J/A+A/708/A157 Identifying CT-AGNs in the COSMOS. II (Guo+, 2026)
Identifying Compton-thick AGNs in the COSMOS.
II. Searching among mid-infrared selected AGNs.
Guo X., Gu Q., Fang G., Lu S., Lyu F., Chen Y., Ding N., Zhang M., Yu X.,
Wang H.
<Astron. Astrophys. 708, A157 (2026)>
=2026A&A...708A.157G 2026A&A...708A.157G (SIMBAD/NED BibCode)
ADC_Keywords: Active gal. nuclei ; X-ray sources
Keywords: galaxies: active - galaxies: nuclei - infrared: galaxies -
X-rays: diffuse background
Abstract:
Compton-thick active galactic nuclei (CT-AGNs), defined by column
density NH≥1.5x1024cm-2, are so heavily absorbed that their
X-ray emission is often feeble, even undetectable by X-ray
instruments. Nevertheless, their radiation is expected to be a
substantial contributor to the cosmic X-ray background (CXB),
predicting that CT-AGNs should comprise at least 30% of the total AGN
population.
In the Cosmological Evolution Survey (COSMOS), the identified CT-AGN
fraction falls far below theoretical expectations, indicating that a
substantial population of CT-AGNs is hidden due to their low photon
counts or their flux below the current flux limits of X-ray
instruments. This work focuses on identifying CT-AGNs hidden in
mid-infrared (MIR)-selected AGNs.
First, we selected a sample of 1,104 MIR-selected AGNs that were
covered but individually undetected by X-ray. Next, we reduced the
X-ray data in the COSMOS and analyzed multiwavelength data in our
sample to derive the key physical parameters required for CT-AGN
identification.
Using MIR diagnostics, we first find out 7 to 23 CT-AGN candidates.
Their subsequent X-ray stacking analysis reveals a clear detection at
>3sigma significance in the soft band and only >1sigma significance in
the hard band. We fit the stacked soft- and hard-band fluxes with a
physical model and confirm that these sources are absorbed by
Compton-thick material. However, CT-AGNs constitute only 2.1%
(23/1104) of our sample, significantly below the fraction predicted by
CXB synthesis models, indicating that a considerable population of
CT-AGNs remains missed by our selection. A comparison of host-galaxy
properties between CT-AGNs and non-CT-AGNs reveals no significant
differences.
Description:
We combined MIR diagnostics with X-ray stacking analysis to identify
CT-AGNs that were not detected by X-ray. Firstly, we constructed a
sample containing 1,104 MIR-selected AGNs.
The table lists the parameters adopted for CT-AGN selection from the
MIR-selected AGN sample and the identified CT-AGNs.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tablea1.dat 112 1104 Parameters adopted for CT-AGN selection from the
MIR-selected AGN sample and the identified CT-AGNs
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See also:
J/ApJS/258/11 : The COSMOS2020 catalog (Weaver+, 2022)
J/A+A/694/A241 : Identifying CT-AGNs in the COSMOS. I. (Guo+, 2025)
Byte-by-byte Description of file: tablea1.dat
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Bytes Format Units Label Explanations
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1- 6 I6 --- ID IDs from the COSMOS2020 FARMER catalog (1)
8- 17 F10.6 deg RAdeg Right ascension (J2000) (1)
19- 26 F8.6 deg DEdeg Declination (J2000) (1)
28- 35 F8.6 --- z Redshift (1)
37- 38 I2 --- B3 The background counts of the source within
a radius of 3 arcsec
40- 49 F10.6 [s] Texp The exposure time of the source
51- 61 F11.6 [10-7W/cm2] Flux2-10 The upper limit of 2-10keV flux
63- 71 F9.6 [10-7W] logL2-10 ?=- Log of the upper limit of
2-10keV luminosity
73- 81 F9.6 [10-7W] logL6um ?=- Log of 6um luminosity for an AGN
in the rest-frame
83- 90 F8.6 --- alphaAGN ?=- Confidence of AGN components
for SED fitting
92-100 F9.6 [Msun] logMstar ?=- Log of the stellar mass
102-110 F9.6 [Msun/yr] logSFR ?=- Log of the star formation rate
112 I1 --- CTflag [0/1] CT-AGN flag; 1 = CT-AGN identified
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Note (1): from Weaver et al., 2022ApJS..258...11W 2022ApJS..258...11W, Cat. J/ApJS/258/11.
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Acknowledgements:
Xiaotong Guo, guoxiaotong(at)aqnu.edu.cn
References:
Guo et al., Paper I 2025A&A...694A.241G 2025A&A...694A.241G Cat. J/A+A/694/A241
(End) Patricia Vannier [CDS] 05-Mar-2026