Abstract

Poster - Splinter Galaxies   (MW-0250)

Study of the formation of Hα and Hβ Emission in Little Red Dots with Radiative Transfer in Multiphase Gas

Seok-Jun Chang, Max Gronke, Jorryt Matthee, Charlotte Mason
Max Planck Institute for Astrophysics

Little Red Dots (LRDs), compact galaxies at z > 5 discovered with JWST, show broad Balmer emission lines, narrow absorption troughs, and strong Balmer breaks — signatures of dense, optically thick hydrogen gas whose small-scale structure has remained unknown. Using our 3D Monte Carlo radiative transfer code RT-scat, we show how scattering processes impact the formation of Balmer lines, Hα and Hβ. Building on this framework, we jointly model the Hα and Hβ profiles of 11 LRDs with JWST spectroscopy and independently constrain the n=2 hydrogen column density from the Balmer-break continuum. We uncover a fundamental puzzle: LRD spectra show deeper Hβ than Hα absorption, an ordering that is physically impossible in any homogeneous medium given the ~ 6 times larger Hα cross-section. We show that a multiphase medium composed of dense, optically thick clumps embedded in a diffuse, hot intercloud medium simultaneously resolves this discrepancy and reconciles the line-derived column density with the independent Balmer-break constraint, closing a 10^1-3 gap in the n = 2 column density left by homogeneous models. This multiphase structure is a necessary ingredient for any viable LRD model, regardless of the nature of the central engine, and is consistent with LRDs hosting proto-broad-line-region clouds assembling during a super-Eddington growth phase of an early supermassive black hole.