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This study examines "little red dots" (LRDs), compact broad-line sources at high redshift, and presents evidence of Paschen jumps in their spectra consistent with hydrogen recombination emission from low-temperature ionized gas. The researchers find that the observed continuum emission, extreme hydrogen-alpha equivalent widths, and tight correlations between emission lines and continuum are all consistent with nebular (gas) emission rather than thermal emission from stellar-like atmospheres or direct light from active galactic nuclei. This provides a unified explanation for multiple observed properties of LRDs without requiring multiple emission components.
Why it matters
Understanding the nature of LRDs helps astronomers interpret observations of distant galaxies and active galactic nuclei in the early universe. The findings suggest that nebular emission may dominate the optical and near-infrared light from these objects, which has implications for how we estimate their masses, ages, and the nature of supermassive black holes at high redshift.
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⚠️ Preprint – Noch nicht peer-reviewed
Dieser Artikel wurde noch nicht von unabhängigen Experten begutachtet. Die Ergebnisse sind vorläufig und sollten mit Vorsicht interpretiert werden.
Abstract: Little red dots (LRDs) are broad-line sources at high redshift that were initially identified by their compact morphologies, red colours, and inflection near the Balmer wavelength. 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. We report evidence of Paschen jumps in a subset of LRDs with a high signal-to-noise ratio, consistent with free-bound recombination to hydrogen $n=3$. The Paschen and Brackett continuum shapes in the LRD sample are consistent with minimally reddened nebular emission from low-temperature gas with $T_elesssim10,000$ K, while the 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 active galactic nucleus accretion component and any stellar-atmosphere-like component, and also on the fraction of line emission that can be thermalised as it traverses the cocoon. Ultimately, ionised gas emission provide a self-consistent explanation of the continuum, line strengths, and line profiles without requiring multiple separately fitted components.
Source: Paschen Jumps in Little Red Dots: Evidence for Nebular Continua