Paschen Jumps in Little Red Dots: Evidence for Nebular Continua
Fuente:
arXiv
Saved in:
| Main Authors: | , , , , , , , , |
|---|---|
| Format: | Preprint |
| Published: |
2026
|
| Subjects: | |
| Online Access: | |
| Tags: |
Add Tag
No Tags, Be the first to tag this record!
|
| _version_ | 1866911582424924160 |
|---|---|
| author | Sneppen, Albert Matthews, James H. Watson, Darach Cameron, Alex J. Sim, Stuart A. Witstok, Joris Brammer, Gabriel B. Heintz, Kasper E. Nikopoulos, Georgios |
| author_facet | Sneppen, Albert Matthews, James H. Watson, Darach Cameron, Alex J. Sim, Stuart A. Witstok, Joris Brammer, Gabriel B. Heintz, Kasper E. Nikopoulos, Georgios |
| contents | ''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. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2604_09399 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Paschen Jumps in Little Red Dots: Evidence for Nebular Continua Sneppen, Albert Matthews, James H. Watson, Darach Cameron, Alex J. Sim, Stuart A. Witstok, Joris Brammer, Gabriel B. Heintz, Kasper E. Nikopoulos, Georgios Astrophysics of Galaxies ''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. |
| title | Paschen Jumps in Little Red Dots: Evidence for Nebular Continua |
| topic | Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2604.09399 |