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

A notable achievement of the first generation of JWST surveys was the discovery of an abundance of faint high-redshift (z > 4) broad-line active galactic nuclei (AGN) in the Lbol < 1045 erg s−1 regime, previously only accessible at z < 1. The high prevalence of absorption features in their broad hydrogen lines is one of the peculiarities of a significant fraction of this new population. In this paper, we conduct a broad census of Balmer absorption in a sample of 47 Type-1 AGN spanning 2 < z < 7. Accounting for incompleteness of JWST spectroscopic data, we estimate that 44₋₆⁺²¹ % of Little Red Dots (LRDs) have absorption in Hα while the incidence rate is <25 % (at 2σ) in Little Blue Dots (LBDs). Additionally, R1000 JWST/NIRSpec data is strongly resolution limited, implying an inability to disentangle Doppler broadening in the absorption from optical depth effects. This is alleviated with R2700 observations, although some degeneracies remain. We find a significant correlation between the velocity of the Balmer absorption and the [OIII]5007 narrow line luminosity, suggesting a common driving mechanism. We do not find any other significant correlations (and do not confirm previously claimed correlations) between Balmer absorption and other spectral properties of LRDs (except for the expected correlation between Hα and Hβ absorption velocities). Comparing LRDs, quasars and stellar Hα absorbers, we establish that absorption in both LRDs and broad absorption line quasars is consistent with radiatively driven outflows, echoing the physics of variable stars yet occurring at vastly different scales.