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1.
We report the detection of extended X-ray emission around several powerful 3CR quasars with redshifts out to 0.73. The ROSAT HRI images of the quasars have been corrected for spacecraft wobble and compared with an empirical point-spread function. All the quasars examined show excess emission at radii of 15 arcsec and more, the evidence being strong for the more distant objects and weak only for the two nearest ones, which are known from other wavelengths not to lie in strongly clustered environments. The spatial profile of the extended component is consistent with thermal emission from the intracluster medium of moderately rich host clusters to the quasars. The total luminosities of the clusters are in the range ∼4×1044–3×1045 erg s−1, assuming a temperature of 4 keV. The inner regions of the intracluster medium are, in all cases, dense enough to be part of a cooling flow.  相似文献   

2.
We describe the selection of candidate radio-loud quasars obtained by cross-matching radio source positions from the low-frequency (151-MHz) 7C survey with optical positions from five pairs of EO POSS-I plates scanned with the Cambridge Automatic Plate-measuring Machine (APM). The sky region studied is centred at RA 10h 28m, Dec.+41° and covers ≈0.057 sr. We present VLA observations of the quasar candidates, and tabulate various properties derived from the radio maps. We discuss the selection criteria of the resulting '7CQ' sample of radio-loud quasars. The 70 confirmed quasars, and some fraction of the 36 unconfirmed candidates, constitute a filtered sample with the following selection criteria: 151-MHz flux density S 151>100 mJy; POSS-I E -plate magnitude E ≈ R <20; POSS-I colour ( O E )<1.8; the effective area of the survey drops significantly below S 151≈200 mJy. We argue that the colour criterion excludes few if any quasars, but note, on the basis of recent work by Willott et al., that the E magnitude limit probably excludes more than 50 per cent of the radio-loud quasars.  相似文献   

3.
The fate of the cooling gas in the central regions of rich clusters of galaxies is not well understood. In one plausible scenario clouds of atomic or molecular gas are formed. However the mass of the cold gas, inferred from measurements of low-energy X-ray absorption, is hardly consistent with the absence of powerful CO or 21-cm emission lines from the cooling flow region. Among the factors which may affect the detectability of the cold clouds are their optical depth, shape and covering fraction. Thus, alternative methods to determine the mass in cold clouds, which are less sensitive to these parameters, are important.   For the inner region of the cooling flow (e.g. within a radius of ∼50–100 kpc) the Thomson optical depth of the hot gas in a massive cooling flow can be as large as ∼ 0.01. Assuming that the cooling time in the inner region is few times shorter than the lifetime of the cluster, the Thomson depth of the accumulated cold gas can be accordingly higher (if most of the gas remains in the form of clouds). The illumination of the cold clouds by the X-ray emission of the hot gas should lead to the appearance of a 6.4-keV iron fluorescent line, with an equivalent width proportional to τT. The equivalent width only weakly depends on the detailed properties of the clouds, e.g. on the column density of individual clouds, as long as the column density is less than a few 1023 cm−2. Another effect also associated exclusively with the cold gas is a flux in the Compton shoulder of bright X-ray emission lines. It also scales linearly with the Thomson optical depth of the cold gas. With the new generation of X-ray telescopes, combining large effective area and high spectral resolution, the mass of the cold gas in cooling flows (and its distribution) can be measured.  相似文献   

