X-ray luminosity function of Compton-thick AGN in the early Universe (z > 3). Robustness and biases of the CTK population

abstract · abstract

The population of Compton-thick (CTK) AGN represents a critical yet elusive phase in the growth of supermassive black holes. Constraining their abundance and evolution at high z is essential for understanding both SMBH growth and the origin of the cosmic X-ray background. We investigate the X-ray luminosity function (XLF) of CTK AGN at z > 3 using one of the largest available samples of X-ray-selected AGN at high z, containing 811 sources from XMM-Newton XXL-N and Chandra CCLS and CDF-S/N surveys. We first selected a subsample of ten high-probability CTK candidates, identified through x-ray spectral fitting. Their multiwavelength properties are examined through SED modelling to assess the robustness of their CTK classification. For most sources, the inferred X-ray luminosities appear overestimated when compared with their IR luminosities. After updating the NH posteriors with IR-informed priors, only three sources remain consistent with the CTK regime. To compute the XLF for the entire CTK AGN population, we used 24 microns photometry to estimate IR luminosities and update the X-ray posteriors for all the remaining sources. Incorporating IR priors systematically reduces the inferred CTK number densities, yielding a more conservative and physically consistent estimate of the XLF. We find that CTK AGN constitute 17 per cent of the total AGN population at 3 < z < 6, consistent with results at lower z. Our analysis reveals no statistically significant evolution in the CTK fraction up to z about 6, suggesting that the most heavily obscured accretion phase remains a persistent component of black hole growth throughout cosmic history. While the overall obscured AGN fraction (NH > 1e23 cm-2) increases toward higher redshifts, the stable CTK fraction supports the interpretation that, at these epochs, the interstellar medium in typical host galaxies cannot produce CTK levels of obscuration.

S1: 1 Introduction / block 1 · html

The broad energy range of NuSTAR detectors (8–-79 keV) makes it a powerful tool for the study of CTK-AGN albeit its sensitivity limits its ability to study obscured AGN in the high redshift ( z > 3 z>3 ) regime. Recent studies based on NuSTAR observations yield similar results for the fraction of CTK AGN in the local Universe ( Boorman et al., 2025 ; Georgantopoulos et al., 2025 ) . Ananna et al. (2019) reported a fraction of CTK-AGN in the local universe ( z ≈ 0.1 z\approx 0.1 ) that can reach up 50 ± 9 50\pm 9 % provided that extremely obscured sources with log ⁡ N H > 25 \log N_{\mathrm{H}}>25 are taken into account. This result is derived from a population synthesis model fitting the observed Cosmic X-ray Background in the 0.5–100 keV energy range.

S4.SS2: 4.2 The CTK AGN luminosity function / block 1 · html

Figure 8 : X-ray luminosity function for the CTK population ( 24 < log ⁡ N H < 26 24<\log N_{\mathrm{H}}<26 ). Red symbols show our result for the parametric XLF in the 3–6 redshift range. The red solid line corresponds to the median value and shaded areas are the 1 ​ σ 1\sigma (darker red) and 2 ​ σ 2\sigma (lighter red) uncertainties. The black-dashed line is the XLF for CTK sources in the local universe ( z < 0.05 z<0.05 ) derived by Georgantopoulos et al. (2025) . The grey, hatched area is the non-parametric XLF in the 3.1–7 redshift range estimated by Buchner et al. (2015) . The area shows the 90% confidence region. The blue-dotted line corresponds to the Aird et al. (2015b) XLF for CTK AGN in the 3–6 redshift range.

