Reliable Identification of Compton‐thick Quasars at<i>z</i>≈ 2:<i>Spitzer</i>Mid‐Infrared Spectroscopy of HDF‐oMD49
Bibliographic record
Abstract
Many models that seek to explain the origin of the unresolved X-ray background predict that Compton-thick active galactic nuclei (AGNs) are ubiquitous at high redshift. However, few distant Compton-thick AGNs have been reliably identified to date. Here we present Spitzer IRS spectroscopy and 3.6-70 μm photometry of a z=2.211 optically identified AGN (HDF-oMD49) that is formally undetected in the 2 Ms Chandra Deep Field-North (CDF-N) survey. The Spitzer IRS spectrum and spectral energy distribution of this object is AGN dominated, and a comparison of the energetics at X-ray wavelengths to those derived from mid-infrared (mid-IR) and optical spectroscopy shows that the AGN is intrinsically luminous (L2-10keV~3×1044 ergs s-1) but heavily absorbed by Compton-thick material (NH>>1024 cm-2) i.e., this object is a Compton-thick quasar. Adopting the same approach that we applied to HDF-oMD49, we found a further six objects at z~2-2.5 in the literature that are also X-ray weak/undetected but have evidence for AGN activity from optical and/or mid-IR spectroscopy, and show that all of these sources are likely to be Compton-thick quasars with L2-10keV>1044 ergs s-1. On the basis of the definition of Daddi et al., these Compton-thick quasars would be classified as mid-IR excess galaxies, and our study provides the first spectroscopic confirmation of Compton-thick AGN activity in a subsample of these z~2 mid-IR-bright galaxies. Using the four objects that lie in the CDF-N field, we estimate the space density of reliably identified Compton-thick quasars [Φ~(0.7-2.5)×10-5 Mpc-3 for L2-10keV>1044 ergs s-1 objects at z~2-2.5] and show that Compton-thick accretion was probably as ubiquitous as unobscured accretion in the distant universe.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.001 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".