A striking excess of red quasars with steep radio spectral slopes: a dusty blow-out phase revealed through AGN-driven shocks?

Year: 2026

Type: article

Abstract: ABSTRACT Red quasars exhibit a higher incidence of compact (galaxy-scale or smaller) radio emission than blue quasars, arising from systems near the radio-loud/radio-quiet threshold. In this paper, we select quasars from Sloan Digital Sky Survey ($0.2 \lt z \lt 2.4$), and use archival radio data (FIRST, VLASS, and LoTSS) to visually determine the radio morphologies of 573 red quasars compared to a control sample of 1278 typical blue quasars. We find an excess of steep-slope radio emission ($\alpha _{1.4-3\text{ GHz}}\sim -1$, where $S_\nu \propto \nu ^\alpha$) from red quasars with compact ($\lt 6\,\mathrm{ arcsec}$) radio morphologies over 144 MHz, and 1.4 and 3 GHz. This excess steep radio emission signature is not seen in normal blue quasars (radio compact or extended) or red quasars with extended low-frequency radio emission, which instead show a broad range of radio spectral slopes consistent with a range of different physical processes. We show that the strength of the excess steep spectral slope component increases with dust extinction, along with an overall increase in the radio-detection fraction. We argue that this excess steep-slope radio emission is due to shocks between quasar-driven winds/jets and the dusty nuclear-host galaxy environment. The majority ($86^{+5}_{-21}\,\mathrm{ per\,cent}$) of the dustiest quasars ($E(B-V)\gt 0.4$ mag) with steep slopes have radio luminosities consistent with the prediction from a wind-shock model with wind efficiencies of up to 7 per cent. This agrees with the scenario where these compact red quasars are undergoing a ‘dusty blow-out’ phase, where compact jets and/or active galactic nucleus-driven winds interact with a dusty interstellar medium, causing shocks, leading to steep spectral slopes and enhanced radio detection rates.

Source: Monthly Notices of the Royal Astronomical Society

DOI: 10.1093/mnras/stag597

Authors: C L Sargent, D M Alexander, C L Greenwell, V A Fawcett, L K Morabito, C M Harrison, M Meenakshi, R C Hickox

Institutions: Durham University, Newcastle University, European Southern Observatory, Leibniz Institute for Astrophysics Potsdam, Dartmouth College

Language: English

Topic: Galaxies: Formation, Evolution, Phenomena

Subfield: Astronomy and Astrophysics

Field: Physics and Astronomy

Domain: Physical Sciences

Open Access status: gold

Estimated APC paid: $3,046

Funders: National Science Foundation, Smithsonian Institution, U.S. Department of Energy, National Aeronautics and Space Administration, Alfred P. Sloan Foundation, Gordon and Betty Moore Foundation, California Department of Fish and Game, Yale University, Carnegie Institution of Washington, Vanderbilt University, Ohio State University, California Institute of Technology, University of Utah, University of Washington, Johns Hopkins University, Carnegie Mellon University, University of Notre Dame, California Earthquake Authority, University of Portsmouth, New Mexico State University, UK Research and Innovation, National Radio Astronomy Observatory, York University, University of Oxford, Commonwealth Scientific and Industrial Research Organisation, Deutsche Forschungsgemeinschaft, Nederlandse Organisatie voor Wetenschappelijk Onderzoek, Ministério da Ciência, Tecnologia e Inovação, Centre National de la Recherche Scientifique, Ministerio de Ciencia e Innovación, Universidad Nacional Autónoma de México, Commissariat à l'Énergie Atomique et aux Énergies Alternatives, Centre de Coopération Internationale en Recherche Agronomique pour le Développement, Max-Planck-Institut für Astronomie, Instituto de Astrofísica de Canarias, Office of Science, Jet Propulsion Laboratory, University of California, Los Angeles, Nurses Organization of Veterans Affairs, University of Colorado Boulder, Science and Technology Facilities Council, Leibniz-Gemeinschaft, Division of Astronomical Sciences, Lawrence Berkeley National Laboratory, H2020 European Research Council

OpenAlex ID: W7140943631