S4.SS2: 4.2 The CTK AGN luminosity function / block 1 · html

Figure 9 : Evolution of the fraction of CTK sources over the total AGN population across redshift. Our results are shown with the red shaded regions that represent the 1 ​ σ 1\sigma (darker red) and 2 ​ σ 2\sigma (lighter red) confidence regions. The gray-hatched area corresponds to the Buchner et al. (2015) results (90% confidence interval) for AGN in the 43.2 ≤ log ⁡ L X ≤ 43.6 43.2\leq\log L_{\mathrm{X}}\leq 43.6 interval. The blue-hatched area shows the Aird et al. (2015b) estimates (99% confidence interval) for sources with log ⁡ L X = 43.5 \log L_{\mathrm{X}}=43.5 . The green squares show the results by Laloux et al. (2023) ; quoted upper-limits are 3 ​ σ 3\sigma . Pink triangles correspond to the results by Lanzuisi et al. (2018) and purple diamond corresponds to a study by Masini et al. (2018) . The black ( Georgantopoulos et al., 2025 ) and yellow ( Burlon et al., 2011 ) circles show the CTK fraction in the local universe using Swift -BAT selected AGN. Red diamonds show the fraction of CTK sources estimated through the integration of the Ananna et al. (2019) luminosity function. Open symbols show the result in the range ( 24 ≤ log ⁡ N H < 26 24\leq\log N_{\mathrm{H}}<26 ), while filled symbols are for ( 24 ≤ log ⁡ N H < 25 24\leq\log N_{\mathrm{H}}<25 ).

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Among these studies, the results of Buchner et al. (2015) and Lanzuisi et al. (2018) stand out for their remarkably high CTK fraction of approximately 40-50%, among the largest values reported in the literature at high redshift. Such high fractions are, however, in clear tension with both our findings and those of other works. For example, Aird et al. (2015b) reported lower CTK fractions that are broadly consistent with our results in the z = 3 − 6 z=3-6 range, although they noted that their highest-redshift bins may be affected by limited statistics. The results of Ananna et al. (2019) show no evolution with redshift and, for the population with log ⁡ N H < 25 \log N_{\mathrm{H}}<25 , the CTK fraction is about 30 per cent, consistent with previous results in similar redshift ranges. The estimate for the high redshift range ( 3 < z < 5 3<z<5 ) aligns with our estimate of the CTK fraction at high redshifts. However, when considering the population with log ⁡ N H > 25 \log N_{\mathrm{H}}>25 , the CTK fraction from Ananna et al. (2019) reaches ∼ 50 % \sim 50\% that is similar to the highest values reported in the literature ( Lanzuisi et al., 2018 ) . This discrepancy may arise from the limited sensitivity to this highly absorbed population in the X-ray energies examined by these studies.

A2: Appendix B X-ray properties / block 1 · html

These figures illustrate the data quality and spectral characteristics of the selected CTK candidates, highlighting the range of absorption levels and luminosities found within the sample. They also provide a visual summary of the Bayesian modelling results, allowing a direct assessment of the uncertainties and degeneracies that affect the determination of N H N_{\mathrm{H}} and L X L_{\mathrm{X}} for individual sources.

S3.SS1: 3.1 Spectral energy distributions of the CTK candidates / block 1 · html

In summary, we employed the stellar synthesis population models of Bruzual and Charlot (2003) , assuming a Salpeter (1955) initial mass function and a fixed metallicity of Z = 0.02 Z=0.02 . The star formation history follows a delayed model of the form SFR ∝ t × e − t / τ \mathrm{SFR}\propto t\times e^{-t/\tau} , incorporating a short starburst phase capped at 50 Myr. Dust emission was modelled using the templates of Dale et al. (2014) , excluding any AGN contribution, while dust attenuation was treated using the extinction law of Charlot and Fall (2000) . For the AGN component, we adopted the SKIRTOR model ( Stalevski et al., 2012 ; Stalevski et al., 2016 ) . The full list of modules and parameters used in our analysis is provided in Table 4 . This setup allowed us to fit the SEDs using a grid comprising over 29 million models.

