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1. The Active Galactic Nuclei in the Hobby–Eberly Telescope Dark Energy Experiment Survey (HETDEX). II. Luminosity Function

   https://doi.org/10.3847/1538-4357/ac8054  

   Liu, Chenxu ; Gebhardt, Karl ; Cooper, Erin Mentuch ; Zhang, Yechi ; Schneider, Donald P. ; Ciardullo, Robin ; Davis, Dustin ; Farrow, Daniel J. ; Finkelstein, Steven L. ; Gronwall, Caryl ; et al ( August 2022 , The Astrophysical Journal)

   We present the Lyαemission line luminosity function (LF) of the active galactic nuclei (AGN) in the first release of the Hobby–Eberly Telescope Dark Energy Experiment Survey (HETDEX) AGN catalog. The AGN are selected either by emission line pairs characteristic of AGN or by a single broad emission line, free of any photometric preselections (magnitude/color/morphology). The sample consists of 2346 AGN spanning 1.88 <z< 3.53, covering an effective area of 30.61 deg². Approximately 2.6% of the HETDEX AGN are not detected at >5σconfidence atr∼ 26 in the deepestr-band images we have searched. The Lyαline luminosity ranges from ∼10^42.3to 10^45.9erg s⁻¹. Our LyαLF shows a turnover luminosity with opposite slopes on the bright end and the faint end: The space density is highest at$L_{\text{Ly}\mathit{\text{α}}}^{\ast }={10}^{43.4}$erg s⁻¹. We explore the evolution of the AGN LF over a broader redshift range (0.8 <z< 3); constructing the rest-frame ultraviolet (UV) LF with the 1450 Å monochromatic luminosity of the power-law component of the continuum (M₁₄₅₀) fromM₁₄₅₀∼ −18 to −27.5. We divide the sample into three redshift bins (z∼ 1.5, 2.1, and 2.6). In all three redshift bins, our UV LFs indicate that the space density of AGN is highest at the turnover luminosity$M_{1450}^{*}$with opposite slopes on the bright end and the faint end. TheM₁₄₅₀LFs in the three redshift bins can be well fit with a luminosity evolution and density evolution model: the turnover luminosity ($M_{1450}^{*}$) increases, and the turnover density (Φ*) decreases with increasing redshift.

    

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2. On the Coevolution of the AGN and Star-forming Galaxy Ultraviolet Luminosity Functions at 3 < z < 9

   https://doi.org/10.3847/1538-4357/ac89eb  

   Finkelstein, Steven L. ; Bagley, Micaela B. ( October 2022 , The Astrophysical Journal)

   Abstract Studies of the rest-frame ultraviolet (UV) luminosity functions (LFs) typically treat star-forming galaxies and active galactic nuclei (AGNs) separately. However, modern ground-based surveys now probe volumes large enough to discover AGNs at depths sensitive enough for fainter galaxies, bridging these two populations. Using these observations as constraints, we present a methodology to empirically jointly model the evolution of the rest-UV LFs at z = 3–9. Our critical assumptions are that both populations have LFs well described by double power laws modified to allow for a flattening at the faint-end, and that all LF parameters evolve smoothly with redshift. This methodology provides a good fit to the observations and makes predictions to volume densities not yet observed, finding that the volume density of bright ( M UV = −28) AGNs rises by ∼10 5 from z = 9 to z = 3, while bright ( M UV = −21) star-forming galaxies rise by only ∼10 2 across the same epoch. The observed bright-end flattening of the z = 9 LF is unlikely to be due to AGN activity, and rather is due to a shallowing of the bright-end slope, implying a reduction of feedback in bright galaxies at early times. The intrinsic ionizing emissivity is dominated by star-forming galaxies at z > 3, even after applying a notional escape fraction. We find decent agreement between our AGN LFs and predictions based on different black hole seeding models, though all models underpredict the observed abundance of bright AGNs. We show that the wide-area surveys of the upcoming Euclid and Roman observatories should be capable of discovering AGNs to z ∼ 8. 

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3. Supermassive black holes in cosmological simulations – II: the AGN population and predictions for upcoming X-ray missions

   https://doi.org/10.1093/mnras/stab3147  

   Habouzit, Mélanie ; Somerville, Rachel S ; Li, Yuan ; Genel, Shy ; Aird, James ; Anglés-Alcázar, Daniel ; Davé, Romeel ; Georgiev, Iskren Y ; McAlpine, Stuart ; Rosas-Guevara, Yetli ; et al ( November 2021 , Monthly Notices of the Royal Astronomical Society)

