Luminosity-dependent Density Evolution Research Articles

The [O iii] emission line luminosity function of optically selected type-2 AGN from zCOSMOS$^{\rm,}$

We present a catalog of 213 type-2 AGN selected from the zCOSMOS survey. The selected sample covers a wide redshift range (0.15<z<0.92) and is deeper than any other previous study, encompassing the luminosity range 10^{5.5} < Lsun< L[OIII] < 10^{9.1} Lsun. We explore the intrinsic properties of these AGN and the relation to their X-ray emission (derived from the XMM-COSMOS observations). We study their evolution by computing the [OIII]5007A line luminosity function (LF) and we constrain the fraction of obscured AGN as a function of luminosity and redshift. The sample was selected on the basis of the optical emission line ratios, after applying a cut to the signal-to-noise ratio (S/N) of the relevant lines. We used the standard diagnostic diagrams [OIII]/Hbeta versus [NII]/Halpha and ([OIII]/Hbeta versus [SII]/Halpha) to isolate AGN in the redshift range 0.15<z<0.45 and the diagnostic diagram [OIII]/Hbeta versus [OII]/Hbeta to extend the selection to higher redshift (0.5<z<0.92). Combining our sample with one drawn from SDSS, we found that the best description of the evolution of type-2 AGN is a luminosity-dependent density evolution model. Moreover, using the type-1 AGN LF we were able to constrain the fraction of type-2 AGN to the total (type-1 + type-2) AGN population. We found that the type-2 fraction decreases with luminosity, in agreement with the most recent results, and shows signs of a slight increase with redshift. However, the trend with luminosity is visible only after combining the SDSS+zCOSMOS samples. From the COSMOS data points alone, the type-2 fraction seems to be quite constant with luminosity.

The evolution of the hard X-ray luminosity function of AGN

We present new observational determinations of the evolution of the 2-10keV X-ray luminosity function (XLF) of AGN. We utilise data from a number of surveys including both the 2Ms Chandra Deep Fields and the AEGIS-X 200ks survey, enabling accurate measurements of the evolution of the faint end of the XLF. We combine direct, hard X-ray selection and spectroscopic follow-up or photometric redshift estimates at z<1.2 with a rest-frame UV colour pre-selection approach at higher redshifts to avoid biases associated with catastrophic failure of the photometric redshifts. Only robust optical counterparts to X-ray sources are considered using a likelihood ratio matching technique. A Bayesian methodology is developed that considers redshift probability distributions, incorporates selection functions for our high redshift samples, and allows robust comparison of different evolutionary models. We find that the XLF retains the same shape at all redshifts, but undergoes strong luminosity evolution out to z~1, and an overall negative density evolution with increasing redshift, which thus dominates the evolution at earlier times. We do not find evidence that a Luminosity-Dependent Density Evolution, and the associated flattening of the faint-end slope, is required to describe the evolution of the XLF. We find significantly higher space densities of low-luminosity, high-redshift AGN than in prior studies, and a smaller shift in the peak of the number density to lower redshifts with decreasing luminosity. The total luminosity density of AGN peaks at z=1.2+/-0.1, but there is a mild decline to higher redshifts. We find >50% of black hole growth takes place at z>1, with around half in Lx<10^44 erg/s AGN.

THE OPTX PROJECT. II. HARD X-RAY LUMINOSITY FUNCTIONS OF ACTIVE GALACTIC NUCLEI FORz≲ 5

We use the largest, most uniform, and most spectroscopically complete to faint X-ray flux limits Chandra sample to date to construct hard 2-8 keV rest-frame X-ray luminosity functions (HXLFs) of spectroscopically identified active galactic nuclei (AGNs) to z~5. In addition, we use a new 2-8 keV local sample selected by the very hard (14-195 keV) SWIFT 9-month Burst Alert Telescope (BAT) survey to construct the local 2-8 keV HXLF. We do maximum likelihood fits of the combined distant plus local sample (as well as of the distant sample alone) over the redshift intervals 0<z<1.2, 0<z<3, and 0<z<5 using a variety of analytic forms, which we compare with the HXLFs. We recommend using our luminosity dependent density evolution (LDDE) model fits of the combined distant plus local sample over 0<z<3 for all the spectroscopically identified sources and for the broad-line AGNs.