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The results of our SED fitting are summarized in Table 2 . We report three key parameters relevant to our analysis: the inclination angle of the AGN torus, the AGN fractional contribution to the total IR emission, and the AGN luminosity at 6 ​ μ ​ m 6\,\mathrm{\upmu m} . For each parameter, we provide two estimates: “best” refers to the values obtained from the model with the lowest χ 2 \chi^{2} in the model grid explored by CIGALE , while ‘‘bayes’’ corresponds to the Bayesian estimate (i.e. the probability-weighted mean and standard deviation across all models). 4 4 4 If the “bayes” value is consistent with zero, we quote the 1 ​ σ 1\sigma upper-limit. As an example, Fig. 2 shows the best-fit SED model obtained by CIGALE for source lid_1278.

S3.SS1: 3.1 Spectral energy distributions of the CTK candidates / block 1 · html

Our results are consistent with the presence of heavily obscured AGN. The high torus inclination angles indicate that the accretion disc emission is hidden in the optical regime. Although the Bayesian estimates for the AGN fraction are consistent with zero for some sources, visual inspection of the best-fitting SED models clearly shows that an AGN component is required to reproduce the observed 24 ​ μ ​ m 24\,\mathrm{\upmu m} emission. The host-galaxy component alone cannot account for such high infrared fluxes, as also indicated by the non-zero “best” AGN fractions reported in Table 2 . This is clearly illustrated in Fig. 2 , where the optical part of the SED is dominated by the host galaxy (grey dotted line), whereas the rest-frame ∼ 3 − 30 ​ μ ​ m \sim 3-30\,\mathrm{\upmu m} range is clearly dominated by the AGN component (red dashed line).

S3.SS2: 3.2 Constraining the X-ray absorption via the L X − L 6 ​ μ ​ m L_{\mathrm{X}}-L_{\mathrm{6\,\upmu m}} correlation / block 1 · html

Figure 3 : Intrinsic X-ray luminosity in the 2–10 keV band versus the monochromatic 6 ​ μ ​ m 6\,\mathrm{\upmu m} luminosity of the AGN component. The results for our CTK sample are shown before (left panel) and after (right panel) applying the infrared luminosity prior, as described in Sect. 3.2 . Grey dots represent the X-ray–selected AGN sample from Laloux et al. (2023) in the CCLS field. Large circles indicate our final CTK candidates, colour-coded by their parent X-ray survey (yellow: CDF; red: CCLS; blue: XXL-N). The black dashed line marks the L X − L 6 ​ μ ​ m L_{\mathrm{X}}-L_{\mathrm{6\,\upmu m}} relation from Stern (2015) , while the grey dashed lines denote the 2 ​ σ 2\sigma dispersion of the Laloux et al. (2023) sample with respect to that relation.

abstract / block 1 · abstract

Recent observations expanded our understanding of galaxy formation and evolution, yet key challenges persist in the X-ray regime, crucial for studying Active Galactic Nuclei (AGN). These limitations drive the development of next-generation observatories such as ESA's NewAthena. Now in phase B (preliminary design), the mission requires extensive testing to ensure compliance with its scientific goals, particularly given the uncertainties surrounding high redshift AGN. This work leverages the IllustrisTNG cosmological simulation to build an X-ray AGN mock catalogue and assess the performance of NewAthena's WFI. We created a Super Massive Black Hole (SMBH) light cone, spanning 10 deg2, with corrections to account for the limited resolution of the simulation and X-ray properties derived in post-processing. The resulting catalogue reveals a 5* overabundance of faint AGN compared to current X-ray constraints, an inconsistency potentially resolved by invoking a higher Compton-thick (CTK) fraction and intrinsic X-ray weakness, as suggested by recent JWST findings. An end-to-end survey simulation using SIXTE predicts 250000 AGN detections, including 20,000 at z > 3 and 35 in the Epoch of Reionization (z > 6); notably, only AGN with LX > 43.5 erg/s are detectable at z > 6. The analysis also forecasts a significant population of detectable CTK AGN, even beyond z > 4. These findings suggest X-ray observations will, for the first time, probe a significant AGN population in the EoR, offering new insights into SMBH growth. They also provide key input for refining NewAthena's mission design and optimizing its survey strategy.

