The warm outer layer of a little red dot as the source of [Fe II] and collisional Balmer lines with scattering wings
作者：Alberto Torralba, Jorryt Matthee, Gabriele Pezzulli, P. Rohan Naidu, Yuzo Ishikawa, B. Gabriel Brammer, Seok-Jun Chang, John Chisholm, Anna de Graaff, Francesco D’Eugenio, Claudia Di Cesare, Anna–Christina Eilers, E. Jenny Greene, Max Gronke, Edoardo Iani, Vasily Kokorev, Gauri Kotiwale, Ivan Kramarenko, Yilun Ma, Sara Mascia, Benjamín Navarrete, Erica Nelson, Pascal A. Oesch, A. Robert Simcoe, Stijn Wuyts · 发表于：Astronomy and Astrophysics · 年份：2026 · DOI：10.1051/0004-6361/202557537 · 被引用次数：13 · 研究领域：Astrophysical Phenomena and Observations、Galaxies: Formation, Evolution, Phenomena、Astronomy and Astrophysical Research
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 H α and H β have an exponential profile due to electron scattering in this same layer. The high H α : H β : H γ flux ratio of ≈10.4 : 1 : 0.14 is an indicator of collisional excitation and resonant scattering dominating the Balmer line emission. A narrow H γ 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 ...