Calcium Triplet Absorption is Common around Little Red Dots
Abstract
We present new evidence for an optically thick atmosphere surrounding Little Red Dots (LRDs) in the form of Ca absorption at rest-frame 8500A, the calcium triplet (CaT). Building on the detection of CaT absorption in one local LRD analog (the "Egg", Lin et. al. 2025), we investigate the region around rest-frame 8500A for an archival sample of 17 LRDs (2 < z < 5) with JWST/NIRSpec grating data. We detect CaT absorption in three individual sources, and find that on average there is an absorption equivalent width (EW) EWCaT ≈ −5 A. Among the three detections, two are approaching the deepest absorption seen in integrated light from stars. Given that the continuum around CaT is probably dominated by the central engine, we argue that the absorbers are likely to be associated with the LRD. Such absorption has not historically been associated with any components of an AGN central engine, but a cool, optically thick photosphere as proposed for LRDs would naturally produce such absorption. The EWCaT that we observe can be matched by hydrostatic atmosphere models at relatively low metallicity ([M/H]<−1) combined with an effective temperature Teff > 4500 K. Alternatively, the distribution can be matched at a low photospheric gas density ρph<10−11~g cm−3 that requires a non-hydrostatic gas structure on dynamical grounds. In the future, metal absorption lines should be a powerful complementary probe of the gas conditions, and possibly the enclosed mass, of LRDs.
Related
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- ATLAS. III. Dust Around Little Red Dots: Hydrogen Line Ratios beyond Dust-free Non-Case B Models
- A Black Hole Star at Cosmic Noon: Extreme Balmer Break, Photospheric Continuum, and Broad Absorption by Thick Winds in a Little Red Dot at z = 1.7
BibTeX
@misc{greene2026calcium,
title = {{Calcium Triplet Absorption is Common around Little Red Dots}},
author = {Jenny E. Greene and Hanpu Liu and Raphael E. Hviding and Rohan P. Naidu and David J. Setton and Bingjie Wang and Ivo Labbe and Vasily Kokorev and Xiaojing Lin and Hollis B. Akins and Anna de Graaff and Joel Leja and Jorryt Matthee and Alberto Torralba and Karl Glazebrook and Rachel Bezanson and Leindert A. Boogaard and Gabriel Brammer and Caitlin M. Casey and John Chisholm and Xiaohui Fan and Lukas Furtak and Seiji Fujimoto and Yan-Fei Jiang and Yilun Ma and Themiya Nanayakkara and Erica J. Nelson and Richard Pan and Eliot Quataert and Anthony J. Taylor and Christina Williams and Adi Zitrin},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.25349},
eprint = {2609.25349},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.25349},
}
RIS
TY - UNPB
TI - Calcium Triplet Absorption is Common around Little Red Dots
AU - Jenny E. Greene
AU - Hanpu Liu
AU - Raphael E. Hviding
AU - Rohan P. Naidu
AU - David J. Setton
AU - Bingjie Wang
AU - Ivo Labbe
AU - Vasily Kokorev
AU - Xiaojing Lin
AU - Hollis B. Akins
AU - Anna de Graaff
AU - Joel Leja
AU - Jorryt Matthee
AU - Alberto Torralba
AU - Karl Glazebrook
AU - Rachel Bezanson
AU - Leindert A. Boogaard
AU - Gabriel Brammer
AU - Caitlin M. Casey
AU - John Chisholm
AU - Xiaohui Fan
AU - Lukas Furtak
AU - Seiji Fujimoto
AU - Yan-Fei Jiang
AU - Yilun Ma
AU - Themiya Nanayakkara
AU - Erica J. Nelson
AU - Richard Pan
AU - Eliot Quataert
AU - Anthony J. Taylor
AU - Christina Williams
AU - Adi Zitrin
PY - 2026
DA - 2026/09/21
PB - arXiv
DO - 10.48550/arxiv.2609.25349
UR - https://arxiv.org/abs/2609.25349
AB - We present new evidence for an optically thick atmosphere surrounding Little Red Dots (LRDs) in the form of Ca absorption at rest-frame 8500A, the calcium triplet (CaT). Building on the detection of CaT absorption in one local LRD analog (the "Egg", Lin et. al. 2025), we investigate the region around rest-frame 8500A for an archival sample of 17 LRDs (2 < z < 5) with JWST/NIRSpec grating data. We detect CaT absorption in three individual sources, and find that on average there is an absorption equivalent width (EW) EW_CaT ≈ −5 A. Among the three detections, two are approaching the deepest absorption seen in integrated light from stars. Given that the continuum around CaT is probably dominated by the central engine, we argue that the absorbers are likely to be associated with the LRD. Such absorption has not historically been associated with any components of an AGN central engine, but a cool, optically thick photosphere as proposed for LRDs would naturally produce such absorption. The EW_CaT that we observe can be matched by hydrostatic atmosphere models at relatively low metallicity ([M/H]<−1) combined with an effective temperature T_eff > 4500 K. Alternatively, the distribution can be matched at a low photospheric gas density ρ_ph<10^−11~g cm^−3 that requires a non-hydrostatic gas structure on dynamical grounds. In the future, metal absorption lines should be a powerful complementary probe of the gas conditions, and possibly the enclosed mass, of LRDs.
ER -