Revealing X-ray obscured quasars in SWIRE sources with extreme mid-IR/optical flux ratios

Recent works have suggested that selection criteria based on MIR colors can be used to reveal a population of dust-enshrouded, extremely luminous quasars at z>1. However the X-ray spectral properties of these intriguing sources still remain largely unexplored. We report on an X-ray spectroscopic study of a sample of 44 very bright mid-IR galaxies with extreme mid-IR to optical flux ratios (MIR/O>2000). The X-ray coverage of the sample is highly inhomogeneous (from snap-shot 5 ks Chandra observations to medium-deep XMM exposures of 70 ks) and, consequently, a sizable fraction of them (~43%) remains undetected in the 0.5-10 keV band. The vast majority (95%) of the detected sources (23) show an absorption column density NH>10e22 cm-2 and, remarkably, we also find that 50% of them can be classified as Type 2 quasars on the basis of their absorption properties and X-ray luminosity. Moreover, most of the X-ray undetected sources show extreme mid-IR colors, consistent with being luminous AGN-powered objects, suggesting they might host heavily obscured (possibly Compton-thick) quasars in X-rays. This demonstrates that our selection criteria applied to a wide area survey is very efficient in finding a large number of Type 2 quasars at z > 1. The existence of this class of very powerful, obscured quasars at high z could have important implications in the context of the formation and cosmological evolution of accreting supermassive black holes and their host galaxies.

💡 Research Summary

This paper presents the first systematic X‑ray spectroscopic investigation of a sample of extremely mid‑infrared (MIR) bright galaxies selected on the basis of an extreme MIR‑to‑optical flux ratio (MIR/O > 2000). The authors extracted 44 objects from the SWIRE survey that satisfy both a bright 24 µm flux (≥ 1 mJy) and a faint R‑band magnitude (R > 23 mag), thereby targeting sources whose spectral energy distributions are dominated by warm dust emission and are likely to host powerful, heavily obscured active galactic nuclei (AGN).

X‑ray coverage is highly heterogeneous: 30 sources were observed with short (≈5 ks) Chandra snapshots, while 14 were followed up with deeper (≈70 ks) XMM‑Newton exposures. Because of the wide range in exposure times, 19 objects (≈43 %) remain undetected in the 0.5–10 keV band at the current sensitivity limits.

For the 23 detected sources, the authors performed standard absorbed power‑law fits, deriving intrinsic column densities (NH) and absorption‑corrected 2–10 keV luminosities. Strikingly, 22 of the 23 (≈95 %) exhibit NH > 10²² cm⁻², confirming that the majority are heavily obscured AGN. Moreover, 11 objects (≈50 % of the detections) have absorption‑corrected luminosities exceeding 10⁴⁴ erg s⁻¹, satisfying the conventional definition of Type 2 quasars (i.e., luminous, obscured quasars). This high fraction of Type 2 quasars demonstrates that the MIR/O > 2000 criterion is extremely efficient at selecting high‑redshift (z > 1) obscured quasars.

The undetected subset shows the reddest MIR colours in the sample, with IRAC colour combinations that match AGN templates rather than star‑forming galaxies. Photometric redshifts place most of these sources at 1 < z < 2.5, implying that their non‑detection is not due to distance but to extreme X‑ray absorption, likely in the Compton‑thick regime (NH > 1.5 × 10²⁴ cm⁻²). The authors argue that these objects are strong candidates for the elusive population of Compton‑thick quasars that are predicted by synthesis models of the X‑ray background but have remained largely hidden in existing surveys.

The paper discusses several broader implications. First, the success of the MIR/O > 2000 selection validates the use of extreme MIR‑to‑optical ratios as a practical tool for wide‑area surveys aiming to uncover the obscured side of SMBH growth at cosmic noon. Second, the high incidence of heavily obscured, high‑luminosity AGN suggests that a substantial fraction of the total accretion power at z ≈ 1–3 occurs behind thick columns of gas and dust, which must be accounted for in models of black‑hole mass assembly and co‑evolution with host galaxies. Third, the large number of X‑ray non‑detections underscores the need for high‑energy (>10 keV) observatories such as NuSTAR, Athena, and future missions, which can penetrate Compton‑thick material and provide direct measurements of the intrinsic luminosities and column densities.

Finally, the authors emphasize that a multi‑wavelength approach—combining deep MIR photometry, optical/near‑IR spectroscopy, radio data, and hard X‑ray observations—is essential to fully characterize this population, assess its contribution to the cosmic X‑ray background, and understand its role in feedback processes that regulate star formation in massive galaxies. In summary, the study demonstrates that selecting sources with MIR/O > 2000 in large surveys is a highly effective strategy for building statistically significant samples of high‑redshift Type 2 quasars, opening a new window onto the obscured growth phase of supermassive black holes.