   ABSTRACT In large-scale hydrodynamical cosmological simulations, the fate of massive galaxies is mainly dictated by the modelling of feedback from active galactic nuclei (AGNs). The amount of energy released by AGN feedback is proportional to the mass that has been accreted on to the black holes (BHs), but the exact subgrid modelling of AGN feedback differs in all simulations. While modern simulations reliably produce populations of quiescent massive galaxies at z ≤ 2, it is also crucial to assess the similarities and differences of the responsible AGN populations. Here, we compare the AGN populations of the Illustris, TNG100, TNG300, Horizon-AGN, EAGLE, and SIMBA simulations. The AGN luminosity function (LF) varies significantly between simulations. Although in agreement with current observational constraints at z = 0, at higher redshift the agreement of the LFs deteriorates with most simulations producing too many AGNs of $L_{\rm x, 2\!-\!10 \, keV}\sim 10^{43\!-\!44}\, \rm erg\, s^{-1}$. AGN feedback in some simulations prevents the existence of any bright AGN with $L_{\rm x, 2\!-\!10 \, keV}\geqslant 10^{45}\rm \,erg\, s^{-1}$ (although this is sensitive to AGN variability), and leads to smaller fractions of AGN in massive galaxies than in the observations at z ≤ 2. We find that all the simulations fail at producing a number density of AGN in good agreement with observational constraints for both luminous ($L_{\rm x, 2\!-\!10 \, keV}\sim 10^\text{43-45}\, \rm erg\, s^{-1}$) and fainter ($L_{\rm x, 2\!-\!10 \, keV}\sim 10^\text{42-43}\, \rm erg\, s^{-1}$) AGNs and at both low and high redshifts. These differences can aid us in improving future BH and galaxy subgrid modelling in simulations. Upcoming X-ray missions (e.g. Athena, AXIS, and LynX) will bring faint AGNs to light and new powerful constraints. After accounting for AGN obscuration, we find that the predicted number density of detectable AGNs in future surveys spans at least one order of magnitude across the simulations, at any redshift. 

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4. Identifying Active Galactic Nuclei at z ∼ 3 from the HETDEX Survey Using Machine Learning

   https://doi.org/10.3847/1538-3881/acba92  

   Tardugno Poleo, Valentina ; Finkelstein, Steven L. ; Leung, Gene ; Mentuch Cooper, Erin ; Gebhardt, Karl ; Farrow, Daniel J. ; Gawiser, Eric ; Zeimann, Greg ; Schneider, Donald P. ; Morabito, Leah ; et al ( March 2023 , The Astronomical Journal)

   Abstract We used data from the Hobby–Eberly Telescope Dark Energy Experiment (HETDEX) to study the incidence of AGN in continuum-selected galaxies at z ∼ 3. From optical and infrared imaging in the 24 deg 2 Spitzer HETDEX Exploratory Large Area survey, we constructed a sample of photometric-redshift selected z ∼ 3 galaxies. We extracted HETDEX spectra at the position of 716 of these sources and used machine-learning methods to identify those which exhibited AGN-like features. The dimensionality of the spectra was reduced using an autoencoder, and the latent space was visualized through t-distributed stochastic neighbor embedding. Gaussian mixture models were employed to cluster the encoded data and a labeled data set was used to label each cluster as either AGN, stars, high-redshift galaxies, or low-redshift galaxies. Our photometric redshift (photo z ) sample was labeled with an estimated 92% overall accuracy, an AGN accuracy of 83%, and an AGN contamination of 5%. The number of identified AGN was used to measure an AGN fraction for different magnitude bins. The ultraviolet (UV) absolute magnitude where the AGN fraction reaches 50% is M UV = −23.8. When combined with results in the literature, our measurements of AGN fraction imply that the bright end of the galaxy luminosity function exhibits a power law rather than exponential decline, with a relatively shallow faint-end slope for the z ∼ 3 AGN luminosity function. 

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5. A cosmic UV/X-ray background model update

   https://doi.org/10.1093/mnras/staa302  

   Faucher-Giguère, Claude-André ( April 2020 , Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We present an updated model of the cosmic ionizing background from the UV to the X-rays. Relative to our previous model, the new model provides a better match to a large number of up-to-date empirical constraints, including: (1) new galaxy and AGN luminosity functions; (2) stellar spectra including binary stars; (3) obscured and unobscured AGN; (4) a measurement of the non-ionizing UV background; (5) measurements of the intergalactic H i and He ii photoionization rates at z ∼ 0−6; (6) the local X-ray background; and (7) improved measurements of the intergalactic opacity. In this model, AGN dominate the H i ionizing background at z ≲ 3 and star-forming galaxies dominate it at higher redshifts. Combined with the steeply declining AGN luminosity function beyond z ∼ 2, the slow evolution of the H i ionization rate inferred from the high-redshift H i Ly α forest requires an escape fraction from star-forming galaxies that increases with redshift (a population-averaged escape fraction of $\approx 1{{\ \rm per\ cent}}$ suffices to ionize the intergalactic medium at z = 3 when including the contribution from AGN). We provide effective photoionization and photoheating rates calibrated to match the Planck 2018 reionization optical depth and recent constraints from the He ii Ly α forest in hydrodynamic simulations. 

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