Balmer Absorption Series and Broad Metal Lines in Two Luminous Little Red Dots

Bingjie Wang, Jenny E. Greene, Hanpu Liu, Nicholas Kaaz, Gabriel B. Brammer, Raphael E. Hviding, Ivo Labbé, Joel Leja, Jorryt Matthee, Rohan P. Naidu, Alberto Torralba, Josephine F. W. Baggen, Nikko J. Cleri, Seiji Fujimoto, Lukas J. Furtak, Anna de Graaff, Michaela Hirschmann, Vasily Kokorev, Erini Lambrides, Ian McConachie, Erica J. Nelson, Adèle Plat, Weichen Wang, Adi Zitrin

First listed 2026-09-22 | Last updated 2026-09-18

Abstract

Balmer absorption is common among little red dots (LRDs), but absorbers at or redward of systemic are rare, occurring in only $\sim10-15$\% of H$α$ 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 H$α$ through H$δ$, 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 H$α$ trough rather than the more redshifted higher-order Balmer lines. We further report the detection of a broad base in [Ne\,{\sc{iii}}]\,$λ$3870, along with broad [O\,{\sc{iii}}]\,$λ$4364, [O\,{\sc{iii}}]\,$\lambda5008$, and He\,{\sc{i}}\,$\lambda5877,\lambda7067$, while He\,{\sc{ii}}\,$λ$4687 remains undetected or weak. Standard AGN photoionization models cannot reproduce the observed line ratios, whereas AGNs with high gas densities provide a consistent explanation, as also indicated by the anomalously high He\,{\sc{i}}\,$\lambda7067/\lambda5877$ 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.

Short digest

Using deep JWST/NIRSpec spectra of the exceptionally red, luminous LRDs RUBIES-EGS-49140 at z=6.68 and lensed UNCOVER-A2744-45924 at z=4.46, Wang et al. resolve near-systemic Balmer absorption from Hα through Hδ and broad metal-line emission. The Balmer troughs shift progressively redward at higher order while their equivalent widths decline much more weakly than a single foreground screen predicts; Ca II K instead follows Hα, pointing to stratified absorbing gas. Broad [Ne III], [O III], and He I emission, together with weak or absent He II and an unusually high He I λ7067/λ5877 ratio, favors very dense AGN gas over standard low-density photoionization models. The authors argue that optically thick re-emission can reshape the Balmer-series absorption strengths, while the velocity progression may trace a stratified, possibly failed-wind inflow geometry.

Key figures to inspect

• Figure 2. Shows the resolved Hα–Hδ profile decompositions in both LRDs, establishing the deep near-systemic absorption and the distinct narrow, broad, and absorption components on which the paper’s Balmer-series results rest.
• Figure 3. Condenses the velocity offsets and widths of all fitted components, making the mild but systematic redward shift of higher-order Balmer absorption immediately visible relative to narrow [O III] systemic redshifts.
• Figure 6. Presents the metal-line fits that reveal broad [Ne III] and [O III] emission as well as Ca II K absorption, connecting the Balmer absorber to independent gas-phase diagnostics.
• Figure 7. Provides the conclusion-driving photoionization comparison: both LRDs fall outside standard low-density AGN grids, while higher-density narrow-line-region models reach the observed ratios and support dense AGN gas.
• Figure 8. Illustrates the proposed failed-wind interpretation in which Hα traces slow outer gas and higher-order Balmer lines probe deeper, faster infalling material, naturally producing the observed redward velocity progression.