S5: V Spectral Fitting Results / block 1 · html

We found 10 (38.5%), 6 (23.1%), and 6 (23.1%) sources classified as Uncertain by MyTorus , Borus02 , and UXCLUMPY , respectively. Among the classified sources, MyTorus identified a single CT-AGN candidate (ESO 362 − - 8; 3.8%), Borus02 identified three (ESO 362 − - 8, 2MASX J02420381+0510061, and 2MASX J06363227 − - 2034532; 11.5%), and UXCLUMPY identified two (MRK 1073 and ESO 362 − - 8; 7.7%). The agreement across all three models highlights ESO 362 − - 8 as a robust CT-AGN candidate, consistent with earlier findings [ 66 ] . In addition to the CT classification, we examined the distribution of sources below the CT threshold (excluding the uncertain cases). We found that 10 (38.5%), 10 (38.5%), and 11 (42.3%) sources in MyTorus , Borus02 , and UXCLUMPY , respectively, have 10 23 ≤ N H , los < 10 24 10^{23}\leq N_{\rm H,los}<10^{24} cm − 2 \rm{cm^{-2}} , while 5 (19.2%), 7 (26.9%), and 7 (26.9%) sources fall below 10 23 10^{23} cm − 2 \rm{cm^{-2}} .

A1: Appendix A Source description / block 1 · html

3C 371: This is the only BL LAC source analyzed in this work. This source, located at z z = 0.0495, was observed once by NuSTAR in 2022, and once by Chandra in 2000. The three torus models yielded broadly consistent results, with MyTorus giving N H , los N_{\rm H,los} = 0.022 − 0.006 + 0.006 {}^{+0.006}_{-0.006} × \times 10 24 cm -2 , Borus02 yielding N H , los N_{\rm H,los} = 0.034 − 0.018 + 0.010 {}^{+0.010}_{-0.018} × \times 10 24 cm -2 , and UXCLUMPY estimated N H , los N_{\rm H,los} = 0.025 − 0.010 + 0.001 {}^{+0.001}_{-0.010} × \times 10 24 cm -2 . the N H , avg N_{\rm H,avg} obtained from the MyTorus and Borus02 fits are 0.50 − 0.088 + 0.010 {}^{+0.010}_{-0.088} × \times 10 24 cm -2 and 0.54 − 0.088 + 0.010 {}^{+0.010}_{-0.088} × \times 10 24 cm -2 , respectively. While the derived column densities differ–log N H , los / cm − 2 N_{\rm H,los/cm^{-2}} = 23.14 from [ 6 ] and 22.10 from [ 65 ] – both studies ultimately classified the source as Compton-thin. The soft part of the spectra is modeled using a mekal component with kT e \rm{kT_{e}} ∼ \sim 0.35 keV (see Table 3 ).

S5.SS1: V.1 Are we missing the majority of Little Red Dots? / block 1 · html

Finally, the F277W ​ – ​ F444W \mathrm{F277W\text{--}F444W} color distribution shows that very red sources (e.g., ≳ 1.5 \gtrsim 1.5 mag; Akins et al. 1 ) constitute only a minor fraction of the LRD population ( ≲ 25 % \lesssim 25\% ), consistent with recent spectroscopic results ( Pérez-González et al. 107 ). The majority instead occupy significantly bluer optical/NIR colors (e.g., ≈ 55 \approx 55 % with F277W ​ – ​ F444W = 0.5 ​ – ​ 1 \rm F277W\text{--}F444W=0.5\text{--}1 mag). This region of parameter space remains largely unexplored, likely biasing current samples toward an extreme subset of LRDs [ 107 ] rather than the full population. If current selection criteria, and subsequent follow-up efforts, are preferentially sensitive to the reddest systems, then our current view of LRDs is inherently biased toward a specific sub-population, preventing both (i) a meaningful inference of their evolutionary trends across cosmic time and (ii) a robust physical interpretation of the LRD phenomenon, by disproportionately emphasizing a single sub-type of a broader, heterogeneous population (e.g., Barro et al. 14 , Pérez-González et al. 107 ), potentially favoring ad hoc models and hindering the development of a unified framework that captures their full diversity and possible evolutionary pathways.

