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1.
In a pilot search for high-redshift radio quasi-stellar objects (QSOs), we have obtained spectra of 55 FIRST sources     with very red     starlike optical identifications. 10 of the candidates are QSOs with redshifts     (four were previously known), six with     . The remaining 45 candidates comprise: one     broad-absorption-line (BAL) QSO; three low-redshift galaxies with narrow emission lines; 18 probable radio galaxies; and 23 M stars (mainly misidentifications). The success rate (high-redshift QSOs / spectroscopically-observed candidates) for this search is 1/2 for     , and 1/9 for     . With an effective search area of 4030 deg2, the surface density of high-redshift     QSOs discovered with this technique is 0.0015 deg−2.  相似文献   

2.
Periodicities and selection effects in the redshift (z) distribution of QSOs have been debated for a long time in the literature. Here we show that peaks and troughs in the redshift distribution of three new samples, claimed to demonstrate the existence of a periodicity, can be interpreted in terms of known selection effects. This analysis confirms earlier findings that the presence of such selection effects seriously weakens any suggestion for periodicity of the form Δl n (1 + z) = constant.  相似文献   

3.
I had the privilege, many years ago, to be supervised by Alfonso Cavaliere in my Master thesis work. I remember the great patience and enthusiasm with which, in a warm summer day, he presented to me a score of exciting astrophysical problems. We finally determined to study the relationship of QSOs and BL Lacs—then considered rather exotic object—with normal galaxies. In the years I have continued on this line of research, reacting far too slowly to the wealth of new ideas that Alfonso has been continuously injecting in our conversations. As examples I will report two recent results on the chemical abundances of QSO host galaxies and on the space density of high-z QSOs, which are closely related to our shared interest on AGN.  相似文献   

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5.
We report results of the observations at the Vainu Bappu Observatory and the Uttar Pradesh State Observatory of eight radio-quiet quasi-stellar objects (RQQSOs) during 1996–99. This is a part of our ongoing programme to search for intranight optical variability in RQQSOs. Additional evidence for very rapid variability in three of the five optically bright and very luminous RQQSOs we had observed earlier, 1049−006, 1444+408 and 1630+377, was found. Of the three newly observed RQQSOs, the data for 0043+039 are too noisy to allow conclusions about variability to be drawn, but 0748+294 and 0824+098 show strong hints of microvariability. We also present a summary of the results from our entire programme to date, which includes observations of 16 radio-quiet QSOs and one radio-weak QSO, and compare the general properties of rapid variability in radio-quiet versus radio-loud AGN as determined from our work and that of several other groups. Observations of this kind are likely to play a key role in understanding the relative contributions of accretion discs and relativistic plasma jets to rapid optical fluctuations of AGN.  相似文献   

6.
The colours of high-redshift Type II quasi-stellar objects (QSOs) are synthesized from observations of moderate-redshift systems. It is shown that Type II QSOs are comparable to starbursts at matching the colours of z 850-dropouts and i 775-drops in the Hubble UltraDeep Field , and more naturally account for the bluest objects detected. Type II QSOs may also account for some of the i 775-drops detected in the Great Observatories Origins Deep Survey (GOODS) fields. It is shown that by combining imaging data from the Hubble Space Telescope and the James Webb Space Telescope , it will be possible to clearly separate Type II QSOs from Type I QSOs and starbursts based on their colours. Similarly, it is shown that the United Kingdom Infrared Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS) ZYJ filters may be used to discriminate high-redshift Type II QSOs from other objects. If Type II QSOs are prevalent at high redshifts, then active galactic nuclei (AGNs) may be major contributors to the re-ionization of the intergalactic medium.  相似文献   

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8.
We present an analysis of X-ray variability in a sample of 156 radio-quiet quasars taken from the ROSAT archive, covering a redshift range  0.12)  in the sense that QSOs of the same X-ray luminosity are more variable at  z>2  . We discuss possible explanations for this effect. The simplest explanation may be that high-redshift QSOs are accreting at a larger fraction of the Eddington limit than local AGNs.  相似文献   

