共查询到20条相似文献，搜索用时 31 毫秒
1.
Margrethe Wold Mark Lacy Per B. Lilje Stephen Serjeant 《Monthly notices of the Royal Astronomical Society》2001,323(1):231-247
We quantify the galaxy environments around a sample of 0.5≤ z ≤0.8 radio-quiet quasars using the amplitude of the spatial galaxy–quasar correlation function, B gq . The quasars exist in a wide variety of environments; some sources are located in clusters as rich as Abell class 1–2 clusters, whereas others exist in environments comparable to the field. We find that, on average, the quasars prefer poorer clusters of ≈Abell class 0, which suggests that quasars are biased tracers of mass compared with galaxies. The mean B gq for the sample is found to be indistinguishable from the mean amplitude for a sample of radio-loud quasars matched in redshift and optical luminosity. These observations are consistent with recent studies of the hosts of radio-quiet quasars at low to intermediate redshifts, and suggest that the mechanism for the production of powerful radio jets in radio-loud quasars is controlled by processes deep within the active galactic nucleus itself, and is unrelated to the nature of the hosts or their environments. 相似文献
2.
R. J. Smith B. J. Boyle S. J. Maddox 《Monthly notices of the Royal Astronomical Society》2000,313(2):252-262
An angular correlation of low significance (2 σ ) is observed between 0.3< z <0.5 QSOs and V 23 galaxies. Overall, the cross-correlation function between 82 intermediate-redshift (0.3< z <0.7) X-ray selected QSOs and V ≲24 galaxies is investigated, but no signal is detected for the z >0.5 QSOs. After converting to an excess of galaxies physically associated with the QSO, this lack of strong correlation is shown to be consistent with the clustering of normal galaxies at the same moderate redshifts. Combined with previous observations, these results imply that the environments of radio-quiet QSOs do not undergo significant evolution with respect to the galaxy population over a wide range of redshifts (0< z <1.5). This is in marked contrast to the rapid increase in the richness of the environments associated with radio-loud QSOs over the same redshift range. 相似文献
3.
4.
Chris Simpson Steve Rawlings 《Monthly notices of the Royal Astronomical Society》2000,317(4):1023-1028
We present JHKL ' photometry of a complete sample of steep-spectrum radio-loud quasars from the revised 3CR catalogue in the redshift range 0.65 z <1.20. After correcting for contributions from emission lines and the host galaxies, we investigate their spectral energy distributions (SEDs) around 1 μm. About 75 per cent of the quasars are tightly grouped in the plane of optical spectral index, α opt , versus near-infrared spectral index, α IR , with the median value of α opt close to the canonical value, and the median α IR slightly flatter. We conclude that the fraction of moderately obscured, red quasars decreases with increasing radio power, in accordance with the 'receding torus' model which can also explain the relatively flat median near-infrared spectra of the 3CR quasars. Two of the red quasars have inverted infrared spectral indices, and we suggest that their unusual SEDs might result from a combination of dust-scattered and transmitted quasar light. 相似文献
5.
6.
R.G. Sharp R.G. McMahon M.J. Irwin S.T. Hodgkin 《Monthly notices of the Royal Astronomical Society》2001,326(4):L45-L49
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
7.
8.
9.
High‐resolution observations, made with the Very Large Array (VLA) at 330 MHz, 1.4 GHz and 8.4 GHz and with the Ryle Telescope at 15 GHz, are presented of a sample of 23 sources which are variable at 151 MHz, concluding the observations of an unbiased sample of 40 such sources. The 8.4 GHz emission of almost all of the sources is dominated by structure on a scale ≲0.1 arcsec – the spectra of these compact components are such that they will also dominate the emission at 151 MHz; the number of sources for which this is not the case is consistent with the number of spurious variables expected to be found in the sample. About two-thirds of the sources have the self-absorbed, or flat, spectra expected from their size. The majority (∼75 per cent) of the sources belong to the compact steep-spectrum (CSS) class, but are generally more compact than other CSS sources; the CSS variables also appear to exhibit more spectral ageing than typical CSS sources, which may indicate a difference in the nature of the sources. 相似文献
10.
11.
I. Waddington R. A. Windhorst J. S. Dunlop D. C. Koo J. A. Peacock 《Monthly notices of the Royal Astronomical Society》2000,317(4):801-824
The results are presented of an extensive programme of optical and infrared imaging of radio sources in a complete subsample of the Leiden–Berkeley Deep Survey. The LBDS Hercules sample consists of 72 sources observed at 1.4 GHz, with flux densities S 1.4 1.0 mJy, in a 1.2 deg2 region of Hercules. This sample is almost completely identified in the g , r , i and K bands, with some additional data available at J and H . The magnitude distributions peak at r ≃22 mag, K ≃16 mag and extend down to r ≃26 mag, K ≃21 mag. The K -band magnitude distributions for the radio galaxies and quasars are compared with those of other radio surveys. At S 1.4 GHz ≲1 Jy, the K -band distribution does not change significantly with radio flux density. The sources span a broad range of colours, with several being extremely red ( r − K ≳6). Though small, this is the most optically complete sample of mJy radio sources available at 1.4 GHz, and is ideally suited for studying the evolution of the radio luminosity function out to high redshifts. 相似文献
12.
13.