4.
We combine photometric observations of high-redshift     quasars, obtained at submillimetre to millimetre wavelengths, to obtain a mean far-infrared (rest-frame) spectral energy distribution (SED) of the thermal emission from dust, parametrized by a single temperature ( T ) and power-law emissivity index ( β ). The best-fitting values are     and     . Our method exploits the redshift spread of this set of quasars, which allows us to sample the SED at a larger number of rest wavelengths than is possible for a single object: the wavelength range extends down to ∼60 μm, and therefore samples the turnover in the greybody curve for these temperatures. This parametrization is of use to any studies that extrapolate from a flux at a single wavelength, for example to infer dust masses and far-infrared luminosities.
We interpret the cool, submillimetre component as arising from dust heated by star formation in the host galaxy of the quasar, although we do not exclude the presence of dust heated directly by the active galactic nucleus (AGN). Applying the mean SED to the data, we derive consistent star formation rates ∼1000 M⊙ yr−1 and dust masses ∼109 M⊙, and investigate a simple scheme of AGN and host galaxy co-evolution to account for these quantities. The time-scale for formation of the host galaxy is     , and the luminous quasar phase occurs towards the end of this period, just before the reservoir of cold gas is depleted. Given the youth of the Universe at     (1.6 Gyr), the coexistence of a massive black hole and a luminous starburst at high redshifts is a powerful constraint on models of quasar host galaxy formation.  相似文献   

5.
We consider a model for quasar formation in which massive black holes are formed and fuelled largely by the accretion of hot gas during the process of galaxy formation. In standard hierarchical collapse models, objects about the size of normal galaxies and larger form a dense hot atmosphere when they collapse. We show that if such an atmosphere forms a nearly 'maximal' cooling flow, then a central black hole can accrete at close to its Eddington limit. This leads to exponential growth of a seed black hole, resulting in a quasar in some cases. In this model, the first quasars form soon after the first collapses to produce hot gas. The hot gas is depleted as time progresses, mostly by cooling, so that the accretion rate eventually falls below the threshold for advection-dominated accretion, at which stage radiative efficiency plummets and any quasar turns off. A simple implementation of this model, incorporated into a semi-analytical model for galaxy formation, overproduces quasars when compared with observed luminosity functions, but is consistent with models of the X-ray background, which indicate that most accretion is obscured. It produces few quasars at high redshift owing to the lack of time needed to grow massive black holes. Quasar fuelling by hot gas provides a minimum level, sufficient to power most quasars at redshifts between one and two, to which other sources of fuel can be added. The results are sensitive to feedback effects, such as might result from radio jets and other outflows.  相似文献   

6.
We report the discovery of two radio-loud quasars with redshifts greater than 4: GB1428+4217, with z =4.72, and GB1713+2148 with z =4.01. This doubles the number of published radio-selected quasars with z >4, bringing the total to 4. GB1428+4217 is the third most distant quasar known and the highest redshift radio and X-ray source currently known. It has a radio flux density at 5 GHz of 259±31 mJy and an optical magnitude of R ∼20.9. The rest frame absolute UV magnitude, Mv (1450 Å), is −26.7, similar to that of the archetypal radio-selected quasar 3C273 [ z =0.158; Mv (1450 Å)=−26.4]. GB1428+4217 is tentatively detected in ROSAT PSPC observations, which has been confirmed by more recent ROSAT observations described in a companion paper by Fabian et al. Both quasars were discovered during the CCD imaging phase of an investigation into the evolution of the space density of radio-loud quasars at high redshift. Combined with our earlier survey results, these objects give a lower limit on the space density of quasars with radio power P 5 GHz>5.8×1026 W Hz−1 sr−1 between z =4 and z =5 of 1.4±0.9×10−10 Mpc−3. This can be compared to 2.9±0.2×10−10 Mpc−3 at z =2 from Dunlop & Peacock for flat-spectrum sources of the same luminosity.  相似文献   