The X-ray luminosity function of AGN at z ∼ 3

We combine Lyman-break colour selection with ultradeep (≳200 ks) Chandra X-ray imaging over a survey area of ∼0.35 deg2 to select high-redshift active galactic nuclei (AGN). Applying careful corrections for both the optical and X-ray selection functions, the data allow us to make the most accurate determination to date of the faint end of the X-ray luminosity function (XLF) at z∼ 3. Our methodology recovers a number density of X-ray sources at this redshift which is at least as high as previous surveys, demonstrating that it is an effective way of selecting high z AGN. Comparing to results at z= 1, we find no evidence that the faint slope of the XLF flattens at high z, but we do find significant (factor ∼3.6) negative evolution of the space density of low luminosity AGN. Combining with bright end data from very wide surveys we also see marginal evidence for continued positive evolution of the characteristic break luminosity L*. Our data therefore support models of luminosity-dependent density evolution between z= 1 and z= 3. A sharp upturn in the the XLF is seen at the very lowest luminosities (LX≲ 1042.5erg s−1), most likely due to the contribution of pure X-ray starburst galaxies at very faint fluxes.

The VVDS type-1 AGN sample: the faint end of the luminosity function

In a previous paper (Gavignaud et al. 2006, A&A, 457, 79), we presented the type-1 Active Galactic Nuclei (AGN) sample obtained from the first epoch data of the VIMOS-VLT Deep Survey (VVDS). The sample consists of 130 faint, broad-line AGN with redshift up to and , selected on the basis of their spectra. The sample is thus free of the morphological and color selection biases, that lead to significant incompleteness in the optical surveys of faint AGN. In this paper we present the measurement of the Optical Luminosity Function up to derived from this sample, we compare our results with previous results from brighter samples both at low and at high redshift and finally, through the estimate of the bolometric luminosity function, we compare them also with the results from X-ray and mid-IR selected samples. Our data, more than one magnitude fainter than previous optical surveys, allow us to constrain the faint part of the luminosity function up to high redshift. A comparison of our data with the 2dF sample at low redshift () shows that the VVDS data can not be well fitted with the PLE models derived by previous samples. Qualitatively, this appears to be due to the fact that our data suggest the presence of an excess of faint objects at low redshift () with respect to these models. By combining our faint VVDS sample with the large sample of bright AGN extracted from the SDSS DR3 (Richards et al. 2006b, AJ, 131, 2766) and testing a number of different evolutionary models, we find that the model which better represents the combined luminosity functions, over a wide range of redshift and luminosity, is a luminosity dependent density evolution (LDDE) model, similar to those derived from the major X-surveys. Such a parameterization allows the redshift of the AGN space density peak to change as a function of luminosity and explains the excess of faint AGN that we find at . On the basis of this model we find, for the first time from the analysis of optically selected samples, that the peak of the AGN space density shifts significantly towards lower redshift going to lower luminosity objects. The position of this peak moves from for to for . This result, already found in a number of X-ray selected samples of AGN, is consistent with a scenario of “AGN cosmic downsizing”, in which the density of more luminous AGN, possibly associated to more massive black holes, peaks earlier in the history of the Universe (i.e. at higher redshift), than that of low luminosity ones, which reaches its maximum later (i.e. at lower redshift).

Gamma-ray luminosity function of blazars and the cosmic gamma-ray background: evidence for the luminosity-dependent density evolution