S3.SS2.SSS5: III.2.5 The final sample / block 1 · html

As a sanity check, motivated by recent results showing that photometrically selected LRDs can also be contaminated by quiescent galaxies (in addition to brown dwarfs and strong-line emitters), we cross-matched our sample with the catalog of (photometrically selected) quiescent galaxies in the JADES fields presented by Baker et al. [11] [ 10 , using the criteria of] . We find three matches. Among these three sources, two show clear photometric excess in the medium bands at the wavelengths expected for H β \beta +[O iii ] and H α \alpha (IDs 152330 and 226386; JADES-GS+53.1027-27.7444 and JADES-GS+53.1912-27.8357), indicating the presence of emission lines and therefore disfavoring a quiescent interpretation [ 25 , 66 , 10 , although an AGN within a quiescent galaxy remain a potential explanation; e.g.,] . The third source is excluded from our sample, as we cannot draw firm conclusions on its nature. For JADES-GS+53.1027 − - 27.7444, a NIRCam/WFSS spectrum from FRESCO is available and reveals a broad H α \alpha component with a full width at half maximum (FWHM) of 2068.78 ± 40.82 2068.78\pm 40.82 km s -1 , which was not reported in previous analyses based on FRESCO data such as Matthee et al. [92] .

abstract · abstract

The study of heavily obscured supermassive black hole (SMBH) growth in late-stage galaxy mergers is challenging: column densities $N_{\mathrm{H}}>10^{24},\mathrm{cm}^{-2}$ can block most nuclear emission, leaving significant gaps in the SMBH growth census. Millimeter-wave continuum emission offers a potential window into this obscured phase, as it can trace Active Galactic Nuclei (AGN) activity through mechanisms less affected by dust extinction. In this work, we test whether the observed correlation between millimeter ($\sim200,\mathrm{GHz}$) and hard X-ray (14 - 150,keV) luminosities can be used to plausibly identify hidden AGN in local (Ultra)Luminous Infrared Galaxies (U)LIRGs, including systems hosting confirmed dual AGN. We identify three sources -- one confirmed AGN and two strong candidates -- presenting significant evidence of AGN activity. The confirmed dual AGN lie within $\sim3σ$ of the mm--X-ray correlation, suggesting this relation can be used to identify hidden pairs. By combining the position of each source relative to this correlation with independent star formation rate constraints, we propose a method to disentangle AGN and star formation contributions for sources with measured column densities. While our analysis is based on a small, heterogeneous local sample and relies on empirical scaling relations, these results indicate that millimeter continuum emission may provide a useful complementary diagnostic for obscured SMBH growth. ALMA observations at high angular resolutions are particularly valuable for this approach, while future facilities such as the ngVLA will be essential to test its robustness in larger and more distant samples.

abstract · abstract

JWST has revealed an abundance of supermassive black holes (BHs) in the early Universe, and yet the lowest mass seed black holes that gave rise to these populations remain elusive. Here we present a systematic search for broad-line Active Galactic Nuclei (AGNs) in some of the faintest high-$z$ galaxies surveyed yet by combining ultra-deep JWST/NIRSpec G395M spectroscopy with the strong lensing aid in Abell S1063. By employing the profile of the [OIII]$λ5007$ emission lines as a template for narrow-line components and carefully cross-validating with mock observations, we identify a sample of ten broad-line AGNs at $4.5<z<7.0$ (eight secure, two tentative). The inferred BH masses from the broad H$α$ line explore the intermediate BH mass regime down to $\sim 10^{5.5}\,M_\odot$. The stellar mass ($M_*$) is estimated with a galaxy+AGN composite model, and we find the BH to stellar mass ratio spans down to $M_{\rm BH}/M_*\lesssim 0.1\%$, unveiling populations on the empirical $M_{\rm BH}-M*$ relation observed in the local universe. We also derive the black hole mass function and investigate its low-mass end at this epoch. While we confirm the agreement of our results with previous studies at $M_{\rm BH}\gtrsim10^{6.5}M_{\odot}$, we find the mass range of $\sim 10^{5.5}\,M_\odot$ features an enhanced abundance with respect to the extrapolated best-fit Schechter function. Comparison with theoretical models suggests that a possible origin for this enhanced abundance is the direct-collapse BH formation, supporting the scenario that the direct collapse of massive gas clouds is a significant pathway for the earliest supermassive BHs.

