2512.11050v1
Balmer Transition Signatures from Gas-Enshrouded, Dust-Poor Active Galactic Nuclei
Digest
Radiative-transfer calculations with CLOUDY through dust-free, dense gas show that LRD-like Balmer features—prominent absorption, strong Balmer breaks, and very large broad-line decrements—can arise without dust. At n_H ≳ 10^8–10^10 cm^-3, Balmer resonance scattering boosts Hα relative to higher-order lines so that multiple Balmer ratios converge to values that mimic dust reddening, resolving MIRI dust-budget tensions. When the Balmer break and broad Balmer lines originate in the same dense gas, their linked strengths constrain the density structure and imply a very small BLR gas mass of order 10 M_sun. This framework explains why LRD spectra resemble obscured AGN while remaining dust-poor and supports an early, gas-enshrouded black-hole growth phase with little recent nuclear star formation.
Key figures to inspect
- Fig. 1 (left): Transmitted SEDs through a dust-free slab at fixed N_H but increasing n_H; compare flux just blueward (≈3646 Å) and redward (≈4200 Å) to see where the Balmer break ramps up as the n=2 population rises.
- Fig. 1 (right): Balmer-break strength versus n_H for several N_H; identify the narrow density range where the break steepens rapidly (n_H ~10^8–10^10 cm^-3) to calibrate break depth as a density diagnostic.
- Fig. 2 (left): Emitted spectra stacked by n_H show evolving Hα, Hβ, Hγ and [O III]; read off the printed Hα/Hβ values to see how line ratios and continuum change as the gas approaches optical thickness in Balmer lines.
- Fig. 2 (right): Hα/Hβ versus n_H across N_H tracks with a dashed Case B line at 2.86 and markers where Hα and Pa become optically thick; inspect where the curves flatten to dust-like decrements, evidencing resonance scattering saturation.