ATLAS. III. Dust Around Little Red Dots: Hydrogen Line Ratios beyond Dust-free Non-Case B Models

Tomokazu Kiyota, Masami Ouchi, Hiroto Yanagisawa, Makoto Ando, Yuichi Harikane, Yuta Kageura, Minami Nakane, Yurina Nakazato, Yoshiaki Ono, Yui Takeda

First listed 2026-08-12 | Last updated 2026-08-12

Abstract

Short digest

This ATLAS installment measures broad Balmer decrements in 20 LRDs, comprising five JWST/NIRSpec high-redshift sources and 15 local analogues, with careful fits that account for absorption and blended neighboring lines. Broad Hα/Hβ spans 6–30, far above Case B; joint Balmer–Paschen Cloudy modeling shows that high density can explain one local LRD, but the other two systems with Paschen constraints require substantial additional extinction even after non-Case-B radiative-transfer effects are included. Because the narrow-line decrements do not imply comparably strong reddening, the paper places the obscuring dust near the broad-line region and argues that dust obscuration may occur in at least roughly half of LRDs, potentially as a lower-column counterpart to an AGN dusty torus.

Key figures to inspect

• Figure 3. This is the population-level measurement that establishes the central anomaly: LRD broad Hα/Hβ ratios reach 6–30 and several exceed the maximum attainable in the dust-free Cloudy grid. It also places the high-redshift JWST sample, local LRDs, lower limits, and comparison SDSS AGNs on the same diagnostic.
• Figure 5. The Balmer–Paschen diagnostic directly breaks the dust-versus-density degeneracy. Comparing the three Paschen-measured LRDs with the dust-free Cloudy grid and reddening vector shows why non-Case-B gas alone succeeds for only one source while others require extinction.
• Figure 9. This figure connects the line-ratio inference to the available infrared evidence in the three local LRDs. Its comparison of inferred minimum broad-line attenuation, narrow-line attenuation, dust-template temperatures, and line widths supports dust associated with the compact broad-line-region environment rather than galaxy-scale reddening.
• Figure 10. The concluding schematic distills the paper's physical interpretation: an LRD can retain visible broad lines while line-of-sight dusty material attenuates the broad-line and continuum emission. It provides the clearest visual synthesis of the proposed lower-column-density analogue of a classical AGN torus.