scattering of the UV continuum is disfavoured given the absence of strong full width at half-maximum variations across transitions. Raman scattering of higher Lyman-series emission can produce H$\alpha$/H$\beta$ wing width ratios of $\gtrsim 1.28$, agreeing with observations. Thomson scattering can reproduce the observed $\gtrsim 1000~\rm km\, s^{-1}$ wings under plausible conditions – e.g. $T_{\rm e} \sim 10^4$ K and $N_{\rm e}\sim 10^{24}\rm ~cm^{-2}$ – and lead to black hole mass overestimates by factors $\gtrsim 10$. Our results provide a framework for interpreting hydrogen lines in LRDs and similar systems.