7.
We present near-infrared spectra of seven radio-loud quasars with a median redshift of 2.1, five of which were previously known to have Ly α nebulae. Extended [O  iii ] λ 5007 and H α emission are evident around six objects, at the level of a few times 10−16 erg cm−2 arcsec−2 s−1 within ≃2 arcsec of the nucleus (≡16 kpc in the adopted cosmology). Nuclear [O  ii ] λ 3727 is detected in three of the five quasars studied at this wavelength and clearly extended in one of them.
The extended [O  iii ] tends to be brighter on the side of the nucleus with the stronger, jet-like radio emission, indicating at least that the extranuclear gas is distributed anisotropically. It is also typically redshifted by several hundred km s−1 from the nuclear [O  iii ], perhaps because of the latter being blueshifted from the host galaxy's systemic velocity. Alternatively, the velocity shifts could be due to infall (which is suggested by linewidths ∼1000 km s−1 FWHM) in combination with a suitable dust geometry. Ly α /H α ratios well below the case B value suggest that some dust is present.
Photoionization modelling of the [O  iii ]/[O  ii ] ratios in the extended gas suggests that its pressure is around or less than a few times 107 cm−3 K; any confining intracluster medium is thus likely to host a strong cooling flow. A comparison with lower redshift work suggests that there has been little evolution in the nuclear emission-line properties of radio-loud quasars between redshifts 1 and 2.  相似文献   

8.
We report the first results of an observational programme designed to determine the luminosity density of high-redshift quasars     quasars) using deep multicolour CCD data. We report the discovery and spectra of three     high-redshift     quasars, including one with     . At     , this is the fourth highest redshift quasar currently published. Using these preliminary results we derive an estimate of the         quasar space density in the redshift range     of     . When completed, the survey will provide a firm constraint on the contribution to the ionizing UV background in the redshift range     from quasars by determining the faint-end slope of the quasar luminosity function. The survey uses imaging data taken with the 2.5-m Isaac Newton Telescope as part of the Public Isaac Newton Group Wide Field Survey (WFS). This initial sample of objects is taken from two fields of effective area ∼12.5 deg2 from the final ∼100 deg2.  相似文献   

9.
We present Hubble Space Telescope ( HST ) images of seven low-redshift quasars (six taken with the Planetary Camera, one with the Wide Field Camera). These complete the sample of 14 quasars observed by the Faint Object Camera Investigation Definition Team (FOC IDT). Following subtraction of the quasar nuclear light, host galaxies can be seen in all seven cases. A combination of the optical morphology and luminosity profiles of the residual host galaxies and the results of 2D cross-correlation model fitting implies that five of the objects have elliptical host galaxies and two have disc host galaxies. The luminosities vary from slightly fainter than L * to about 1.3 mag brighter than L *.   We discuss the properties of the complete sample of 14 quasars. Nine of the objects appear to have elliptical host galaxies (all six of the radio-loud quasars in the sample as well as three radio-quiet quasars). Two further radio-quiet quasars appear to lie in disc galaxies. The other three objects (radio-quiet, ultraluminous infrared quasars) all lie in violently interacting systems. The sample as a whole has an average luminosity about 0.8 mag brighter than L *, although the radio-loud objects have hosts on average 0.7 mag brighter than the radio-quiet objects.   We compare our results with those from HST imaging of quasars by other authors. Taken together, our observations are in broad agreement with those of Bahcall et al. Radio-loud quasars appear to lie in luminous elliptical galaxies whereas radio-quiet quasars are found to lie in either elliptical or spiral hosts. Host galaxy luminosities (of radio-quiet and radio-loud quasars) are much brighter than would be expected if they followed a Schechter luminosity function.  相似文献   