We present a comprehensive study of the gamma-ray luminosity function (GLF) of blazars and their contribution to the extragalactic diffuse gamma-ray background (EGRB). Radio and gamma-ray luminosity correlation is introduced with a modest dispersion, consistent with observations, to take into account the radio detectability, which is important for blazar identification. Previous studies considered only pure luminosity evolution (PLE) or pure density evolution, but here we introduce the luminosity-dependent density evolution (LDDE) model, which is favored on the basis of the evolution of the X-ray luminosity function (XLF) of AGNs. The model parameters are constrained by likelihood analyses of the observed redshift and gamma-ray flux distributions of the EGRET blazars. Interestingly, we find that the LDDE model gives a better fit to the observed distributions than the PLE model, indicating that the LDDE model is also appropriate for gamma-ray blazars and that the jet activity is universally correlated with the accretion history of AGNs. We then find that only 25–50% of the EGRB can be explained by unresolved blazars with the best-fit LDDE parameters. Unresolved blazars can account for all the EGRB only with a steeper index of the faint-end slope of the GLF, which is marginally consistent with the EGRET data but inconsistent with XLF data. Therefore, unresolved AGNs cannot be the dominant source of the EGRB, unless there is a new population of gamma-ray emitting AGNs that evolves differently from the XLF of AGNs. Predictions for the GLAST mission are made, and we find that the best-fit LDDE model predicts about 3000 blazars in the entire sky, which is considerably fewer (by a factor of more than 3) than a previous estimate.

A Spectroscopic Survey of Faint Quasars in the SDSS Deep Stripe. I. Preliminary Results from the Co-added Catalog

In this paper we present the first results of a deep spectroscopic survey of faint quasars in the Sloan Digital Sky Survey (SDSS) Southern Survey, a deep survey carried out by repeatedly imaging a 270 deg^2 area. Quasar candidates were selected from the deep data with good completeness over 0 2.0) at the faint end, indicating a break in the QLF slope. Using a luminosity-dependent density evolution model, we find that the quasar density at M_{g}<-22.5 peaks at z~2, which is later in cosmic time than the peak of z~2.5 found from surveys of more luminous objects. The SFQS QLF is consistent with the results of the 2dF QSO Redshift Survey, the SDSS, and the 2dF-SDSS LRG and QSO Survey, but probes fainter quasars. We plan to obtain more quasars from future observations and establish a complete faint quasar sample with more than 1000 objects over 10 deg^2.

Gamma‐Ray Luminosity Function of Blazars and the Cosmic Gamma‐Ray Background: Evidence for the Luminosity‐Dependent Density Evolution

We present a comprehensive study for the gamma-ray luminosity function (GLF) of blazars and their contribution to the extragalactic diffuse gamma-ray background (EGRB). Radio and gamma-ray luminosity correlation is introduced to take into account the radio detectability which is important for the blazar identification. Previous studies considered only pure luminosity evolution (PLE) or pure density evolution, but we introduce the luminosity dependent density evolution (LDDE) model, which is favored from the evolution of X-ray luminosity function (XLF) of AGNs. The model parameters are constrained by likelihood analyses about the observed redshift and gamma-ray flux distributions of the EGRET blazars. We find that the LDDE model gives a better fit to the observed distributions than the PLE model, indicating that the LDDE model is also appropriate for gamma-ray blazars, and that the jet activity is universally correlated with the accretion history of AGNs. The normalization between the GLF and XLF is consistent with the unified picture of AGNs, when the beaming and a reasonable duty cycle of jet activity are taken into account. We then find that only 25--50% of the EGRB can be explained by unresolved blazars with the best-fit LDDE parameters. Unresolved blazars can account for all the EGRB only with a steeper index of the faint-end slope of the GLF, which is marginally consistent with the EGRET data but inconsistent with that of the XLF. Therefore unresolved AGNs cannot be the dominant source of the EGRB, unless there is a new population of gamma-ray emitting AGNs that evolves differently from the XLF of AGNs. Predictions for the GLAST mission are made, and we find that the best-fit LDDE model predicts about 3000 blazars in the entire sky, which is considerably fewer than a previous estimate.

