BASS. XXIII. A New Mid-infrared Diagnostic for Absorption in Active Galactic Nuclei

Ryan W. Pfeifle, Claudio Ricci, Peter G. Boorman, Marko Stalevski, Daniel Asmus, Benny Trakhtenbrot, Michael J. Koss, Daniel Stern, Federica Ricci, Shobita Satyapal, Kohei Ichikawa, David J. Rosario, Turgay Caglar, Ezequiel Treister, Meredith Powell, Kyuseok Oh, C. Megan Urry, Fiona Harrison

Research output: Contribution to journalArticlepeer-review


In this study, we use the Swift/BAT AGN sample, which has received extensive multiwavelength follow-up analysis as a result of the BAT AGN Spectroscopic Survey, to develop a diagnostic for nuclear obscuration by examining the relationship between the line-of-sight column densities (N H), the 2-10 keV to 12 μm luminosity ratio, and WISE mid-infrared colors. We demonstrate that heavily obscured AGNs tend to exhibit both preferentially "redder"mid-infrared colors and lower values of L X,Obs./L 12 μm than less obscured AGNs, and we derive expressions relating N H to the L X,Obs./L 12 μm and L 22 μm/L 4.6 μm luminosity ratios, as well as develop diagnostic criteria using these ratios. Our diagnostic regions yield samples that are ≳80% complete and ≳60% pure for AGNs with log(N H/cm-2) ≥ 24, as well as ≳85% pure for AGNs with log(NH/cm-2)≳23.5 . We find that these diagnostics cannot be used to differentiate between optically star-forming galaxies and active galaxies. Further, mid-IR contributions from host galaxies that dominate the observed 12 μm emission can lead to larger apparent X-ray deficits and redder mid-IR colors than the AGNs would intrinsically exhibit, though this effect helps to better separate less and more obscured AGNs. Finally, we test our diagnostics on two catalogs of AGNs and infrared galaxies, including the XMM-Newton XXL-N field, and we identify several known Compton-thick AGNs, as well as a handful of candidate heavily obscured AGNs based upon our proposed obscuration diagnostics.

Original languageEnglish
Article number3
JournalAstrophysical Journal, Supplement Series
Issue number1
StatePublished - 1 Jul 2022


FundersFunder number
European Union's Horizon 2020 innovation program793499
Fondecyt IniciacionFB210003, 11190831
Millennium Nucleus NCN19_058
National Science Foundation
National Aeronautics and Space AdministrationNNH16CT03C
Alfred P. Sloan Foundation
Office of Science
Horizon 2020 Framework Programme950533
Science and Technology Facilities Council
European Research Council
Japan Society for the Promotion of Science18K13584, 20H01939
Grantová Agentura České Republiky19-05599Y
Fondo Nacional de Desarrollo Científico y Tecnológico1190818, 1200495, 3180506
National Research Foundation of KoreaNRF-2020R1C1C1005462
Israel Science Foundation1849/19
Ministarstvo Prosvete, Nauke i Tehnološkog Razvoja451-03-68/2020/14/20002
Science Fund of the Republic of Serbia
Agencia Nacional de Investigación y DesarrolloCATA-Basal AFB-170002, ACE210002


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