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

Abstract

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 À1 ; ''optically narrow'' AGNs) show a wide range of absorbing column densities. We determine hard X-ray luminosity functions for all spectral types with L X ! 10 42 ergs s À1 and for broad-line AGNs alone. At z < 1:2, both are well described by pure luminosity evolution, with L * evolving as (1 þ z) 3:2AE0:8 for all spectral types and as (1 þ z) 3:0AE1:0 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 submillimeterby 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 ...

Supermassive Black Holes in Galaxies

“…The relative HXLFs of the total and the broad-line AGNs from Barger et al (2005) reproduce the results of Steffen et al (2003), who showed that the dominant population at these higher X-ray luminosities, where the redshift identifications are very complete, is broad-line AGNs, while at the lower X-ray luminosities, the non-broad-line AGNs dominate (hereafter, I refer to this as the Steffen effect). Since broad-line AGNs are 466 A. J. Barger straightforward to identify spectroscopically, and the bulk of the X-ray sources at these luminosities have now been observed, one does not need to worry that broad-line AGNs are making up a substantial fraction of the unidentified population.…”

“…The observation that there are almost no low X-ray luminosity, broad-line AGNs is not a galaxy dilution effect, ruling out the simple unified model. The Barger et al (2005) hard X-ray luminosity function determinations of the supermassive black hole mass densities are a factor of almost two lower than what Yu & Tremaine (2002) found due to the Yu & Tremaine extrapolation of the optical QSO luminosity function outside the optically observed luminosity range. Comparing this result with the local 470 A. J. Barger supermassive black hole mass density, there is now room for obscured accretion by the optically-narrow AGNs.…”

“…With the advent of the Chandra and XMM data, there have been a number of computations of the evolution of the HXLF with redshift (e.g., Cowie et al 2003;Steffen et al 2003;Ueda et al 2003;Hasinger 2003;Fiore et al 2003;Barger et al 2005), while Sazonov & Revnivtsev (2004) have used the RXTE data to compute the local 3-20 keV luminosity function. The latest HXLF determinations by Barger et al (2005) use only the spectroscopically identified X-ray sources with L 2−8 keV = 10 42 ergs s −1 in the CDF-N, CDF-S, and CLASXS fields. They assume that any source more luminous than L 2−8 keV = 10 42 ergs s −1 is very likely to be an AGN on energetic grounds (Zezas et al 1998;Moran et al 1999).…”

“…Since these results suggest a luminosity dependence in optical spectral type, the simple unified model for AGNs, in which there is no luminosity or redshift dependence of the obscuration, does not appear to be valid. Figure 3 shows the measured HXLFs from Barger et al (2005) for two low redshift intervals (open squares -z = 0.2-0.4; solid diamonds -z = 0.8-1.2). Poissonian 1σ uncertainties are based on the number of galaxies in each luminosity bin.…”

“…That is, might galaxy dilution (see, e.g., Moran et al 2002) be a partial explanation of the Steffen effect? With the highresolution ACS GOODS-North data (Giavalisco et al 2004), Barger et al (2005) were able to separate the nuclear component of each source from the host galaxy light, even at the higher redshifts, in order to analyze the nuclear colors. It is well-known that the nuclear UV magnitudes and the X-ray fluxes for broad-line AGNs are strongly correlated.…”