The Evolution in the Faint‐End Slope of the Quasar Luminosity Function

Using a new model for quasar lifetimes and light curves derived from numerical simulations of galaxy mergers that incorporate black hole growth, we study the faint-end slope of the quasar luminosity function (QLF) and its evolution with redshift. Our model motivates a new interpretation of the QLF in which the bright end consists of quasars radiating near their peak luminosities but the faint end is mostly made up of brighter peak luminosity quasars seen in less luminous phases of evolution. The faint-end slope of the QLF is then set by the behavior of the lifetime (light curve) of quasars with peak luminosities near the observed break when they are in less luminous stages of evolution. We determine the faint-end slope of the QLF from the quasar lifetime, based on a set of simulations that encompass a wide range of host galaxy, merger, black hole, and interstellar gas properties. Brighter peak luminosity (higher black hole mass) systems undergo more violent evolution, and gas is expelled and heated more rapidly in the final stages of quasar evolution, resulting in a flatter faint-end slope (as these objects fall below the observed break in the QLF more rapidly). Therefore, as the QLF break luminosity moves to higher luminosities with increasing redshift, implying a larger typical quasar peak luminosity, the faint-end QLF slope flattens. From our model, we predict the evolution of the faint-end slope of the QLF and find good agreement with observations. Although black holes grow in an antihierarchical manner (with lower mass black holes formed primarily at lower redshifts), in our picture the observed change in slope and differential or luminosity-dependent density evolution in the QLF is determined by the nontrivial, luminosity-dependent quasar lifetime and physics of quasar feedback, and not by changes in the shape of the underlying peak luminosity or active black hole mass distributions.

The HELLAS2XMM Survey. VII. The Hard X‐Ray Luminosity Function of AGNs up toz= 4: More Absorbed AGNs at Low Luminosities and High Redshifts

We have determined the cosmological evolution of the density of active galactic nuclei (AGNs) and of their NH distribution as a function of the unabsorbed 2-10 keV luminosity up to redshift 4. We used the HELLAS2XMM sample combined with other published catalogs, yielding a total of 508 AGNs. Our best fit is obtained with a luminosity-dependent density evolution (LDDE) model where low-luminosity (LX ~ 1043 ergs s-1) AGNs peak at z ~ 0.7, while high-luminosity AGNs (LX > 1045 ergs s-1) peak at z ~ 2.0. A pure luminosity evolution model (PLE) can instead be rejected. There is evidence that the fraction of absorbed (NH > 1022 cm-2) AGNs decreases with the intrinsic X-ray luminosity and increases with the redshift. Our best-fit solution provides a good fit to the observed counts, the cosmic X-ray background, and to the observed fraction of absorbed AGNs as a function of the flux in the 10-15 ergs cm-2 s-1 < S2-10 < 10-10 ergs cm-2 s-1 range. We find that the absorbed, high-luminosity (LX > 1044 ergs s-1) AGNs have a density of 267 deg-2 at fluxes S2-10 > 10-15 ergs cm-2 s-1. Using these results, we estimate a density of supermassive black holes in the local universe of ρBH = 3.2 h × 105 M☉ Mpc-3, which is consistent with the recent measurements of the black hole mass function in the local galaxies.

Luminosity-dependent evolution of soft X-ray selected AGN

We present new results on the cosmological evolution of unabsorbed (type-1) active galactic nuclei (AGN) selected in the soft (0.5-2 keV) X-ray band. From a variety of ROSAT, XMM-Newton and Chandra surveys we selected a total of ~1000 AGN with an unprecedented spectroscopic and photometric optical/NIR identification completeness. For the first time we are able to derive reliable space densities for low-luminosity (Seyfert-type) X-ray sources at cosmological redshifts. The evolutionary behaviour of AGN shows a strong dependence on X-ray luminosity: while the space density of high-luminosity AGN reaches a peak around , similar to that of optically selected QSO, the space density of low-luminosity AGNs peaks at redshifts below . This confirms previous ROSAT findings of a luminosity-dependent density evolution. Using a rigorous treatment of the optical identification completeness we are able to show that the space density of AGN with X-ray luminosities erg s-1 declines significantly towards high redshifts.

