Inside the cocoon: a comprehensive explanation of the spectra of Little Red Dots
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arXiv
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| Main Authors: | , , , , , , , , , , , , |
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| Format: | Preprint |
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2026
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| _version_ | 1866915757373259776 |
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| author | Sneppen, A. Watson, D. Matthews, J. H. Nikopoulos, G. Allen, N. Brammer, G. Damgaard, R. Heintz, K. E. Knigge, C. Long, K. S. Rusakov, V. Sim, S. A. Witstok, J. |
| author_facet | Sneppen, A. Watson, D. Matthews, J. H. Nikopoulos, G. Allen, N. Brammer, G. Damgaard, R. Heintz, K. E. Knigge, C. Long, K. S. Rusakov, V. Sim, S. A. Witstok, J. |
| contents | JWST has revealed a population of compact galaxies in the early Universe with broad emission lines and strong Balmer breaks; among them the so-called ''little red dots'' (LRDs). Their nature remains uncertain with hypotheses including exotic phenomena. We assemble a sample of LRD-like objects at $z>3$ and use self-consistent radiative-transfer calculations to show that a supermassive black hole accreting from a dense gas cocoon accurately reproduces the detailed spectra. We show that the cocoons must be non-spherical, with comparable amounts of inflowing and outflowing material. And we predict correlations between Balmer break strength, Balmer line-absorption and scattering line width, which we confirm in our observed sample. We reproduce all LRD-like properties without requiring star-like atmospheres and we determine the typical black hole in our sample to be of order a million solar masses, with ionized cocoon masses of tens of solar masses potentially supplied from a much larger cold-gas reservoir. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2601_18864 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Inside the cocoon: a comprehensive explanation of the spectra of Little Red Dots Sneppen, A. Watson, D. Matthews, J. H. Nikopoulos, G. Allen, N. Brammer, G. Damgaard, R. Heintz, K. E. Knigge, C. Long, K. S. Rusakov, V. Sim, S. A. Witstok, J. Astrophysics of Galaxies High Energy Astrophysical Phenomena JWST has revealed a population of compact galaxies in the early Universe with broad emission lines and strong Balmer breaks; among them the so-called ''little red dots'' (LRDs). Their nature remains uncertain with hypotheses including exotic phenomena. We assemble a sample of LRD-like objects at $z>3$ and use self-consistent radiative-transfer calculations to show that a supermassive black hole accreting from a dense gas cocoon accurately reproduces the detailed spectra. We show that the cocoons must be non-spherical, with comparable amounts of inflowing and outflowing material. And we predict correlations between Balmer break strength, Balmer line-absorption and scattering line width, which we confirm in our observed sample. We reproduce all LRD-like properties without requiring star-like atmospheres and we determine the typical black hole in our sample to be of order a million solar masses, with ionized cocoon masses of tens of solar masses potentially supplied from a much larger cold-gas reservoir. |
| title | Inside the cocoon: a comprehensive explanation of the spectra of Little Red Dots |
| topic | Astrophysics of Galaxies High Energy Astrophysical Phenomena |
| url | https://arxiv.org/abs/2601.18864 |