The Cambridge Low-Frequency Synthesis Telescope has been used to produce a representative sample of low-frequency variable sources. 20 fields, each covering an area of approximately 9°×9° cosec δ , have been observed at 151 MHz at between 2 and 10 epochs over the period from 1984 to 1996. At each epoch, maps were made with rms noise levels of typically 10–15 mJy beam−1 . From a total of ∼6000 sources detected on these maps, 207 are found for which the flux density variations between at least two epochs appear significant at greater than the 3 σ level. A numerical model is used to assess the true significance of the variability, given the analysis method adopted. This shows that for about half of the sources which appear to vary by >3 σ the variability is genuine. For the other half it is caused by random statistical fluctuations; most of the spurious variables vary by ≲3.5 σ between a single pair of epochs. A catalogue of the variable sources is presented, which includes an estimate of the probability that a given source is a genuine variable. Fractional flux density variations of between 5 and 100 per cent (typically 15–25 per cent) have been detected on a range of time-scales from 1 to 12 years. 相似文献
14.
Minfeng Gu Zhaoyu Chen Xinwu Cao 《Monthly notices of the Royal Astronomical Society》2009,397(3):1705-1710
The relationship between the black hole mass and velocity dispersion indicated with [O iii ] linewidth is investigated for a sample of 87 flat-spectrum radio quasars selected from the Sloan Digital Sky Survey Data Release 3 quasar catalogue. We found that the M BH −σ[O III] relation is different from the Tremaine et al. relation for nearby inactive galaxies, with a larger black hole mass at given velocity dispersion. There is no strong evidence of cosmology evolution in the M BH −σ[O III] relation up to z ∼ 0.8 . A significant correlation between the [O iii ] luminosity and broad-line region (BLR) luminosity is found. When transferring the [O iii ] luminosity to narrow-line region (NLR) luminosity, the BLR luminosity is, on average, larger than the NLR one by about one order of magnitude. We found a strong correlation between the synchrotron peak luminosity and NLR luminosity, which implies a tight relation between the jet physics and accretion process. 相似文献
15.
16.
17.
18.
A. C.Fabian K.Iwasawa A.Celotti W. N.Brandt R. G.McMahon I. M.Hook 《Monthly notices of the Royal Astronomical Society》1998,295(1):l25-l28
The X-ray-luminous quasar GB 1428+4217 at redshift 4.72 has been observed with ASCA . The observed 0.5–10 keV flux is 3.2 Å– 10−12 erg cm−2 s−1 . We report here on the intrinsic 4 − 57 keV X-ray spectrum, which is very flat (photon index 1.29). We find no evidence for flux variability within the ASCA data set or between it and ROSAT data. We show that the overall spectral energy distribution of GB 1428+4217 is similar to that of lower redshift MeV blazars, and present models that fit the available data. The Doppler beaming factor is likely to be at least 8. We speculate on the number density of such high-redshift blazars, which must contain rapidly formed massive black holes. 相似文献
19.
K. K. Knudsen P. P. van der Werf J.-P. Kneib 《Monthly notices of the Royal Astronomical Society》2008,384(4):1611-1626
We have conducted a submillimetre mapping survey of faint, gravitationally lensed sources, where we have targeted 12 galaxy clusters and additionally the New Technology Telescope (NTT) Deep Field. The total area surveyed is 71.5 arcmin2 in the image plane; correcting for gravitational lensing, the total area surveyed is 40 arcmin2 in the source plane for a typical source redshift z ≈ 2.5. In the deepest maps, an image plane depth of 1σ rms ∼0.8 mJy is reached. This survey is the largest survey to date to reach such depths. In total 59 sources were detected, including three multiply imaged sources. The gravitational lensing makes it possible to detect sources with flux density below the blank field confusion limit. The lensing-corrected fluxes range from 0.11 to 19 mJy. After correcting for multiplicity, there are 10 sources with fluxes <2 mJy of which seven have submJy fluxes, doubling the number of such sources known. Number counts are determined below the confusion limit. At 1 mJy, the integrated number count is ∼104 deg−2 , and at 0.5 mJy it is ∼2 × 104 deg−2 . Based on the number counts, at a source plan flux limit of 0.1 mJy, essentially all of the 850-μm background emission has been resolved. The dominant contribution (>50 per cent) to the integrated background arises from sources with fluxes S 850 between 0.4 and 2.5 mJy, while the bright sources S 850 > 6 mJy contribute only 10 per cent. 相似文献
20.
Eleni Vardoulaki Steve Rawlings Chris Simpson David G. Bonfield R. J. Ivison Eduardo Ibar 《Monthly notices of the Royal Astronomical Society》2008,387(2):505-535
We study the 37 brightest radio sources in the Subaru/ XMM–Newton Deep Field. We have spectroscopic redshifts for 24 of 37 objects and photometric redshifts for the remainder, yielding a median redshift z med for the whole sample of z med ≃ 1.1 and a median radio luminosity close to the 'Fanaroff–Riley type I/type II (FR I/FR II)' luminosity divide. Using mid-infrared (mid-IR) ( Spitzer MIPS 24 μm) data we expect to trace nuclear accretion activity, even if it is obscured at optical wavelengths, unless the obscuring column is extreme. Our results suggest that above the FR I/FR II radio luminosity break most of the radio sources are associated with objects that have excess mid-IR emission, only some of which are broad-line objects, although there is one clear low-accretion-rate object with an FR I radio structure. For extended steep-spectrum radio sources, the fraction of objects with mid-IR excess drops dramatically below the FR I/FR II luminosity break, although there exists at least one high-accretion-rate 'radio-quiet' QSO. We have therefore shown that the strong link between radio luminosity (or radio structure) and accretion properties, well known at z ∼ 0.1, persists to z ∼ 1. Investigation of mid-IR and blue excesses shows that they are correlated as predicted by a model in which, when significant accretion exists, a torus of dust absorbs ∼30 per cent of the light, and the dust above and below the torus scatters ≳1 per cent of the light. 相似文献