摘要:

The cores of active galactic nuclei (AGN) harbor some of the most extreme conditions of matter and energy in the Universe. One of the major goals of high-energy astrophysics is to probe these extreme environments in the vicinity of supermassive black holes, which are intimately linked to the mechanisms that produce the continuum emission in AGN. X-ray studies seek to understand the physics responsible for the continuum emission, its point of origin, how nuclear activity is fueled, and how supermassive black holes evolve. The key to finding answers to these questions lies in measuring the intrinsic luminosities and spectral shapes, the relation of these properties to other wavebands, and now the source properties change with redshift. This article reviews X-ray observations of AGN from redshifts of∼0.1−3with the goal of summarizing our current knowledge of their X-ray spectral characteristics. Results are evaluated in terms of their robustness and are examined in the light of current theoretical predictions of energy release via processes associated with the accretion mechanism. A possible evolutionary scenario is discussed, along with the importance of AGN studies at high redshift as they relate to the total energetics of the Universe. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.58607

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

Deep X-ray surveys using ROSAT, ASCA and BeppoSAX have resolved a significant fraction of the cosmic X-ray background (XRB) into discrete sources and optical identifications are demonstrating that the XRB is largely due to accretion onto massive black holes, integrated over cosmic time. The deep soft X-ray surveys have detected a larger surface density of AGN than in any other waveband and find significant evolution in the space density of high-luminosity AGN contrary to the pure luminosity evolution which was the standard assumption so far. Considerable uncertainties still exist for the evolution of low-luminosity AGN. These findings are consistent with the notion that most larger galaxies contain black holes many of which have been active in the past. Of particular interest in the context of this conference is the space density of high-redshift AGN, which is consistent with a constant value in the range2<z<5.X-ray surveys are therefore prone to yield valuable insight into the formation and accretion history of black holes in the early universe. However, the characteristic hard spectrum of the XRB can only be explained if most AGN spectra are heavily absorbed. Thus as much as 80–90&percent; of the light produced by accretion may be absorbed by gas and dust clouds, which according to recent models could reside in nuclear starburst regions that feed the AGN. This scenario has important consequences for the current attempts to understand black hole and galaxy formation and evolution: The absorbed AGN will suffer severe extinction and therefore, unlike classical QSOs, will not be prominent at optical wavelengths. If most of the accretion power is being absorbed by gas and dust, it will have to be reradiated in the FIR range and be redshifted into the sub-mm band. AGN could therefore contribute a substantial fraction to the recently discovered cosmic FIR/sub-mm background which has already partly been resolved by deep SCUBA surveys. The AGN light therefore needs to be taken into account when studying the star formation history in the early universe. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.58608

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

The continuum morphologies of high redshift radio galaxies and quasars can be modeled as enormous bipolar reflection nebulae from shells of dust swept up by bipolar outflows. If the observed shape of a particular object is fit with an analytic function, then the velocity of the shell is specified by the equations of motion. The predicted kinematics can be compared with the observed emission line velocity field, and the resulting fit is excellent. The implications for massive galaxies at high redshift include the requirement of an initial epoch of star formation that creates dust distributed throughout a very large, diffuse, nearly virialized halo. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.58609

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

Previously we have discussed models for strong compact and extended radio sources. In this paper we describe how the nature of these models is expected to depend on redshift and make a preliminary attempt to connect theory with observations. In particular we predict that the number of strong compact radio sources should decline more rapidly with redshift than the number of quasars. We also give physical explanations for the steepening of the average spectral index of radio sources with redshift as well as their smaller dimensions. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.58610

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

Observations of resolved Hell Lyman alpha absorption in spectra of two QSO’s suggest that the epoch of helium ionization occurred atz≈3.Proximity zones in the spectra of the quasars(z=3.18,3.285)at 304 Å resemble Stromgren spheres, suggesting that the intergalactic medium is only singly ionized in helium. We present models of the proximity effect which include the full physics of the ionization, heating and cooling and an accurately simulated inhomogeneous gas distribution. In these models the underdense intergalactic medium is heated to at least 10,000–20,000 K after cooling to as low as a few 1000K due to cosmological expansion, with higher temperatures achieved farther away from the quasar due to absorption-hardened ionizing spectra. The quasars turn on for a few×107years with a fairly steady flux output at 228 Å comparable to the 304 Å flux output directly observed with HST. The recoveries in the spectra occur naturally due to voids in the IGM and may provide a fairly model-independent probe of the baryon density. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.58611

