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We present a new modeling of the X-ray luminosity function (XLF) of active galactic nuclei (AGNs) out to z ∼ 3, dissecting the contributions of main-sequence (MS) and starburst (SB) galaxies. For each galaxy population, we convolved the observed galaxy stellar mass (M⋆) function with a grid of M⋆-independent Eddington ratio (λEDD) distributions, normalized via empirical black hole accretion rate (BHAR) to star formation rate (SFR) relations. Our simple approach yields an excellent agreement with the observed XLF since z ∼ 3. We find that the redshift evolution of the observed XLF can only be reproduced through an intrinsic flattening of the λEDD distribution and with a positive shift of the break λ*, consistent with an antihierarchical behavior. The AGN accretion history is predominantly made by massive (1010 < M⋆ < 1011M⊙) MS galaxies, while SB-driven BH accretion, possibly associated with galaxy mergers, becomes dominant only in bright quasars, at log(LX/erg s−1) > 44.36 + 1.28 × (1 + z). We infer that the probability of finding highly accreting (λEDD > 10%) AGNs significantly increases with redshift, from 0.4% (3.0%) at z = 0.5%–6.5% (15.3%) at z = 3 for MS (SB) galaxies, implying a longer AGN duty cycle in the early universe. Our results strongly favor a M⋆-dependent ratio between BHAR and SFR, as BHAR/SFR ∝ , supporting a nonlinear BH buildup relative to the host. Finally, this framework opens potential questions on super-Eddington BH accretion and different λEDD prescriptions for understanding the cosmic BH mass assembly. [ABSTRACT FROM AUTHOR]

We explore the kinematics of ionized gas via the [O iii] λ5007 emission lines in active galactic nuclei (AGNs) selected on the basis of their mid-infrared (IR) emission, and split into obscured and unobscured populations based on their optical−IR colors. After correcting for differences in redshift distributions, we provide composite spectra of spectroscopically and photometrically defined obscured/Type 2 and unobscured/Type 1 AGNs from 3500 to 7000 Å. The IR-selected obscured sources contain a mixture of narrow-lined Type 2 AGNs and intermediate sources that have broad Hα emission and significantly narrower Hβ. Using both [O iii] luminosities and AGN luminosities derived from optical−IR spectral energy distribution fitting, we find evidence for enhanced large-scale obscuration in the obscured sources. In matched bins of luminosity we find that the obscured population typically has broader, more blueshifted [O iii] emission than in the unobscured sample, suggestive of more powerful AGN-driven outflows. This trend is not seen in spectroscopically classified samples, and is unlikely to be entirely explained by orientation effects. In addition, outflow velocities increase from small to moderate AGN E(B − V) values, before flattening out (as traced by FWHM) and even decreasing (as traced by blueshift). While difficult to fully interpret in a single physical model, due to both the averaging over populations and the spatially averaged spectra, these results agree with previous findings that simple geometric unification models are insufficient for the IR-selected AGN population, and may fit into an evolutionary model for obscured and unobscured AGNs. [ABSTRACT FROM AUTHOR]

We discuss the spectral analysis of a sample of 63 active galactic nuclei (AGN) detected above a limiting flux of in the multi-tiered NuSTAR extragalactic survey program. The sources span a redshift range (median ). The spectral analysis is performed over the broad 0.5–24 keV energy range, combining NuSTAR with Chandra and/or XMM-Newton data and employing empirical and physically motivated models. This constitutes the largest sample of AGN selected at to be homogeneously spectrally analyzed at these flux levels. We study the distribution of spectral parameters such as photon index, column density (), reflection parameter (), and 10–40 keV luminosity (). Heavily obscured () and Compton-thick (CT; ) AGN constitute ∼25% (15–17 sources) and ∼2–3% (1–2 sources) of the sample, respectively. The observed distribution agrees fairly well with predictions of cosmic X-ray background population-synthesis models (CXBPSM). We estimate the intrinsic fraction of AGN as a function of , accounting for the bias against obscured AGN in a flux-selected sample. The fraction of CT AGN relative to AGN is poorly constrained, formally in the range 2–56% (90% upper limit of 66%). We derived a fraction (fabs) of obscured AGN () as a function of in agreement with CXBPSM and previous X-ray determinations. Furthermore, fabs at and agrees with observational measurements/trends obtained over larger redshift intervals. We report a significant anti-correlation of R with (confirmed by our companion paper on stacked spectra) with considerable scatter around the median R values. [ABSTRACT FROM AUTHOR]