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Vaida et al. Little Red Dots in the JWST Era Lovell, C. C., Harrison, I., Harikane, Y., Tacchella, S., and Wilkins, S. M. (2023). Extreme value statistics of the halo and stellar mass distributions at high redshift: are jwst results in tension with λcdm? Monthly Notices of the Royal Astronomical Society 518, 2511–2520 Lu, Y., Picker, Z. S., and Kusenko, A. (2024). Direct collapse supermassive black holes from relic particle decay. Physical Review Letters 133, 091001 Madau, P. (2025). Chasing the light: Shadowing, collimation, and the super-eddington growth of infant black holes in jwst-discovered agns. arXiv preprint arXiv:2501.09854 Maiolino, R., Risaliti, G., Signorini, M., Trefoloni, B., Juodžbalis, I., Scholtz, J., et al. (2025). Jwst meets chandra: a large population of compton thick, feedback-free, and intrinsically x-ray weak agn, with a sprinkle of sne. Monthly Notices of the Royal Astronomical Society 538, 1921–1943 Matthee, J., Naidu, R. P., Brammer, G., Chisholm, J., Eilers, A.-C., Goulding, A., et al. (2024). Little red dots: an abundant population of faint active galactic nuclei at z 5 revealed by the eiger and fresco jwst surveys. The Astrophysical Journal 963, 129 Mazzolari, G., Gilli, R., Maiolino, R., Prandoni, I., Delvecchio, I., Norman, C., et al. (2024). The radio properties of the jwst-discovered agn. arXiv preprint arXiv:2412.04224 McElwain, M. W., Feinberg, L. D., Perrin, M. D., Clampin, M., Mountain, C. M., Lallo, M. D., et al. (2023). The james webb space telescope mission: optical telescope element design, development, and performance. Publications of the Astronomical Society of the Pacific 135, 058001 Naidu, R. P., Matthee, J., Katz, H., de Graaff, A., Oesch, P., Smith, A., et al. (2025). A” black hole star” reveals the remarkable gas-enshrouded hearts of the little red dots. arXiv preprint arXiv:2503.16596 Oesch, P. A., Brammer, G., Naidu, R., Bouwens, R., Chisholm, J., Illingworth, G., et al. (2023). The jwst fresco survey: legacy nircam/grism spectroscopy and imaging in the two goods fields. Monthly Notices of the Royal Astronomical Society 525, 2864–2874 Onoue, M., Inayoshi, K., Ding, X., Li, W., Li, Z., Molina, J., et al. (2023).

