Paschen Jumps in Little Red Dots: Evidence for Nebular Continua

TL;DR

This study presents evidence of Paschen jumps in high-redshift 'Little Red Dots', supporting nebular recombination as the primary source of their optical-to-near-infrared emission.

Contribution

The paper provides observational evidence for Paschen jumps in LRDs, constraining emission mechanisms and supporting nebular recombination models over thermalized stellar atmospheres.

Findings

Paschen jumps are consistent with low-temperature nebular emission.

Hα equivalent widths correlate with the continuum, indicating recombination origin.

Nebular models explain the spectra without multiple components.

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$\alpha$ equivalent widths and the tight observed correlation between H$\alpha$ 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.