ATLAS. III. Dust Around Little Red Dots: Hydrogen Line Ratios beyond Dust-free Non-Case B Models
Year: 2026
Type: preprint
Abstract: We investigate broad hydrogen line ratios in little red dots (LRDs) using five high-redshift ($z>2$) sources from JWST/NIRSpec medium/high-resolution spectra in the DAWN JWST Archive and fifteen low-redshift sources ($z=0.1$--$0.9$) from the literature, all with broad H$β$ detected at $>5σ$. After carefully measuring the broad-line fluxes while accounting for absorption features and neighboring emission lines, we find that the broad H$α$/H$β$ ratios are very high, ranging from 6 to 30, well above the Case B recombination value. Using plane-parallel \textsc{Cloudy} photoionization models with the total line emission from both sides of the slab, we investigate the physical origin of the broad hydrogen line ratios beyond Case B, jointly modeling the Balmer and Paschen line ratios for the subset of one high-$z$ and two low-$z$ LRDs with detected broad Paschen lines. We find that one low-$z$ LRD is reproduced by a high gas number density ($\log (n_{\mathrm H}/{\rm cm^{-3}})\sim10$--11) on the broad-line H$α$/H$β$--Pa$β$/Pa$γ$ plane, whereas the remaining two LRDs additionally require substantial dust extinction of $E(B-V)\gtrsim0.2$, even after accounting for non-Case B effects. Since the narrow H$α$/H$β$ ratios do not indicate such large dust extinction, these results demonstrate that the obscuring dust is spatially associated with the broad-line region. Even without Paschen-line measurements, two and eight LRDs in the high- and low-redshift samples, respectively, exhibit H$α$/H$β>13$, which cannot be reproduced by the non-Case B models, suggesting that dust obscuration might be common among LRDs, occurring in at least about half of the population. Such dust may represent a lower-column-density counterpart of the dusty torus in AGNs, reconciling the weak hot-dust emission.
Source: arXiv (Cornell University)
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Funders: National Aeronautics and Space Administration, Space Telescope Science Institute, Flatiron Health, National Research Foundation, Ministry of Education, Culture, Sports, Science and Technology, Danmarks Grundforskningsfond, National Astronomical Observatory of Japan, Japan Society for the Promotion of Science, Institute for Cosmic Ray Research, University of Tokyo
Awards: G103270039, G1338664626, ＪＷＳＴとＰＦＳのα/Ｆｅ測定で探る遠方銀河の超新星爆発と天の川銀河の恒星の起源, 徹底的なJWST研究で迫る初代銀河と極金属欠乏初期銀河の統計的性質, 可視・近赤外分光で探る宇宙再電離の起源天体, G256731684, Uncovering Early Cosmic Evolution with JWST and Subaru/PFS Spectroscopy and Theoretical Modeling, Cosmic Dawn Center (DAWN), Investigating the first star formation and reionization process of the Universe by MWA, G5568641063, G6531013582, G6603402766, Comprehensive understanding of the formation history of structures in the Universe, 極金属欠乏銀河のヘリウム組成比で探る宇宙のレプトン非対称性, First galaxy formation probed by thorough analysis of JWST data
OpenAlex ID: W7202292090