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

Carolina Andonie, David M. Alexander, David Rosario, Brivael Laloux, Antonis Georgakakis, Leah K. Morabito, Carolin Villforth, Mathilda Avirett-Mackenzie, Gabriela Calistro Rivera, Agnese Del Moro, Sotiria Fotopoulou, Chris Harrison, Andrea Lapi, James Petley, Grayson Petter, Francesco Shankar

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12 Citations (SciVal)


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.

Original languageEnglish
Pages (from-to)2577-2598
Number of pages22
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
Early online date3 Oct 2022
Publication statusPublished - 1 Dec 2022

Bibliographical note

Funding Information:
We thank the referee for his positive and constructive comments. We would also like to express our gratitude to Philip Hopkins for useful research discussions. We want to thank Mara Salvato for sharing her redshift compilation in the COSMOS field, and Stefano Marchesi and Giorgio Lanzuisi for sharing their X-ray spectral fitting results in COSMOS. This work has been supported by the EU H2020-MSCAITN-2019 Project 860744 ‘BiD4BESt: Big Data applications for black hole Evolution STudies.’ DMA thanks the Science Technology Facilities Council (STFC) for support from the Durham consolidated grant (ST/T000244/1). CH acknowledges funding from an United Kingdom Research and Innovation grant (code: MR/V022830/1). AL is partly supported by the PRIN MIUR 2017 prot. 20173ML3WW 002 ‘Opening the ALMA window on the cosmic evolution of gas, stars, and massive black holes’. JP acknowledges support from STFC grants ST/T506047/1 and ST/V506643/1.

DATA AVAILABILITY The data sets generated and/or analysed in this study are available from the corresponding author on reasonable request.


  • galaxies: active
  • infrared: galaxies
  • quasars: general

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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