Constraints on the distribution of absorption in the X-ray-selected AGN population found in the 13HXMM-Newton/Chandradeep field

We present an analysis of the X-ray properties of sources detected in the 13 H XMM‐Newton deep (200-ks) field. In order to constrain the absorbed active galactic nuclei (AGN) population, we use extensive Monte Carlo simulations to compare directly the X-ray colours of observed sources with those predicted by several model distributions. In particular, we have carried out our comparisons over the entire 0.2‐10 keV energy range of the XMM‐Newton cameras, making our analysis sensitive to a large range of absorbing column densities. We have tested the simplest form of the unified scheme, whereby the intrinsic luminosity function of absorbed AG Ni sset to be the same as that of their unabsorbed brethren, coupled with various model distributions of absorption. Of the tested distributions, the best-fitting model has the fraction of AGN with absorbing column N H, proportional to (log N H) 8 .W e h ave also tested two extensions to the unified scheme: an evolving absorption scenario, in which the fraction of absorbed sources is larger at higher redshifts, and a luminosity-dependent model in which high-luminosity AGN are less likely to be absorbed. Both of these models provide poorer matches to the observed X-ray colour distributions than the best-fitting simple unified model. We find that a luminosity-dependent density evolution luminosity function reproduces poorly the 0.5‐2 keV source counts seen in the 13 H field. Field-to-field variations could be the cause of this disparity. Computing the X-ray colours with a simple absorbed power-law spectral model is found to overpredict, by a factor of 2, the fraction of hard sources that are completely absorbed below 0.5 keV, implying that an additional source of soft-band flux must be present in a number of the absorbed sources. The tested synthesis models predict that around 16 per cent of the detections in the 13 H field are due to AGN at z > 3. However, so far, only a single AGN with z > 3 has been identified in our approximately 50 per cent complete optical spectroscopy follow-up programme. Finally, we use our simulations to demonstrate the efficacy of a hardness ratio selection scheme at selecting absorbed sources for further study. Using this selection scheme, we show that around 40 per cent of the 13 H sample are expected to be AGN with N H > 10 22 cm −2 . Ke yw ords: surveys ‐ galaxies: active ‐ quasars: general ‐ X-rays: galaxies.

The space density of moderate-luminosity active galaxies at z = 3

Abstract We present an estimate of the space density of active galactic nuclei (AGN) at z = 3. Combining deep X-ray data with Lyman break galaxy (LBG) colour selection in the rest-frame ultraviolet makes for highly efficient identification of AGN in a narrow redshift range (z ∼ 2.5–3.5). Using Chandra data from the Groth–Westphal Strip (GWS) and the Hubble Deep Field North (HDF-N), we find a total of 15 X-ray-detected LBGs at z ∼ 3, the majority of which are unlikely to have been identified in blanket follow-up surveys of X-ray-detected objects. We find the comoving space density of moderate-luminosity AGN (MLAGN; LX = 1043–44.5 erg s−1) at z = 3 to be a factor of ∼10 higher than that of the most powerful objects. The available data are consistent with a roughly constant space density of MLAGN from z = 0.5 to 3, and they are also consistent with a mild decline in the space density above z = 1 as predicted by the luminosity-dependent density evolution models of Ueda et al. This strong AGN activity at z = 3 argues against previous suggestions that the majority of black hole accretion occurs at low redshift. A further implication of our investigation is that, as far as can be determined from current data, the majority of the AGN population at z ∼ 3 is selected by the LBG dropout technique. Although this is sensitive to AGN with an ultraviolet excess arising from an accretion disc, it predominantly identifies star-forming galaxies. A significant fraction of X-ray sources seem likely to be hosted by these more typical LBGs, further strengthening the starburst–AGN connection at high redshift.

The XMM-Newton/2dF Survey -- VI. Clustering and bias of the soft X-ray point sources

