Balmer Absorption Series and Broad Metal Lines in Two Luminous Little Red Dots
Abstract
Balmer absorption is common among little red dots (LRDs), but absorbers at or redward of systemic are rare, occurring in only ∼10−15\% of Hα absorbers. In this paper, we study two such exceptional cases with deep JWST/NIRSpec spectroscopy: 15 hr of high-resolution (G395H) observations of RUBIES-EGS-49140 (z=6.68), resolving the absorption in all four transitions from Hα through Hδ, and medium-resolution spectroscopy (10 hr of G235M, 2 hr of G395M) of UNCOVER-A2744-45924 (z=4.46; 1.7x magnification). Both sources are among the optically reddest and most luminous LRDs known, and both show deep, near-systemic Balmer absorption troughs. We find two systematic trends along the Balmer series: the absorption centroids become more redshifted toward higher-order transitions, while the absorbed equivalent widths decline only weakly with increasing order, far less than expected from the atomic optical-depth ratios for a single attenuating screen. Ca\,{\sc{ii}}\,K is detected in absorption in both sources, whose offset follows the Hα trough rather than the more redshifted higher-order Balmer lines. We further report the detection of a broad base in [Ne\,{\sc{iii}}]\,λ3870, along with broad [O\,{\sc{iii}}]\,λ4364, [O\,{\sc{iii}}]\,λ5008, and He\,{\sc{i}}\,λ5877,λ7067, while He\,{\sc{ii}}\,λ4687 remains undetected or weak. Standard AGN photoionization models cannot reproduce the observed line ratios, whereas AGNs with high gas densities provide a consistent explanation, as also indicated by the anomalously high He\,{\sc{i}}\,λ7067/λ5877 ratio. A possible explanation for the relative strengths of the Balmer absorption lines could be a dense, optically thick medium whose re-emission modifies their apparent absorption strengths, while the velocity progression may arise from stratification in the absorbing gas.
Related
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BibTeX
@misc{wang2026balmer,
title = {{Balmer Absorption Series and Broad Metal Lines in Two Luminous Little Red Dots}},
author = {Bingjie Wang and Jenny E. Greene and Hanpu Liu and Nicholas Kaaz and Gabriel B. Brammer and Raphael E. Hviding and Ivo Labbé and Joel Leja and Jorryt Matthee and Rohan P. Naidu and Alberto Torralba and Josephine F. W. Baggen and Nikko J. Cleri and Seiji Fujimoto and Lukas J. Furtak and Anna de Graaff and Michaela Hirschmann and Vasily Kokorev and Erini Lambrides and Ian McConachie and Erica J. Nelson and Adèle Plat and Weichen Wang and Adi Zitrin},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.22623},
eprint = {2609.22623},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.22623},
}
RIS
TY - UNPB
TI - Balmer Absorption Series and Broad Metal Lines in Two Luminous Little Red Dots
AU - Bingjie Wang
AU - Jenny E. Greene
AU - Hanpu Liu
AU - Nicholas Kaaz
AU - Gabriel B. Brammer
AU - Raphael E. Hviding
AU - Ivo Labbé
AU - Joel Leja
AU - Jorryt Matthee
AU - Rohan P. Naidu
AU - Alberto Torralba
AU - Josephine F. W. Baggen
AU - Nikko J. Cleri
AU - Seiji Fujimoto
AU - Lukas J. Furtak
AU - Anna de Graaff
AU - Michaela Hirschmann
AU - Vasily Kokorev
AU - Erini Lambrides
AU - Ian McConachie
AU - Erica J. Nelson
AU - Adèle Plat
AU - Weichen Wang
AU - Adi Zitrin
PY - 2026
DA - 2026/09/18
PB - arXiv
DO - 10.48550/arxiv.2609.22623
UR - https://arxiv.org/abs/2609.22623
AB - Balmer absorption is common among little red dots (LRDs), but absorbers at or redward of systemic are rare, occurring in only ∼10−15\% of Hα absorbers. In this paper, we study two such exceptional cases with deep JWST/NIRSpec spectroscopy: 15 hr of high-resolution (G395H) observations of RUBIES-EGS-49140 (z=6.68), resolving the absorption in all four transitions from Hα through Hδ, and medium-resolution spectroscopy (10 hr of G235M, 2 hr of G395M) of UNCOVER-A2744-45924 (z=4.46; 1.7x magnification). Both sources are among the optically reddest and most luminous LRDs known, and both show deep, near-systemic Balmer absorption troughs. We find two systematic trends along the Balmer series: the absorption centroids become more redshifted toward higher-order transitions, while the absorbed equivalent widths decline only weakly with increasing order, far less than expected from the atomic optical-depth ratios for a single attenuating screen. Ca\,{\sc{ii}}\,K is detected in absorption in both sources, whose offset follows the Hα trough rather than the more redshifted higher-order Balmer lines. We further report the detection of a broad base in [Ne\,{\sc{iii}}]\,λ3870, along with broad [O\,{\sc{iii}}]\,λ4364, [O\,{\sc{iii}}]\,λ5008, and He\,{\sc{i}}\,λ5877,λ7067, while He\,{\sc{ii}}\,λ4687 remains undetected or weak. Standard AGN photoionization models cannot reproduce the observed line ratios, whereas AGNs with high gas densities provide a consistent explanation, as also indicated by the anomalously high He\,{\sc{i}}\,λ7067/λ5877 ratio. A possible explanation for the relative strengths of the Balmer absorption lines could be a dense, optically thick medium whose re-emission modifies their apparent absorption strengths, while the velocity progression may arise from stratification in the absorbing gas.
ER -