10.
Using cosmological hydrodynamic simulations, we measure the mean transmitted flux in the Lyα forest for quasar sightlines that pass near a foreground quasar. We find that the trend of absorption with pixel quasar separation distance can be fitted using a simple power-law form including the usual correlation function parameters r 0 and γ, so that     . From the simulations, we find the relation between r 0 and quasar host mass, and formulate this as a way to estimate quasar host dark matter halo masses, quantifying uncertainties due to cosmological and IGM parameters, and redshift errors. With this method, we examine data for ∼9000 quasars from the Sloan Digital Sky Survey (SDSS) Data Release 5, assuming that the effect of ionizing radiation from quasars (the so-called transverse proximity effect) is unimportant (no evidence for it is seen in the data). We find that the best-fitting host halo mass for SDSS quasars with mean redshift z = 3 and absolute G -band magnitude −27.5 is  log  M /M⊙= 12.68+0.81−0.67  . We also use the Lyman-Break Galaxy (LBG) and Lyα forest data of Adelberger et al. in a similar fashion to constrain the halo mass of LBGs to be  log10  M /M⊙= 11.41+0.54−0.59  , a factor of ∼20 lower than the bright quasars. In addition, we study the redshift distortions of the Lyα forest around quasars, using the simulations. We use the quadrupole to monopole ratio of the quasar Lyα forest correlation function as a measure of the squashing effect. We find its dependence on halo mass difficult to measure, but find that it may be useful for constraining cosmic geometry.  相似文献   

11.
We present J , H and K -band spectroscopy of Cygnus A, spanning 1.0–2.4 μm in the rest-frame and hence several rovibrational H2, H recombination and [Fe  ii ] emission lines. The lines are spatially extended by up to 6 kpc from the nucleus, but their distinct kinematics indicate that the three groups (H, H2 and [Fe  ii ]) are not wholly produced in the same gas. The broadest line, [Fe  ii ] λ 1.644, exhibits a non-Gaussian profile with a broad base (FWHM≃1040 km s−1), perhaps because of the interaction with the radio source. Extinctions to the line-emitting regions substantially exceed earlier measurements based on optical H recombination lines.
Hard X-rays from the quasar nucleus are likely to dominate the excitation of the H2 emission. The results of Maloney, Hollenbach & Tielens are thus used to infer the total mass of gas in H2 v=1–0 S(1)-emitting clouds as a function of radius, for gas densities of 103 and 105 cm−3, and stopping column densities N H=1022–1024 cm−2. Assuming azimuthal symmetry, at least 2.3×108 M⊙ of such material is present within 5 kpc of the nucleus, if the line-emitting clouds see an unobscured quasar spectrum. Alternatively, if the bulk of the X-ray absorption to the nucleus inferred by Ueno et al. actually arises in a circumnuclear torus, the implied gas mass rises to ∼1010 M⊙. The latter plausibly accounts for 109 yr of mass deposition from the cluster cooling flow, for which within this radius.  相似文献   

12.
We present the spectra and redshifts of 62 quasars, from observations made with the Cerro Tololo Inter-American Observatory (CTIO) 4-m Blanco Telescope. These quasars form part of a total sample of 118 (with 56 having been published previously), which is being used for analysis of structure in the early universe. Quasars of particular interest are noted, including eight broad absorption line (BAL) quasars and two quasars with unusual emission spectra. Finally, we include a short summary of the present status of the large quasar group (LQG) that was discovered by Clowes & Campusano from the earlier observations. The quasars are from an area ∼25.3 deg2 of ESO/SERC field 927, which is centred at (1950) 10h40m00s, 05°00'00'.  相似文献   

13.
We propose the apparent shrinking criterion (ASC) to interpret the spatial extent, R w, of transmitted flux windows in the absorption spectra of high- z quasars. The ASC can discriminate between the two regimes in which R w corresponds either to the physical size,   R H  ii   , of the quasar H  ii region or to the distance,   R maxw  , at which the transmitted flux drops to  =0.1  and a Gunn–Peterson (GP) trough appears. In the first case [H  ii region (HR) regime], one can determine the intergalactic medium mean H  i fraction,   x H I  ; in the second [proximity region (PR) regime], the value of R w allows one to measure the local photoionization rate and the local enhancement of the photoionization rate,  ΓG  , due to nearby/intervening galaxies. The ASC has been tested against radiative transfer+smoothed particle hydrodynamics numerical simulations, and applied to a sample of 15 high-   z ( z > 5.8  ) quasar spectra. All sample quasars are found to be in the PR regime; hence, their observed spectral properties (inner flux profile, extent of transmission window) cannot reliably constrain the value of   x H  i   . Four sample quasars show evidence for a local enhancement (up to 50 per cent) in the local photoionization rate possibly produced by a galaxy overdensity. We discuss the possible interpretations and uncertainties of this result.  相似文献   

