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

Research output: Contribution to journalArticlepeer-review


To understand the Active Galactic Nuclei (AGN) 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 multi-wavelength data in the COSMOS field to select a complete sample of 578 infrared (IR) quasars (⁠LAGN,IR>1045ergs−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 L2−10keV – L6μm 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>1022cm−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 ≈0.2dex 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
Article numberstac2800
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
Early online date3 Oct 2022
Publication statusPublished - 31 Dec 2022


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