Calcium Triplet Absorption is Common around Little Red Dots

Jenny E. Greene, Hanpu Liu, Raphael E. Hviding, Rohan P. Naidu, David J. Setton, Bingjie Wang, Ivo Labbe, Vasily Kokorev, Xiaojing Lin, Hollis B. Akins, Anna de Graaff, Joel Leja, Jorryt Matthee, Alberto Torralba, Karl Glazebrook, Rachel Bezanson, Leindert A. Boogaard, Gabriel Brammer, Caitlin M. Casey, John Chisholm, Xiaohui Fan, Lukas Furtak, Seiji Fujimoto, Yan-Fei Jiang, Yilun Ma, Themiya Nanayakkara, Erica J. Nelson, Richard Pan, Eliot Quataert, Anthony J. Taylor, Christina Williams, Adi Zitrin

First listed 2026-09-23 | Last updated 2026-09-21

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) EW$_{\rm CaT} \approx -5$ 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$_{\rm CaT}$ that we observe can be matched by hydrostatic atmosphere models at relatively low metallicity ([M/H]$<-1$) combined with an effective temperature $T_{\rm eff} > 4500$ K. Alternatively, the distribution can be matched at a low photospheric gas density $ρ_{\rm ph}<10^{-11}{\rm~g~cm^{-3}}$ 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.

Short digest

Using JWST/NIRSpec grating spectra of 17 spectroscopically identified little red dots at 2 < z < 5, Greene et al. search the rest-frame 8500 Å region for calcium-triplet absorption, a stellar-atmosphere-like feature previously seen in the local LRD analog the Egg. They detect CaT absorption in three individual sources, including the robust JADES-28074/Rosetta Stone case, and obtain a stacked mean equivalent width of roughly −5 Å, arguing that the absorption is associated with the central engine rather than host starlight. The result supplies direct evidence for cool, optically thick gas around LRDs: the observed strengths can arise in low-metallicity hydrostatic photospheres with T_eff > 4500 K, or in lower-density gas whose required structure is dynamically non-hydrostatic.

Key figures to inspect

• Figure 1. Shows the individual CaT detections and matched non-detections, making the line-model decomposition and the strength of the JADES-28074/Rosetta Stone evidence immediately visible. It also conveys the important fit sensitivity in GLIMPSE ID17775 and the role of broad CaT emission in setting the local continuum.
• Figure 2. Presents the stack across the full grating sample, demonstrating that CaT absorption persists at similar depth even beyond the individually detected objects. This is the clearest population-level evidence that the feature is common around LRDs rather than confined to a few exceptional spectra.
• Figure 3. Places individual CaT measurements, upper limits, the stack, and the low-redshift analogs on a common equivalent-width versus optical-luminosity plane. The figure shows that the limited current sample does not establish a significant CaT–luminosity trend despite the unusually strong absorption in the Egg.
• Figure 4. Connects the observed CaT equivalent-width distribution to atmosphere calculations, isolating the temperature, metallicity, and surface-gravity or density conditions compatible with the data. It is the conclusion-driving physical diagnostic and makes clear the systematic uncertainty from unknown triplet line ratios and possible line or continuum infill.