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

The results of an optical an infrared investigation of a complete subsample of the Leiden-Berkeley Deep Survey are presented. Optical counterparts have been identified for 69 of the 73 sources in the two Hercules fields, and redshifts obtained for 49 of them. Photometric redshifts are computed from the griK data for the remaining 21 sources. This complete sample is compared with the radio luminosity functions (RLFs) of Dunlop and Peacock (1990) [1]. The RLF models successfully trace the evolution of the radio sources with redshift, but there is some disagreement between the luminosity-dependence of the models and the data. The observed RLF for the lower luminosity population(log10P<26)shows evidence for a cut-off at lower redshifts(z∼0.5–1.5)than for the more powerful objects. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.58647

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

The Sloan Digital Sky Survey has obtained images in five broad-band colors for several hundred square degrees. We present color-color diagrams for stellar objects, and demonstrate that quasars are easily distinguished from stars by their distinctive colors. Follow-up spectroscopy in less than ten nights of telescope time has yielded 22 new quasars, 9 of them atz>3.65,and one withz=4.75,the second highest-redshift quasar yet known. Roughly 80&percent; of the high-redshift quasar candidates selected by color indeed turn out to be high-redshift quasars. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.58612

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

The alignment of optical emission along the axis of radio emission in high redshift radio galaxies is now commonly observed. In many cases this aligned component can be shown to be significantly polarized, a result that has led to models for the emission that employ scattering of light emitted from the AGN. Both electron scattering and dust scattering have been proposed, but one aspect of dust scattering that has not been explored previously is the response of the dust grains to the passage of the strong shock associated with the radio source. The survival of dust grains in such an environment after the passage of a high speed shock associated with the radio jet is calculated for a wide range of parameters. It is found that for most configurations the grains are destroyed as a scattering population by sputtering processes in a time much less than the minimum radio source lifetime of ∼ 10 million years. Thus polarization due to scattering by anin situpopulation of grains is somewhat problematic. Alternate methods for providing the needed grain population, either by grain replenishment via outward convection from a dust-rich galactic interior or by local production from a population of stars formed by passage of the radio jet are described. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.58648

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

In this calculation we estimate the contribution of massive X-ray binaries to the hard X-ray background (XRB). Traditionally, due to their short life-times, MXRBs were thought not to be significant contributors to the XRB. However, given the recent progress in the observational determination of the global star-formation rate as a function of redshift probed both via emitted blue-light and that re-radiated in the sub-mm, assuming that the formation rate of MXRBs tracks the global star formation rate, their integrated contribution to the XRB can be computed and is shown to be non-negligible. While the origin of the bulk of the hard XRB can be attributed to obscured AGNs, we show that MXRBs can account for 10–30&percent;. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.58613

出版商:AIP    

年代:1999

数据来源: AIP

摘要:

Following the detection of a puzzling emission-line feature in the X-ray spectrum of thez=0.654high-luminosity quasar PKS 0637–752 (centered at 1.6 keV in the quasar frame) we examined the spectra of allASCAquasars (public as of 1998 July 7) with1<z<5to search for any new, unexpected emission-line features. Out of 35 sources, we found only one source (PKS 2149–302,z=2.345) with a significant emission-line feature, centered at ∼ 17 keV in the quasar frame. The most likely explanation is blueshifted Fe-K emission (the EW is∼300±200&hthinsp;eV). Curiously, if the feature in PKS 0637–752 is due to blueshifted Oxygen VII emission the Doppler factor in both quasars is similar (∼2.7–2.8), implying high velocities of order ∼ 0.75c. ©1999 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.58614

出版商:AIP    

年代:1999

数据来源: AIP