The Cosmic Evolution of Hard X-Ray-selected Active Galactic Nuclei

By A. Barger, L. Cowie, R. Mushotzky, Y. Yang, Wei-Hao Wang, A. Steffen, P. Capak. 2004

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We use highly spectroscopically complete deep and wide-area Chandra surveys to determine the cosmic evolution of hard X-ray–selected active galactic nuclei (AGNs). For the deep fields, we supplement the spectroscopic redshifts with photometric redshifts to assess where the unidentified sources are likely to lie. We find that the median redshifts are fairly constant with X-ray flux at z ∼ 1. We classify the optical spectra and measure the FWHM line widths. Most of the broad-line AGNs show essentially no visible absorption in X-rays, whereas the sources without broad lines (FWHM < 2000 km s-1; "optically narrow" AGNs) show a wide range of absorbing column densities. We determine hard X-ray luminosity functions for all spectral types with LX ≥ 1042 ergs s-1 and for broad-line AGNs alone. At z < 1.2, both are well described by pure luminosity evolution, with L* evolving as (1 + z)3.2±0.8 for all spectral types and as (1 + z)3.0±1.0 for broad-line AGNs alone. Thus, all AGNs drop in luminosity by almost an order of magnitude over this redshift range. We show that this observed drop is due to AGN downsizing rather than to an evolution in the accretion rates onto the supermassive black holes. We directly compare our broad-line AGN hard X-ray luminosity functions with the optical QSO luminosity functions and find that at the bright end they agree extremely well at all redshifts. However, the optical QSO luminosity functions do not probe faint enough to see the downturn in the broad-line AGN hard X-ray luminosity functions and even appear to be missing some sources at the lowest luminosities they probe. We find that broad-line AGNs dominate the number densities at the higher X-ray luminosities, while optically narrow AGNs dominate at the lower X-ray luminosities. We rule out galaxy dilution as a partial explanation for this effect by measuring the nuclear UV/optical properties of the Chandra sources using the Hubble Space Telescope Advanced Camera for Surveys GOODS-North data. The UV/optical nuclei of the optically narrow AGNs are much weaker than expected if the optically narrow AGNs were similar to the broad-line AGNs. We therefore postulate the need for a luminosity-dependent unified model. An alternative possibility is that the broad-line AGNs and the optically narrow AGNs are intrinsically different source populations. We cover both interpretations by constructing composite spectral energy distributions—including long-wavelength data from the mid-infrared to the submillimeter—by spectral type and by X-ray luminosity. We use these spectral energy distributions to infer the bolometric corrections (from hard X-ray luminosities to bolometric luminosities) needed to map the accretion history. We determine the accreted supermassive black hole mass density for all spectral types and for broad-line AGNs alone, using the observed evolution of the hard X-ray energy density production rate and our inferred bolometric corrections. We find that only about one-half to one-quarter of the supermassive black hole mass density was fabricated in broad-line AGNs. Using either recent optical QSO luminosity function determinations or our broad-line AGN hard X-ray luminosity function determinations, we measure an accreted supermassive black hole mass density that is a factor of almost 2 lower than that measured by previous work, assuming ϵ = 0.1. This leaves room for obscured accretion when compared with the local supermassive black hole mass density. In fact, we find reasonable agreement between the accreted supermassive black hole mass density from all spectral types and the local supermassive black hole mass density, assuming ϵ ≈ 0.1–0.2. However, there is little room for further obscured sources or for low-efficiency accretion periods.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/8f2cf9bef821acba75a3c397143c7057f6a009b4

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Citations in the corpus

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The XMM-Newton Wide-Field Survey in the COSMOS Field. III. Optical Identification and Multiwavelength Properties of a Large Sample of X-Ray-Selected Sources (2006, background): “However, according to the most recent modeling of the X–ray luminosity function evolution (e.g. Ueda et al. 2003; Barger et al. 2005; Hasinger, Miyaji & Schmidt 2005; La Franca et al. 2005), the paucity of low-redshift BL AGN and high– redshift obscured NL AGN may be at least partly real and due to…”
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The All-Wavelength Extended Groth Strip International Survey (AEGIS) Data Sets (2006, background): “At the target redshift of the DEEP2 survey, z=1, the AEGIS limiting luminosity of 7×1041 erg s−1 corresponds to ∼0.005×L∗, where L∗ is the characteristic AGN luminosity at this redshift (Barger et al. 2005).”
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Radio and millimeter continuum surveys and their astrophysical implications (2009, result): “The luminosity dependence of the high-z decline is qualitatively similar to the downsizing observed for galaxies and optically and X-ray selected quasars (Cowie et al. 1996; Barger et al. 2005; Pérez-González et al. 2008).”
The synthesis of the cosmic X-ray background in the Chandra and XMM-Newton era (2006, background): “While the total (including both type-1 and type-2 objects) AGN XLF obtained by Barger et al. (2005) is in good agreement with those measured by Ueda et al. (2003) and La Franca et al. (2005), their type-1 XLF appears to be in contrast with that measured by HMS05 especially at low luminosities.”
GeV gamma-ray attenuation and the high-redshift UV background (2009, methodology): “Much more detailed IR background modeling, taking into account the latest Spitzer data, will be presented in Gilmore et al. (2009).”
Spitzer Power-Law Active Galactic Nucleus Candidates in the Chandra Deep Field-North (2007, background): “Stronger evidence exists for a decrease in the obscured fraction with increasing X-ray luminosity (Ueda et al. 2003; Steffen et al. 2003; Hasinger 2004; Szokoly et al. 2004; Barger et al. 2005; Treister et al. 2005; Ballantyne et al. 2006), as expected from the receding torus model (Lawrence 1991).”
Cosmic Evolution of Black Holes and Spheroids. III. The MBH-σ* Relation in the Last Six Billion Years (2008, methodology): “In a galaxy merging scenario, the evolution of the MBHσ∗ relation could be mass-dependent, similarly to the downsizing trends in galaxy evolution (Cowie et al. 1996) and AGN evolution (Barger et al. 2005).”
Chandra and Spitzer Unveil Heavily Obscured Quasars in the Chandra/SWIRE Survey (2006, background): “There is evidence that the fraction of obscured AGNs decreases with higher intrinsic luminosities (Ueda et al. 2003; Szokoly et al. 2004; Barger et al. 2005; Treister & Urry 2005) and possibly increases with redshift (La Franca et al. 2005).”