Paschen Jumps in Little Red Dots: Evidence for Nebular Continua

Albert Sneppen, James H. Matthews, Darach Watson, Alex J. Cameron, Stuart A. Sim, Joris Witstok, Gabriel B. Brammer, Kasper E. Heintz, Georgios Nikopoulos

First listed 2026-04-10 | Last updated 2026-04-10

Abstract

''Little Red Dots'' (LRDs) are broad-line sources at high redshift, initially identified by their compact morphologies, red colours and prominent Balmer breaks. The origin of their optical-to-near-infrared continua is debated, with proposed explanations ranging from direct recombination emission to thermalised blackbodies from stellar-like atmospheres. Here we report evidence for Paschen jumps in a subset of LRDs, consistent with free-bound recombination to hydrogen $n=3$. The Paschen and Brackett continuum shapes across the sample are consistent with minimally reddened emission from low-temperature gas with $T_e\lesssim10\,000$ K, while the presence of Paschen jump signatures limits scenarios in which the emission is thermalised. Further, the extreme H$α$ equivalent widths and the tight observed correlation between H$α$ and the continuum follow naturally if both originate in recombination emission. This provides an observational upper limit on the contribution of any direct AGN accretion component and any stellar-atmosphere-like component, as well as on the fraction of line emission that can be thermalised as it traverses the cocoon. Ultimately, nebular radiative-transfer models provide a self-consistent explanation of the continuum, line strengths and line profiles without requiring multiple separately fitted components.

Short digest

This paper argues that Paschen and Brackett continuum structure in little red dots carries direct evidence for nebular free-bound emission rather than a simple reddened blackbody. The main result is that the observed slope changes near the Paschen jump are reproduced by recombination-dominated models and lead to concrete constraints on temperature and extinction. The paper matters because it reframes the continuum of LRDs as a diagnostic of dense ionized gas, not just of dust reddening or continuum shape.

Key figures to inspect

• Figure 1 is the must-see observational figure: it shows that the Paschen and Brackett continua in the PRISM LRD sample have different slopes, which is the key evidence against a simple reddened blackbody continuum.
• Figure 2 is the paper's Rosetta-stone comparison between the Sirocco model and the best observed example, making clear how the Paschen-jump region is blended with recombination emission.
• Figure 3 broadens the argument beyond the flagship object by showing a little blue dot with separated Balmer, Paschen, and Brackett continuum slopes and the same basic phenomenology.
• Figure 4 turns the continuum-shape argument into physical constraints, showing what electron temperature and dust reddening are allowed if the Paschen and Brackett continua are free-bound emission.