14.
We use a three-dimensional hydrodynamical code to simulate the effect of energy injection on cooling flows in the intracluster medium. Specifically, we compare a simulation of a 1015-M⊙ cluster with radiative cooling only with a second simulation in which thermal energy is injected 31 kpc off-centre, over 64 kpc3 at a rate of     for 50 Myr. The heat injection forms a hot, low-density bubble which quickly rises, dragging behind it material from the cluster core. The rising bubble pushes with it a shell of gas which expands and cools. We find the appearance of the bubble in X-ray temperature and luminosity to be in good qualitative agreement with recent Chandra observations of cluster cores. Toward the end of the simulation, at 600 Myr, the displaced gas begins to fall back toward the core, and the subsequent turbulence is very efficient at mixing the low- and high-entropy gas. The result is that the cooling flow is disrupted for up to ∼ 50 Myr after the injection of energy ceases. Thus this mechanism provides a very efficient method for regulating cooling flows, if the injection events occur with a 1:1 duty cycle.  相似文献   

15.
We use semi-analytic modelling on top of the Millennium simulation to study the joint formation of galaxies and their embedded supermassive black holes. Our goal is to test scenarios in which black hole accretion and quasar activity are triggered by galaxy mergers, and to constrain different models for the light curves associated with individual quasar events. In the present work, we focus on studying the spatial distribution of simulated quasars. At all luminosities, we find that the simulated quasar two-point correlation function is fit well by a single power law in the range  0.5 ≲ r ≲ 20  h −1 Mpc  , but its normalization is a strong function of redshift. When we select only quasars with luminosities within the range typically accessible by today's quasar surveys, their clustering strength depends only weakly on luminosity, in agreement with observations. This holds independently of the assumed light-curve model, since bright quasars are black holes accreting close to the Eddington limit, and are hosted by dark matter haloes with a narrow mass range of a few  1012  h −1 M⊙  . Therefore, the clustering of bright quasars cannot be used to disentangle light-curve models, but such a discrimination would become possible if the observational samples can be pushed to significantly fainter limits. Overall, our clustering results for the simulated quasar population agree rather well with observations, lending support to the conjecture that galaxy mergers could be the main physical process responsible for triggering black hole accretion and quasar activity.  相似文献   

16.
Low-frequency radio surveys are ideal for selecting orientation-independent samples of extragalactic sources because the sample members are selected by virtue of their isotropic steep-spectrum extended emission. We use the new 7C Redshift Survey along with the brighter 3CRR and 6C samples to investigate the fraction of objects with observed broad emission lines – the 'quasar fraction'– as a function of redshift and of radio and narrow-emission-line luminosity. We find that the quasar fraction is more strongly dependent upon luminosity (both narrow-line and radio) than it is on redshift. Above a narrow [O  ii ] emission-line luminosity of log10( L [O  ii ]/W)≳35 [or radio luminosity log10( L 151/W Hz−1 sr−1)≳ 26.5], the quasar fraction is virtually independent of redshift and luminosity; this is consistent with a simple unified scheme with an obscuring torus with a half-opening angle θ trans≈53°. For objects with less luminous narrow lines, the quasar fraction is lower. We show that this is not due to the difficulty of detecting lower luminosity broad emission lines in a less luminous, but otherwise similar, quasar population. We discuss evidence which supports at least two probable physical causes for the drop in quasar fraction at low luminosity: (i) a gradual decrease in θ trans and/or a gradual increase in the fraction of lightly reddened (0≲ A V ≲5) lines of sight with decreasing quasar luminosity; and (ii) the emergence of a distinct second population of low-luminosity radio sources which, like M87, lack a well-fed quasar nucleus and may well lack a thick obscuring torus.  相似文献   

