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

¹ Institute of Science and Technology Austria (ISTA) Am Campus 1 3400 Klosterneuburg, Austria
² Kapteyn Astronomical Institute, University of Groningen Landleven 12 NL-9747 AD Groningen, The Netherlands
³ MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology Cambridge MA 02139, USA
⁴ Cosmic Dawn Center (DAWN), Niels Bohr Institute, University of Copenhagen Jagtvej 128 København N DK-2200, Denmark
⁵ Niels Bohr Institute, University of Copenhagen Jagtvej 128 2200 Copenhagen N, Denmark
⁶ Max Planck Institute for Astrophysics Karl-Schwarzschild-Str. 1 85748 Garching, Germany
⁷ Department of Astronomy, University of Texas at Austin 2515 Speedway Austin Texas 78712, USA
⁸ Max-Planck-Institut für Astronomie Königstuhl 17 D-69117 Heidelberg, Germany
⁹ Kavli Institute for Cosmology, University of Cambridge Madingley Road Cambridge CB3 0HA, United Kingdom
¹⁰ Cavendish Laboratory, Astrophysics Group, University of Cambridge 19 JJ Thomson Avenue Cambridge CB3 0HE, United Kingdom
¹¹ Department of Astrophysical Sciences, Princeton University Princeton NJ 08544, USA
¹² Astronomisches Rechen-Institut, Zentrum für Astronomie, Universität Heidelberg Monchhofstraße 12-14 69120 Heidelberg, Germany
¹³ Department for Astrophysical and Planetary Science, University of Colorado Boulder CO 80309, USA
¹⁴ Department of Astronomy, University of Geneva Chemin Pegasi 51 1290 Versoix, Switzerland
¹⁵ Department of Physics, Massachusetts Institute of Technology Cambridge MA 02139, USA
¹⁶ Department of Physics, University of Bath Claverton Down Bath BA2 7AY, UK

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 3 October 2025
Accepted: 21 December 2025

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_e ≈ 7000 K. The broad wings of Hα and Hβ have an exponential profile due to electron scattering in this same layer. The high Hα : Hβ : Hγ flux ratio of ≈10.4 : 1 : 0.14 is an indicator of collisional excitation and resonant scattering dominating the Balmer line emission. A narrow Hγ component, unseen in the other two Balmer lines due to outshining by the broad components, could trace the ISM of a normal host galaxy with a star formation rate of ∼5 M_⊙ yr⁻¹. The warm layer is mostly opaque to Balmer transitions, producing a characteristic P Cygni profile in the line centers suggesting outflowing motions. This same layer is responsible for shaping the Balmer break. The broadband spectrum can be reasonably matched by a simple photoionized slab model that dominates the λ > 1500 Å continuum and a low-mass (∼10⁸ M_⊙) galaxy that could explain the narrow [O III], with only a subdominant contribution to the UV continuum. Our findings indicate that Balmer lines are not directly tracing the gas kinematics near the SMBH and that the BH mass scale is likely much lower than virial indicators suggest.