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A panchromatic view of infrared quasars: excess star formation and radio emission in the most heavily obscured systems

Andonie, Carolina and Alexander, David M and Rosario, David and Laloux, Brivael and Georgakakis, Antonis and Morabito, Leah K and Villforth, Carolin and Avirett-Mackenzie, Mathilda and Rivera, Gabriela Calistro and Del Moro, Agnese and Fotopoulou, Sotiria and Harrison, Chris and Lapi, Andrea and Petley, James and Petter, Grayson and Shankar, Francesco (2022) 'A panchromatic view of infrared quasars: excess star formation and radio emission in the most heavily obscured systems.', Monthly notices of the Royal Astronomical Society, 517 (2). pp. 2577-2598.


To understand the active galactic nuclei (AGNs) phenomenon and their impact on the evolution of galaxies, a complete AGN census is required; however, finding heavily obscured AGNs is observationally challenging. Here we use the deep and extensive multiwavelength data in the COSMOS field to select a complete sample of 578 infrared (IR) quasars (LAGN,IR > 1045 erg s−1) at z < 3, with minimal obscuration bias, using detailed UV-to-far-IR spectral energy distribution (SED) fitting. We complement our SED constraints with X-ray and radio observations to further investigate the properties of the sample. Overall, 322 of the IR quasars are detected by Chandra and have individual X-ray spectral constraints. From a combination of X-ray stacking and L2−10 kev – L6 μm analyses, we show that the majority of the X-ray faint and undetected quasars are heavily obscured (many are likely Compton thick), highlighting the effectiveness of the mid-IR band to find obscured AGNs. We find that 355 (≈61 per cent) IR quasars are obscured (NH > 1022 cm−2) and identify differences in the average properties between the obscured and unobscured quasars: (1) obscured quasars have star formation rates ≈3 times higher than unobscured systems for no significant difference in stellar mass and (2) obscured quasars have stronger radio emission than unobscured systems, with a radio-loudness parameter ≈ 0.2 dex higher. These results are inconsistent with a simple orientation model but in general agreement with either extreme host-galaxy obscuration towards the obscured quasars or a scenario where obscured quasars are an early phase in the evolution of quasars.

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Publisher statement:© 2022 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society This is an Open Access article distributed under the terms of the Creative Commons Attribution License (, which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.
Date accepted:26 September 2022
Date deposited:26 October 2022
Date of first online publication:03 October 2022
Date first made open access:26 October 2022

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