We present a study of the average X-ray spectral properties of the sources detected by the NuSTAR extragalactic survey, comprising observations of the Extended Chandra Deep Field South (E-CDFS), Extended Groth Strip (EGS), and the Cosmic Evolution Survey (COSMOS). The sample includes 182 NuSTAR sources (64 detected at 8–24 keV), with 3–24 keV fluxes ranging between and 6 × 10−13 erg cm−2 s−1 ( erg cm−2 s−1) and redshifts in the range of . We produce composite spectra from the Chandra + NuSTAR data (, rest frame) for all the sources with redshift identifications (95%) and investigate the intrinsic, average spectra of the sources, divided into broad-line (BL) and narrow-line (NL) active galactic nuclei (AGNs), and also in different bins of X-ray column density and luminosity. The average power-law photon index for the whole sample is , flatter than the typically found for AGNs. While the spectral slope of BL and X-ray unabsorbed AGNs is consistent with the typical values (), a significant flattening is seen in NL AGNs and heavily absorbed sources ( and , respectively), likely due to the effect of absorption and to the contribution from the Compton reflection component to the high-energy flux ( keV). We find that the typical reflection fraction in our spectra is (for ), with a tentative indication of an increase of the reflection strength with X-ray column density. While there is no significant evidence for a dependence of the photon index on X-ray luminosity in our sample, we find that decreases with luminosity, with relatively high levels of reflection () for erg s−1 and for erg s−1 AGNs, assuming a fixed spectral slope of . [ABSTRACT FROM AUTHOR]

We present a sample of 10 low-mass active galactic nuclei (AGNs) selected from the 40-month Nuclear Spectroscopic Telescope Array (NuSTAR) serendipitous survey. The sample is selected to have robust NuSTAR detections at , to be at , and to have optical r-band magnitudes at least 0.5 mag fainter than an galaxy at its redshift. The median values of absolute magnitude, stellar mass, and 2–10 X-ray luminosity of our sample are , , and erg s−1, respectively. Five objects have detectable broad Hα emission in their optical spectra, indicating black hole masses of . We find that of the galaxies in our sample do not show AGN-like optical narrow emission lines, and one of the 10 galaxies in our sample, J115851+4243.2, shows evidence for heavy X-ray absorption. This result implies that a non-negligible fraction of low-mass galaxies might harbor accreting massive black holes that are missed by optical spectroscopic surveys and X-ray surveys. The mid-IR colors of our sample also indicate that these optically normal low-mass AGNs cannot be efficiently identified with typical AGN selection criteria based on Wide Field Infrared Survey Explorer colors. While the hard ( keV) X-ray-selected low-mass AGN sample size is still limited, our results show that sensitive NuSTAR observations are capable of probing faint hard X-ray emission originating from the nuclei of low-mass galaxies out to moderate redshift (), thus providing a critical step in understanding AGN demographics in low-mass galaxies. [ABSTRACT FROM AUTHOR]

We present the 3–8 keV and 8–24 keV number counts of active galactic nuclei (AGNs) identified in the Nuclear Spectroscopic Telescope Array (NuSTAR) extragalactic surveys. NuSTAR has now resolved 33%–39% of the X-ray background in the 8–24 keV band, directly identifying AGNs with obscuring columns up to . In the softer 3–8 keV band the number counts are in general agreement with those measured by XMM-Newton and Chandra over the flux range S(3–8 keV)/ probed by NuSTAR. In the hard 8–24 keV band NuSTAR probes fluxes over the range S(8–24 keV)/, a factor ∼100 fainter than previous measurements. The 8–24 keV number counts match predictions from AGN population synthesis models, directly confirming the existence of a population of obscured and/or hard X-ray sources inferred from the shape of the integrated cosmic X-ray background. The measured NuSTAR counts lie significantly above simple extrapolation with a Euclidian slope to low flux of the Swift/BAT 15–55 keV number counts measured at higher fluxes (S(15–55 keV) ≳ 10−11 ), reflecting the evolution of the AGN population between the Swift/BAT local () sample and NuSTAR’s sample. CXB synthesis models, which account for AGN evolution, lie above the Swift/BAT measurements, suggesting that they do not fully capture the evolution of obscured AGNs at low redshifts. [ABSTRACT FROM AUTHOR]

We present the first direct measurements of the rest-frame 10–40 keV X-ray luminosity function (XLF) of active galactic nuclei (AGNs) based on a sample of 94 sources at 0.1 < z < 3, selected at 8–24 keV energies from sources in the Nuclear Spectroscopic Telescope Array (NuSTAR) extragalactic survey program. Our results are consistent with the strong evolution of the AGN population seen in prior, lower-energy studies of the XLF. However, different models of the intrinsic distribution of absorption, which are used to correct for selection biases, give significantly different predictions for the total number of sources in our sample, leading to small, systematic differences in our binned estimates of the XLF. Adopting a model with a lower intrinsic fraction of Compton-thick sources and a larger population of sources with column densities cm−2 or a model with stronger Compton reflection component (with a relative normalization of R ∼ 2 at all luminosities) can bring extrapolations of the XLF from 2–10 keV into agreement with our NuSTAR sample. Ultimately, X-ray spectral analysis of the NuSTAR sources is required to break this degeneracy between the distribution of absorbing column densities and the strength of the Compton reflection component and thus refine our measurements of the XLF. Furthermore, the models that successfully describe the high-redshift population seen by NuSTAR tend to over-predict previous, high-energy measurements of the local XLF, indicating that there is evolution of the AGN population that is not fully captured by the current models. [ABSTRACT FROM AUTHOR]