OCEANS of Absorption: High-resolution NIRSpec Spectroscopy Reveals Diverse Balmer-line Absorption in Little Red Dots

Kelcey Davis, Madisyn Brooks, Raymond C. Simons, Jonathan R. Trump, Guillermo Barro, Pablo Arrabal Haro, Bren E. Backhaus, Nikko J. Cleri, Alexander de la Vega, Steven L. Finkelstein, Mauro Giavalisco, Norman A. Grogin, Michaela Hirschmann, Taylor A. Hutchison, Dale Kocevski, Anton M. Koekemoer, Erini Lambrides, Mario Llerena, Ray A. Lucas, Madeline A. Marshall, Elizabeth J. McGrath, Casey Papovich, Aidan Starrs, Anthony J. Taylor, Phoebe R. Upton Sanderbeck, Xin Wang, Stijn Wuyts

First listed 2026-06-02 | Last updated 2026-05-29

Abstract

The ``Little Red Dots' (LRDs) that appeared in JWST deep field images have been the subject of significant study since their discovery. In this work, we present high-resolution follow-up spectroscopy from the OCEANS program of 10 LRDs with Ha coverage at 3<z<7 in the CEERS/EGS field. We find Balmer-line absorption in 4 of these LRDs, a detection rate higher than the fractions reported in lower-resolution NIRSpec surveys. All of the absorbers are presented in high-resolution for the first time here and two have Balmer-line absorption detected for the first time. Of the 10 LRDs, 7 are best fit by Ha profiles with exponential wings. We find that absorbers tend to be blue-shifted with a median velocity offset of (-49 km/s) and absorption equivalent width of 5.3 Angstroms. Trends are explored to compare LRD absorption properties along the sequence of LRDs. We confirm an LRD with statistically significant absorption velocity offsets between Ha and Hb. The diversity of absorption properties can be effectively explained by a model with a radial distribution of partial-covering absorbing gas that is often co-located near the broad-line emission regions, along with a radial gradient of close inflow and distant outflow velocities for the absorbing gas. We present other interesting LRDs, including an outflow-dominated LRD and an LRD with relatively blue UV-to-optical colors but clear Balmer-line absorption. This high occurrence of absorbing hydrogen in LRDs, evident by both the Balmer-line absorption features and Balmer break strengths, implies a near-ubiquitous presence of dense, excited n=2 hydrogen.

Short digest

This OCEANS high-resolution NIRSpec study targets 10 CEERS/EGS little red dots with Hα coverage at 3<z<7 and finds Balmer-line absorption in 4 objects, including two first-time detections and a substantially higher absorber fraction than lower-resolution surveys. Seven of the ten LRDs are best fit by Hα profiles with exponential wings, while the absorbers show a median velocity offset of -49 km/s and a median rest-frame equivalent width of 5.3 Å; one source also shows a statistically significant offset between its Hα and Hβ absorption velocities. The authors argue that the diversity of line shapes and absorption kinematics is best explained by partial-covering n=2 hydrogen distributed radially near the broad-line emitting region, with close-in inflow and more distant outflow components. That makes the Balmer absorption and Balmer-break phenomenology a strong sign that dense, excited hydrogen is nearly ubiquitous around LRD central engines.

Key figures to inspect

• Figure 4. This is the paper’s core evidence figure: it shows the four absorbing OCEANS LRDs in Hα, [O III], and Hβ with the full spectral decomposition, making it clear where the Balmer absorption is detected and how the preferred fits separate narrow emission, broad Gaussian or exponential wings, [N II], and absorption. Use this figure to see object-by-object diversity and to verify that the absorber classifications are driven by resolved line-profile structure rather than photometric inference alone.
• Figure 6. This figure directly demonstrates why high-resolution spectroscopy changes the result. By comparing OCEANS G395H data for a source whose absorption is only recovered at high resolution to the medium-resolution RUBIES spectrum and to a degraded version of the OCEANS data, it shows that Balmer absorption can be washed out by lower spectral resolution and supports the paper’s claim that part of the higher absorber fraction is observational.
• Figure 7. This is the cleanest population-level summary of the paper’s survey comparison. It places the OCEANS 4/10 absorber fraction alongside prism, medium-resolution, and other high-resolution NIRSpec samples, making the resolution dependence of Balmer-absorption recovery immediately visible and showing why OCEANS matters in the broader LRD literature.
• Figure 9. This figure quantifies the absorber population that OCEANS is adding to the field. The velocity-offset and rest-frame equivalent-width distributions for OCEANS and other high-resolution absorbers make the typical blue-shifted, moderate-EW Balmer absorber easy to read off and anchor the paper’s reported median kinematic and line-strength values.
• Figure 12. This is the most important later diagnostic figure because it connects absorption velocity and EW to net-to-narrow emission, UV-to-optical color, and Balmer-break strength across absorbing LRDs. It is where the paper’s interpretation becomes physical rather than descriptive: the trends along the LRD sequence support a radially structured absorbing medium with changing inflow and outflow signatures, tying line kinematics to the same excited hydrogen reservoir implicated by the Balmer break.