9.
We have undertaken a pilot survey for faint quasi-stellar objects (QSOs) in the UKIRT Infrared Deep Survey (UKIDSS) Ultra Deep Survey (UDS) Field using the KX selection technique. These observations exploit the very deep near-infrared and optical imaging of this field from United Kingdom Infrared Telescope (UKIRT) and Subaru to select candidate QSOs based on their VJK colours and morphologies. We determined redshifts for 426 candidates using the AAOmega spectrograph on the Anglo-Australian Telescope in service time. We identify 17 QSOs  ( M B ≲−23)  in this pilot survey at z = 1.57–3.29. We combine our sample with an X-ray-selected sample of QSOs in the same field (a large fraction of which also comply with our KX selection) to constrain the surface density of QSOs with K ≤ 20, deriving limits on the likely surface density of 85–150 deg−2. We use the good image quality available from our near-infrared imaging to detect a spatially extended component of the QSO light which probably represents the host galaxies. We also use our sample to investigate routes to improve the selection of KX QSOs at faint limits in the face of the significant contamination by compact, foreground galaxies. The brightest examples from our combined QSO sample will be used in conjunction with a large Very Large Telescope VIMOS spectroscopic survey of high-redshift galaxies in this region to study the structures inhabited by gas, galaxies and growing supermassive black holes at high redshifts in the UKIDSS UDS.  相似文献   

10.
Direct and indirect observational evidence leads to the conclusion that high-redshift QSOs did shine in the core of early-type protogalaxies during their main episode of star formation. Exploiting this fact, we derive the rate of formation of this kind of stellar system at high redshift by using the QSO luminosity function. The elemental proportions in elliptical galaxies, the descendants of the QSO hosts, suggest that the star formation was more rapid in more massive objects. We show that this is expected to occur in dark matter haloes, when the processes of cooling and heating are considered. This is also confirmed by comparing the observed submm counts with those derived by coupling the formation rate and the star formation rate of the spheroidal galaxies with a detailed model for their SED evolution. In this scenario SCUBA galaxies and Lyman-break galaxies are early-type protogalaxies forming the bulk of their stars before the onset of QSO activity.  相似文献   

11.
We present spatially resolved spectrophotometric observations of multiply imaged QSOs, using the Potsdam Multi‐Aperture Spectrophotometer (PMAS), with the intention to search for spectral differences between components indicative of either microlensing or dust extinction. For the quadruple QSO HE 0435–1223 we find that the continuum shapes are indistinguishable, therefore differential extinction is negligible. The equivalent widths of the broad emission lines are however significantly different, and we argue that this is most likely due to microlensing. Contrariwise, the two components of the well‐known object UM 673 have virtually identical emission line properties, but the continuum slopes differ significantly and indicate different dust extinction along both lines of sight. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

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14.
We present an analysis of the proximity effect in a sample of 10 2-Å-resolution QSO spectra of the Ly α forest at     . Rather than investigating variations in the number density of individual absorption lines, we employ a novel technique that is based on the statistics of the transmitted flux itself. We confirm the existence of the proximity effect at the > 99 per cent confidence level. We derive a value for the mean intensity of the extragalactic background radiation at the Lyman limit of     . This value assumes that QSO redshifts measured from high-ionization lines differ from the true systemic redshifts by     . We find evidence at a level of 2.6 σ that the significance of the proximity effect is correlated with QSO Lyman limit luminosity. Allowing for known QSO variability, the significance of the correlation reduces to 2.1 σ .
The QSOs form a close group on the sky and the sample is thus well suited for an investigation of the foreground proximity effect, where the Ly α forest of a background QSO is influenced by the UV radiation from a nearby foreground QSO. From the complete sample we find no evidence for the existence of this effect, implying either that     or that QSOs emit at least a factor of 1.4 less ionizing radiation in the plane of the sky than along the line of sight to Earth. We do, however, find one counter-example. Our sample includes the fortunate constellation of a foreground QSO surrounded by four nearby background QSOs. These four spectra all show underdense absorption within ±3000 km s−1 of the redshift of the foreground QSO.  相似文献   