17.
An empirically motivated model is presented for accretion-dominated growth of supermassive black holes (SMBH) in galaxies, and the implications are studied for the evolution of the quasar population in the Universe. We investigate the core aspects of the quasar population, including space density evolution, evolution of the characteristic luminosity, plausible minimum masses of quasars, the mass function of SMBH and their formation epoch distribution. Our model suggests that the characteristic luminosity in the quasar luminosity function arises primarily as a consequence of a characteristic mass scale above which there is a systematic separation between the black hole and the halo merging rates. At lower mass scales, black hole merging closely tracks the merging of dark haloes. When combined with a declining efficiency of black hole formation with redshift, the model can reproduce the quasar luminosity function over a wide range of redshifts. The observed space density evolution of quasars is well described by formation rates of SMBH above  ∼108  M⊙  . The inferred mass density of SMBH agrees with that found independently from estimates of the SMBH mass function derived empirically from the quasar luminosity function.  相似文献   

18.
We study the effect of quasar feedback on distributions of baryons in galaxy groups using high-resolution numerical simulations. We use the entropy-conserving gadget code that includes gas cooling and star formation, modified to include a physically based model of quasar feedback. For a sample of 10 galaxy group-sized dark matter haloes with masses in the range of  1–5 × 1013 M⊙  h −1  , star formation is suppressed by more than 50 per cent in the inner regions due to the additional pressure support by quasar feedback, while gas is driven from the inner region towards the outer region of the haloes. As a result, the average gas density is 50 per cent lower in the inner region and 10 per cent higher in the outer region in the simulation, compared to a similar simulation with no quasar feedback. Gas pressure is also higher in the outer region, while temperature and entropy are enhanced in the inner region. The total group gas fraction in the two simulations generally differs by less than 10 per cent. We also find a small change of the total thermal Sunyaev–Zeldovich distortion, leading to 10 per cent changes in the microwave angular power spectrum at angular scales below 2 arcmin.  相似文献   

19.
We have observed the galaxy environments around a sample of 21 radio-loud, steep-spectrum quasars at 0.5≤ z ≤0.82, spanning several orders of magnitude in radio luminosity. The observations also include background control fields used to obtain the excess number of galaxies in each quasar field. The galaxy excess was quantified using the spatial galaxy–quasar correlation amplitude, B gq, and an Abell-type measurement, N 0.5. A few quasars are found in relatively rich clusters, but on average, they seem to prefer galaxy groups or clusters of approximately Abell class 0. We have combined our sample with literature samples extending down to z ≈0.2 and covering the same range in radio luminosity. By using the Spearman statistic to disentangle redshift and luminosity dependences, we detect a weak, but significant, positive correlation between the richness of the quasar environment and the radio luminosity of the quasar. However, we do not find any epoch dependence in B gq, as has previously been reported for radio quasars and galaxies. We discuss the radio luminosity–cluster richness link and possible explanations for the weak correlation that is seen.  相似文献   

20.
Starting from the ∼50 000 quasars of the Sloan Digital Sky Survey for which Mg  ii line width and 3000 Å monochromatic flux are available, we aim to study the dependence of the mass of active black holes on redshift. We focus on the observed distribution in the full width at half-maximum–nuclear luminosity plane, which can be reproduced at all redshifts assuming a limiting M BH, a maximum Eddington ratio and a minimum luminosity (due to the survey flux limit). We study the z -dependence of the best-fitting parameters of assumed distributions at increasing redshift and find that the maximum mass of the quasar population evolves as  log ( M BH(max)/M⊙) ∼ 0.3 z + 9  , while the maximum Eddington ratio (∼0.45) is practically independent of cosmic time. These results are unaffected by the Malmquist bias.  相似文献   

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