The little blue and red dot Rosetta stones
Year: 2026
Type: article
Abstract: The population of active galactic nuclei (AGNs) newly discovered by the James Webb Space Telescope (JWST) exhibits peculiar properties that distinguish it from both local type I AGNs and high-redshift quasars. Most of these sources are compact, appearing as ‘little dots’; among them, the sub-class (10−30% of the total) characterised by significantly red optical colours is known as ‘little red dots’ (LRDs). In this work, we introduce the analogous term ‘little blue dots’ (LBDs) to characterise the remaining (bluer) sources (70−90%). We present a comparative analysis of two bright prototypical representatives, which we refer to as the Rosetta stones of the two classes: GN-28074 at z=2.26 as the red Rosetta stone and GS-3073 at z=5.55 as the blue Rosetta stone. Both Rosetta stones exhibit non-Gaussian broad Balmer lines, better described by exponential or double-Gaussian profiles rather than single Gaussians, similarly to normal low-redshift type I AGNs, indicating that non-Gaussian wings are not unique to LRDs. They are both extremely X-ray weak, showing strong auroral λ 4363 emission, weak hot dust mid-IR emission, and no time variability. However, they do differ in terms of excitation diagnostics: the λ 4686 line is undetected in the red Rosetta, but strongly detected in the blue Rosetta in both narrow and broad components, with the latter being much broader than hydrogen Balmer lines. This supports BLR stratification and disfavours the cocoon electron-scattering scenario in LBDs. An additional difference is the presence of prominent Balmer absorption in the red Rosetta, which is indicative of extremely dense gas along the line of sight and also happens to be absent in the blue Rosetta. Taken together, these results suggest that LRDs and LBDs share the same central engine as standard type I AGNs, while differing in terms of the amount and geometry of dense gas surrounding the accretion disk and/or in their accretion properties.
Source: Astronomy and Astrophysics
Authors: M. Brazzini, F. D’Eugenio, R. Maiolino, J. Lyu, C. DeCoursey, H. Übler, X. Ji, I. Juodžbalis, J. Scholtz, G. C. Jones, K. Hainline, E. Dalla Bontà, P. G. Pérez-González, S. Geris, A. Harshan, C. Feruglio, M. Bischetti, G. Mazzolari, G. Rieke, S. Alberts, B. Trefoloni, S. Carniani, E. Parlanti, A. Marconi, G. Risaliti, C. Ramos Almeida, P. Rinaldi, M. Perna, S. Zamora, I. Lamperti, G. Venturi, G. Cresci, E. Bertola, A. J. Bunker, L. R. Ivey
Institutions: University of Trieste, University of Cambridge, Trieste Astronomical Observatory, National Institute for Astrophysics, University College London, University of Arizona, Max Planck Institute for Extraterrestrial Physics, University of Padua, University of Lancashire, Osservatorio Astronomico di Padova, Consejo Superior de Investigaciones Científicas, Centro de Astrobiología, Institute for Fundamental Physics of the Universe, Scuola Normale Superiore, Arcetri Astrophysical Observatory, University of Florence, Universidad de La Laguna, Instituto de Astrofísica de Canarias, Space Telescope Science Institute, University of Oxford
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,865
Funders: UK Research and Innovation, Ministerio de Ciencia, Innovación y Universidades, European Commission, Max-Planck-Gesellschaft, Ministerio de Ciencia e Innovación, Woolf Fisher Trust, University of California Observatories, Science and Technology Facilities Council, European Regional Development Fund, Agencia Estatal de Investigación, European Social Fund, NextGenerationEU
Awards: G121809054, G1622782223, G1812695587, G1969106598, G2005369488, G204809963, G2144673021, G2309174360, G2351152971, G2660406951, G2674430119, G3215856511, G3228396264, G3518538256, G3660973570, G4161023950, G4212894969, G4476781803, G4915654406, G498124819, G5136794905, G531848386, G5368796297, G5404395142, G5441971315, G5480708011, G5635532810, G5663025379, G6307073466, G717256432, G7181133418, G7220758630, G7343478979, G7804586450, G7805626310, G7935691682, G8920660035, G8931419319, G897484643
OpenAlex ID: W7202138113