15.
It is argued that the formation of a dwarf galaxy causes a massive burst of star formation, resulting in the ejection of most of the available gas from the galaxy as a weakly collimated wind. The ejected gas can give rise to a damped Lyα absorber (DLA). Weakly collimated outflows naturally explain the asymmetric profiles seen in low-ionization absorption lines caused by heavy elements associated with DLAs, where absorption is strongest at one edge of the absorption feature. The shape of the distribution of column densities in the model agrees reasonably well with observations. In particular, the break in slope is caused by external photoionization of the wind. A semi-analytical model for galaxy formation is used to show that, for currently acceptable cosmological parameters, dwarf galaxy outflows can account for the majority of DLA systems and their distribution with redshift. This model also predicts a correlation between velocity structure and metallicity of DLA systems, in qualitative agreement with observations. DLAs do not require many large, rapidly rotating disc galaxies to have formed early on, as in other models for their origin.  相似文献   

16.
Older and more recent pieces of observational evidence suggest a strong connection between QSOs and galaxies; in particular, the recently discovered correlation between black hole and galactic bulge masses suggests that QSO activity is directly connected to the formation of galactic bulges. The cosmological problem of QSO formation is analysed in the framework of an analytical model for galaxy formation; for the first time a joint comparison with galaxy and QSO observables is performed. In this model it is assumed that the same physical variable that determines galaxy morphology is able to modulate the mass of the black hole responsible for QSO activity. Both halo spin and the occurrence of a major merger are considered as candidates for this role. The predictions of the model are compared with available data for the type-dependent galaxy mass functions, the star formation history of elliptical galaxies, the QSO luminosity function and its evolution (including the obscured objects contributing to the hard-X-ray background), the mass function of dormant black holes and the distribution of black hole-to-bulge mass ratios. A good agreement with observations is obtained if the halo spin modulates the efficiency of black hole formation, and if the galactic haloes at z =0 have shone in an inverted order with respect to the hierarchical one (i.e., stars and black holes in bigger galactic haloes have formed before those in smaller ones). This inversion of hierarchical order for galaxy formation, which reconciles galaxy formation with QSO evolution, is consistent with many pieces of observational evidence.  相似文献   

17.
We present an analysis of X-ray variability in a flux-limited sample of quasi-stellar objects (QSOs). Selected from our deep ROSAT survey, these QSOs span a wide range in redshift (0.1< z <3.2) and are typically very faint, so we have developed a method to constrain the amplitude of variability in ensembles of low signal-to-noise ratio light curves. We find evidence for trends in this variability amplitude with both redshift and luminosity. The mean variability amplitude declines sharply with luminosity, as seen in local active galactic nuclei (AGN), but with some suggestion of an upturn for the most powerful sources. We find tentative evidence that this is caused by redshift evolution, since the high-redshift QSOs ( z >0.5) do not show the anticorrelation with luminosity seen in local AGN. We speculate on the implications of these results for physical models of AGN and their evolution. Finally, we find evidence for X-ray variability in an object classified as a narrow-emission-line galaxy, suggesting the presence of an AGN.  相似文献   

18.
We report on the analysis of the photometric and spectroscopic properties of a sample of 29 low-redshift  ( z < 0.6)  QSOs for which both Hubble Space Telescope ( HST ) WFPC2 images and ultraviolet HST   FOS spectra are available. For each object we measure the R -band absolute magnitude of the host galaxy, the C  iv (1550 Å) linewidth and the 1350 Å continuum luminosity. From these quantities we can estimate the black hole (BH) mass through the   M BH– L bulge  relation for inactive galaxies, and from the virial method based on the kinematics of the regions emitting the broad-lines. The comparison of the masses derived from the two methods yields information on the geometry of the gas emitting regions bound to the massive BH. The cumulative distribution of the linewidths is consistent with that produced by matter laying in planes with inclinations uniformly distributed between ∼10° and ∼50°, which corresponds to a geometrical factor   f ∼ 1.3  . Our results are compared with those of the literature and discussed within the unified model of active galactic nuclei.  相似文献   

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