articleAstronomy and AstrophysicsJan 5, 2026DIAMOND OA

The warm outer layer of a little red dot as the source of [Fe II] and collisional Balmer lines with scattering wings

ATAlberto TorralbaJMJorryt MattheeGPGabriele PezzulliPRP. Rohan NaiduYIYuzo Ishikawa

Institute of Science and Technology Austria · University of Groningen · +15 more institutions

Indexed inarxivcrossrefdoaj

Abstract

The population of the little red dots (LRDs) may represent a key phase of supermassive black hole (SMBH) growth. A cocoon of dense excited gas is emerging as a key component to explain the most striking properties of LRDs, such as strong Balmer breaks and Balmer absorption, as well as the weak IR emission. To dissect the structure of LRDs, we analyzed new deep JWST/NIRSpec PRISM and G395H spectra of FRESCO-GN-9771, one of the most luminous known LRDs at z = 5.5. These spectra reveal a strong Balmer break, broad Balmer lines, and very narrow [O III ] emission. We revealed a forest of optical [Fe II ] lines, which we argue are emerging from a dense ( n H = 10 9 − 10 cm −3 ) warm layer with electron temperature T…

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Topics & keywords

Topics
Keywords
  • Balmer series
  • Population
  • Scattering
  • Spectral line
  • Line (geometry)
  • Opacity
UN Sustainable Development Goals
  • Life in Land
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