An (in)complete NIRSpec census of Balmer absorption in Type 1 AGN -- radiation-driven outflows in little red dots, quasars and variable stars

Ignas Juodžbalis, Xihan Ji, Francesco D'Eugenio, Jan Scholtz, Roberto Maiolino, Amanda Stoffers, Alessandro Marconi, Elena Bertola, Andrew J. Bunker, Stefano Carniani, Giovanni Cresci, Emma Curtis-Lake, Zheng Ma, Cosimo Marconcini, Eleonora Parlanti, Pierluigi Rinaldi, Brant Robertson, Hannah Übler, Giacomo Venturi, Junyu Zhang

First listed 2026-07-22 | Last updated 2026-07-22

Abstract

Short digest

Using archival JWST/NIRSpec R1000 and R2700 spectroscopy, this paper builds a completeness-aware census of broad Hα Balmer absorption in 47 Type-1 AGN, separating Little Red Dots (LRDs) from Little Blue Dots (LBDs). Absorbers are confined to the LRD region of the UV-optical slope selection, while the completeness-corrected incidence in LBDs is consistent with only an upper limit. The authors find that Balmer-absorption velocity correlates significantly with narrow [O III] λ5007 luminosity and argue, from comparisons with FeLoBAL quasars and stellar Hα absorbers, that LRD absorption traces radiatively driven outflows. A central observational caveat is that R1000 spectra are strongly resolution-limited for recovering absorption widths and optical depths, making R2700 data essential for more reliable physical constraints.

Key figures to inspect

• Figure 1. This selection diagram establishes the population-level result that every identified Balmer absorber occupies the LRD region rather than the LBD region, making the LRD-LBD distinction immediately visible.
• Figure 3. The completeness maps show why instrumental resolution is central to the census: R1000 performs substantially worse than R2700 for recovering narrow absorption features, directly motivating the paper's incompleteness corrections and measurement caveat.
• Figure 5. This is the key demographic synthesis, comparing the inferred Balmer-absorption incidence across the LRD and LBD samples with BAL and FeLoBAL quasars while clearly displaying the LBD constraint as an upper limit.
• Figure 8. This comparison places LRDs on the luminosity-column-density sequence of FeLoBALs, showing that they extend the BAL population toward lower luminosities and higher columns rather than forming an unrelated absorber class.
• Figure 15. This cross-scale velocity versus broad-Hα-luminosity comparison provides the paper's clearest physical synthesis, linking LRDs and FeLoBALs along an AGN sequence while contrasting them with NGC 4151 and luminous blue variable stars.