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 $\sim 10-15$\% of H$\alpha$ 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$\alpha$ through H$\delta$, 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$\alpha$ trough rather than the more redshifted higher-order Balmer lines. We further report the detection of a broad base in [Ne\,{\sc{iii}}]\,$\lambda$3870, along with broad [O\,{\sc{iii}}]\,$\lambda$4364, [O\,{\sc{iii}}]\,$\lambda 5008$, and He\,{\sc{i}}\,$\lambda 5877,\lambda 7067$, while He\,{\sc{ii}}\,$\lambda$4687 remains undetected or weak. Standard AGN photoionization models cannot reproduce the observed line ratios, whereas AGNs with high gas densities provide a consistent explanation, as also indicated by the anomalously high He\,{\sc{i}}\,$\lambda 7067/\lambda 5877$ 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.

20 pages, 11 figures; submitted to ApJ