We study the clustering properties of X-ray sources detected in the wide area (∼2 deg 2 ) bright, contiguous XMM-Newton/2dF survey. We detect 432 objects to a flux limit of 5 x 10 -15 erg cm -2 s -1 in the soft 0.5-2 keV band. Performing the standard angular correlation function analysis, a ∼3σ correlation signal between 0 and 150 arcsec is detected: w(θ < 150 arcsec) ≃ 0.114 ± 0.037. If the angular correlation function is modelled as a power law, w(θ) = (θ 0 /θ) γ-1 , then for its nominal slope of y = 1.8 we estimate, after correcting for the integral constraint and the amplification bias, that θ 0 ≃ 10.4 ±1.9 arcsec. Very similar results are obtained for the 462 sources detected in the total 0.5-8 keV band (θ 0 ≃ 10.8 ± 1.9 arcsec). Using a clustering evolution model which is constant for comoving coordinates (e = -1.2), a luminosity-dependent density evolution (LDDE) model for the X-ray luminosity function and the concordance cosmological model (Ω m = 1 - Ω Λ = 0.3) we obtain, by inverting Limber's integral equation, a spatial correlation length of r 0 ∼ 16 h -1 Mpc. This value is larger than that of previous ROSAT surveys as well as of the optical two-degree quasar redshift survey. Only in models where the clustering remains constant for physical coordinates (e = -3), do we obtain an r 0 value (∼7.5 h -1 Mpc) which is consistent with the above surveys. Finally, comparing the measured angular correlation function with the predictions of the concordance cosmological model, we find for two different bias evolution models that the soft X-ray sources at the present time should be biased with respect to the underline matter fluctuation field with bias values in the range (which depends on the biasing model used): 1.9 ≤b 0 ≤ 2.7 for e = -1.2 or 1 ≤ b 0 ≤ 1.6 for e = -3.

Cosmological Evolution of the Hard X-Ray AGN Luminosity Function: Formation History of Supermassive Black Holes

The cosmological evolution of the hard X-ray luminosity function (HXLF) of Active Galactic Nuclei (AGN) is investigated in the 2-10 keV luminosity range of 10 4 1 . 5 - 10 4 6 . 5 erg s - 1 as a function of redshift up to3. From a combination of surveys conducted at photon energies above 2 keV with HEAO1, ASCA and Chandra, we construct a highly complete (>96%) sample consisting of 247 AGNs over the wide flux range of 10 - 1 0 - 3.8 x 10 - 1 5 erg cm - 2 s - 1 (2-10 keV). We find that (i) the fraction of X-ray absorbed AGNs decreases with the intrinsic luminosity and (ii) the evolution of the HXLF of the whole AGNs (including both type-I and type-II AGNs) is best described with a luminosity dependent density evolution where the cutoff redshift increases with the luminosity. Our results directly constrain the evolution of AGNs that produce a major part of the hard X-ray background, thus solving its origin quantitatively. Based on these results, we discuss the formation history of supermassive black holes in galactic centers. The total accreted mass traced by the HXLF with an accretion mass-to-energy conversion factor of 0.1 is consistent with an estimate of SMBH mass density from the demography of galaxies in nearby universe.

Cosmological Evolution of the Hard X‐Ray Active Galactic Nucleus Luminosity Function and the Origin of the Hard X‐Ray Background

We investigate the cosmological evolution of the hard X-ray luminosity function (HXLF) of active galactic nuclei (AGNs) in the 2-10 keV luminosity range of 1041.5-1046.5 ergs s-1 as a function of redshift up to 3. From a combination of surveys conducted at photon energies above 2 keV with HEAO 1, ASCA, and Chandra, we construct a highly complete (>96%) sample consisting of 247 AGNs over the wide flux range of 10-10 to 3.8 × 10-15 ergs cm-2 s-1 (2-10 keV). For our purpose, we develop an extensive method of calculating the intrinsic (before absorption) HXLF and the absorption (NH) function. This utilizes the maximum likelihood method, fully correcting for observational biases with consideration of the X-ray spectrum of each source. We find that (1) the fraction of X-ray absorbed AGNs decreases with the intrinsic luminosity and (2) the evolution of the HXLF of all AGNs (including both type I and type II AGNs) is best described with a luminosity-dependent density evolution (LDDE) where the cutoff redshift increases with the luminosity. Our results directly constrain the evolution of AGNs that produce a major part of the hard X-ray background, thus solving its origin quantitatively. A combination of the HXLF and the NH function enables us to construct a purely observation-based population synthesis model. We present basic consequences of this model and discuss the contribution of Compton-thick AGNs to the rest of the hard X-ray background.

