Exploring active galactic nuclei and little red dots with the Obelisk simulation

¹ Institut d’Astrophysique de Paris, Sorbonne Université, CNRS, UMR 7095, 98 bis bd Arago, 75014 Paris, France
² Kapteyn Astronomical Institute, University of Groningen, PO Box 800 NL-9700 AV Groningen, The Netherlands
³ Department of Astrophysical Sciences, Princeton University, Princeton, NJ, 08544 USA
⁴ Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
⁵ Zentrum für Astronomie der Universität Heidelberg, ITA, Albert-Ueberle-Str. 2, D-69120 Heidelberg, Germany
⁶ Dipartimento di Scienza e Alta Tecnologia, Università degli Studi dell’Insubria, via Valleggio 11, I-22100 Como, Italy
⁷ INFN, Sezione di Milano-Bicocca, Piazza della Scienza 3, I-20126 Milano, Italy
⁸ Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh, EH9 3HJ UK
⁹ Dipartimento di Fisica, “Sapienza” Università di Roma, Piazzale Aldo Moro 2, 00185 Roma, Italy
¹⁰ INAF/Osservatorio Astronomico di Roma, Via di Frascati 33, 00040 Monte Porzio Catone, Italy
¹¹ INFN, Sezione Roma1, Dipartimento di Fisica, “Sapienza” Università di Roma, Piazzale Aldo Moro 2, 00185 Roma, Italy
¹² Sapienza School for Advanced Studies, Viale Regina Elena 291, 00161 Roma, Italy

^⋆ Corresponding author; martav@iap.fr

Received: 23 August 2024
Accepted: 20 December 2024

Abstract

The James Webb Space telescope has discovered an abundant population of broad line emitters, typical signposts for Active Galactic Nuclei (AGN). Many of these sources have red colors and a compact appearance that has led to them being named “little red dots”. In this paper, we develop a detailed framework to estimate the photometry of AGNs embedded in galaxies extracted from the OBELISK cosmological simulation to understand the properties of color-selected little red dots (cLRDs) in the context of the full AGN and massive black hole population. We find that using realistic spectral energy distributions (SEDs) and attenuation for AGNs we can explain the shape of the cLRD SED as long as galaxies host a sufficiently luminous AGN that is neither overly attenuated nor too little attenuated. When attenuation is too low or too high, AGNs do not enter the cLRD selection, because the AGN dominates over the host galaxy too much in blue filters, or it does not contribute to photometry anywhere, respectively. cLRDs are also characterized by high Eddington ratios, possibility super-Eddington ones, and/or high ratios between the black hole and the stellar mass.