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Vaida et al. Little Red Dots in the JWST Era deep NOEMA observations of z > 7 LRD (Xiao et al., 2025) as well as multi-band Atacama Large Millimeter/Submillimeter Array (ALMA; Wootten and Thompson, 2009) observations of a couple of the brightest LRD known resulted in stringent non-detections (Setton et al., 2025). Observations of a larger sample (∼60) LRD with ALMA in the 1.3 mm continuum band similarly resulted in no detections, even with stacking, suggesting LRD either have modest dust reservoirs (Av ∼2 −4) or otherwise very dense gas 109 cm−3 causes the obscuration. The paucity of dust in LRD extends to a broader range of wavelengths in the far-IR and sub-mm: LRD observations in the rest-frame IR in Spitzer/MIPS 24 µm, JCMT/SCUBA-2 850 µm, ALMA 1.2 & 2.0 mm, and MeerKAT/VLA 1.3 & 3.0 GHz all resulted in non-detections (Bao et al., 2025). Consistent with non-detections at redder wavelengths, Brooks et al. (2025) investigated the dust attenuation of Balmer narrow-line emission and broad-line signatures in 29 z > 3.5 AGN selected from JWST surveys (CEERS, JADES, and RUBIES). The narrow-line emission is consistent with no dust attenuation, which Brooks et al. (2025) consider to be consistent with star formation at larger scales in the galaxy. They place a lower limit on the dust attenuation Av > 3.6 due to non-detection (< 3σ) of the broad Hβ line even after stacking 25 of their sources, which they say is consistent with a dust-obscured AGN. Multi-wavelength observations resulting in non-detections have extended to an increasingly larger sample of LRD. Studying 434 LRD in the JWST COSMOS-Web survey, Akins et al. (2025b) find no detections, even when stacking, in X-ray, mid-IR, far-IR/submillimeter, and radio bands. Their non-detections are consistent with prior work of smaller samples that found LRD are not detected, or quite weakly emitting, in X-ray (Ananna et al., 2024; Yue et al., 2024; Maiolino et al., 2025) and in radio (Mazzolari et al., 2024; Perger et al., 2025; Gloudemans et al., 2025). 4 THEORETICAL INTERPRETATIONS We next consider how LRD spectral properties arise and why AGN and stellar interpretations can both appear. Forward modeling shows that an

abstract · abstract

X-ray surveys of active galactic nuclei (AGNs) provide direct constraints on the properties of individual AGNs, such as their emission, obscuration, and accretion rate. Previous AGN population synthesis models have not addressed such properties self-consistently. Here, we use a simulation-based inference (SBI) approach to constrain the geometrical and physical properties of the AGN population. We perform numerical simulations with our ray-tracing code, RefleX, which allows the self-consistent modelling of the X-ray emission of AGNs with flexible circumnuclear and source geometries. We create our synthetic population by sampling the intrinsic active black hole mass function (BHMF) and Eddington ratio distribution function (ERDF) of local AGNs, and we construct a geometry based on the radiation-regulated model, along with Eddington-ratio-dependent emission spectra. Using the RefleX-simulated emission of the AGN population, we aim to simultaneously reproduce the cosmic X-ray background (CXB), differential AGN number counts, and several observed absorption properties of local AGNs, such as the fraction of $N_\mathrm{H}$ in bins of log($N_\mathrm{H}$), the Compton-thick fraction as a function of limiting flux, and the number of obscured and unobscured AGNs as a function of Eddington ratio. With this approach, we test the consistency of the radiation-regulated model with a very comprehensive set of X-ray observables, while constraining the size and density of the dusty torus and the evolution of the local AGN population. We derive an intrinsic Compton-thick fraction of $40\pm3$%, and find that a simple evolutionary prescription controlling the active fraction of supermassive black holes is sufficient for our synthetic population to reproduce the CXB.

abstract · abstract

Characterizing the growth of supermassive Black Holes (SMBHs) is critical to the evolution of galaxies, however the majority of this activity is obscured, rendering traditional tracers of active SMBHs, such as in the restframe optical/UV, ineffective. The mid-infrared has been particularly successful in revealing obscured AGN activity however much of this work is confined to the local universe due to the lack of a far-IR telescope with the required sensitivity and wavelength coverage. In this work we demonstrate the effectiveness of PRIMA (PRobe far-Infrared Mission for Astrophysics), a concept 1.8m far-IR observatory, to detect and characterize deeply obscured galaxy nuclei over cosmic time. With the PRIMAger instrument covering 25 - 235 $μ$m, we find that we can accurately detect obscured nuclei via the deep silicate absorption at restframe $9.8 μ$m between $z=2-7$. Additionally, the FIRESS spectrograph can produce R$\sim$100 spectra of obscured nuclei out to $z\sim7$, detecting Polycyclic Aromatic Hydrocarbons (PAHs), ices, ionized and molecular gas. With the large number of deeply obscured nuclei PRIMA can detect and characterize, such a mission is critical to understanding the growth of SMBHs.