TheBeppoSAXHigh‐Energy Large‐Area Survey. V. The Nature of the Hard X‐Ray Source Population and Its Evolution

We present optical spectroscopic identifications of hard X-ray (5-10 keV) selected sources belonging to the High-Energy Large-Area Survey sample obtained with BeppoSAX down to a 5-10 keV flux limit of f5-10 keV ~ 3 × 10-14 ergs cm-2 s-1. The sample consists of 118 sources. There are 25 sources that have been identified through correlations with catalogs of known sources. A spectroscopic identification for 49 more has been searched for with the telescope. The results of 13 fields were empty down to R = 21. There were 37 sources identified as type 1 active galactic nuclei (AGNs) and nine as type 2 AGNs. The remaining sources are five narrow emission-line galaxies, six clusters, two BL Lac objects, one radio galaxy, and one star. Combining these objects with other hard X-ray selected AGNs from ASCA and HEAO 1, we find that the local luminosity function of type 1 AGNs (AGN 1s) in the 2-10 keV band is fairly well represented by a double power law function. There is evidence for significant cosmological evolution according to a pure luminosity evolution (PLE) model [LX(z) (1 + z)k], with k = 2.12 and 2.22 (σk 0.14) in a (Ωm, Ωλ) = (1.0, 0.0) and a (Ωm, Ωλ) = (0.3, 0.7) cosmology, respectively. The data show an excess of faint high-redshift type AGN 1s, which is well modeled by a luminosity-dependent density evolution (LDDE), similar to what is observed in soft X-rays. However, in both cosmologies, the statistics are not significant enough to distinguish between the PLE and LDDE models. The fitted models imply a contribution of AGN 1s to the 2-10 keV X-ray background from 35% up to 60%.

The optical luminosity evolution function of quasars. I. A statistical evolution function that includes the large redshift range above redshift 2.2

We have redetermined the parameters in the Heisler-Ostriker quasar evolution formula so as to make it applicable to a larger redshift range. The form of the formula is appropriate because, physically, any variation in the mass of the central black hole and the accretion rate will cause a luminosity evolution while the birth of quasars will imply a luminosity-dependent density evolution. With our new parameters, we can 1) reproduce the quasar magnitude count down to magnitude 22.0, 2) reproduce the quasar redshift distribution in intervals of ΔZ = 0.1 and 3) substantially lessen the excess count for Z > 2 redicted by previous formulae.

Models of the quasar population. I - A new luminosity function

A new functional form for the quasar luminosity function is tested using recent observational results for both bright and faint quasar count and redshift distributions. The form is of a fairly general type based on three free parameters and allows for quasars to undergo a combination of luminosity evolution and luminosity-dependent density evolution; an advantage to this approach is that it does not constrain quasars to follow a single type of evolution. Models of pure luminosity evolution or luminosity-dependent density evolution can be constructed, but the apparent magnitude distribution of observed quasars is best fitted by a combination model. The combination model also gives the correct redshift distribution for quasars with redshifts less than three and predicts that quasars brighter than B = 22 provide a 2-10 keV X-ray flux that is equal to 32 percent of the observed X-ray background. However, the model is flawed in that it predicts more high-redshift quasars than are observed.

The space distribution of faint quasars from the CFHT survey

Basic data are presented for 163 QSOs brighter than B = 20.5 selected from 1000 A/mm CFHT blue grens observations. The data are used to investigate the form of the evolution of QSOs in order to obtain their luminosity functions, and to predict their magnitude and density distributions to allow comparison with the results of other surveys. Less than 5 percent of the QSOs have z greater than 2.5 despite the ease of detecting them up to z = 3.4, so high-redshift QSOs are rare. The surface density of QSOs brighter than m = 20.5 is 30/sq deg. While the luminosity-dependent density evolution models of Schmidt and Green (1983) are found to fit the data well, a smooth cutoff to the evolution at high redshifts is proposed which implies a density maximum at z of roughly 1.7. The clustering properties of the sample QSOs are discussed and it is shown, in agreement with previous studies, that while clustering is not a general phenomenon among QSOs, significant density enhancements apparently do exist. 26 references.