Identifying Compton-thick AGNs in the COSMOS II. Among mid-infrared selected AGNs

Abstract

Compton-thick active galactic nuclei (CT-AGNs), defined by column density $\mathrm{N_H} \geqslant 1.5 \times 10^{24} \ \mathrm{cm}^{-2}$, 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 $\sim$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 $>3σ$ significance in the soft band and only $>1σ$ 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.

Figures (7)

• Figure 1: IRAC color–color diagram of the MIR sources. The small gray dots represent all IRAC sources selected in the COSMOS. The region bounded by red dashed lines marks the MIR-AGN selection region of 2012ApJ...748..142D. The sources within this region are classified as MIR-selected AGNs. The color bar and overlaid contours trace the number density gradient across the diagram.
• Figure 2: Merged Chandra exposure map of the COSMOS, overlaid with the positions of MIR-selected AGNs (black dots) used in this work. The color bar indicates the effective exposure time for the corresponding X-ray coverage.
• Figure 3: Example of best-fitting SED for an AGN. The solid black line indicates the best-fitting model. The dashed blue, solid gold, and solid red lines represent unattenuated stellar, attenuated stellar, and dust emission, respectively. The solid apricot line indicates AGN emission. The strawberry filled circles, pastel-purple open circles, and green open inverted triangles denote model predictions, observed fluxes, and observed upper limits, respectively. The lower panel indicates the residual of the best fitting.
• Figure 4: Observed X-ray luminosity in the rest-frame 2--10 keV band as a function of the 6 $\mu$m luminosity for fit AGN component. The solid purple, blue, and black lines represent the relation for 2017ApJ...837..145C, 2015ApJ...807..129S, and 2009ApJ...693..447F, respectively. The dashed lines indicate the same relationships but where the X-ray luminosities are absorbed by a column density of $\mathrm{N_H}=1.5\times 10^{24}\ \mathrm{cm}^{-2}$. The circles are MIR-selected AGNs with a fit AGN component. The color bar represents the confidence coefficient of the 6 $\mu$m luminosity for a fit AGN component (or the fit AGN components). The cutting stars within the large circles represent CT-AGNs identified using the relationship between MIR and X-ray luminosities.
• Figure 5: X-ray stacking results for the 23 CT-AGNs identified by MIR diagnostics. Left: stacked images in the 0.5--2 keV (top) and 2--8 keV (bottom) bands. Right: corresponding bootstrap histograms of the net count rates; green lines indicate the mean count rates and $1\sigma$ confidence intervals. Detections exceed $3\sigma$ significance in soft band, while only exceed $1\sigma$ significance in hard band.