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Quasars
Terms modified by Quasars Selected AbstractsAre radio-loud Broad Absorption Line Quasars young sources?ASTRONOMISCHE NACHRICHTEN, Issue 2-3 2009F.M. Montenegro-Montes Abstract Studying Broad Absorption Line Quasars (BAL QSOs) is interesting because they probe the physics of the AGN environment, and because AGN outflows are important ingredients in many recent astrophysical puzzles. In the last decade, a substantial population of radio BAL QSOs was discovered and we have started a characterisation of the radio-loud BAL QSO population studying their radio spectra, radio morphology and polarisation properties. Our high-resolution radio maps show that BAL QSOs are compact radio sources with projected linear sizes below 1 kpc. Most BAL QSOs in our sample are unpolarised or weakly polarised at 8.4 GHz. They display convex radio spectra which typically flatten at low frequencies and become steeper at high frequencies, i.e. above 20 GHz. Many of these characteristics are common to the population of young radio sources, like Compact Steep Spectrum (CSS) or Gigahertz-Peaked Spectrum (GPS) sources. The above supports the hypothesis that BAL QSOs might be related to the initial stages in the AGN evolution (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Identifying Compact Symmetric Objects from the VLBA Imaging and Polarization SurveyASTRONOMISCHE NACHRICHTEN, Issue 2-3 2009S.E. Tremblay Abstract Compact Symmetric Objects (CSOs) are small (less than 1 kpc) radio sources which have symmetric double lobes or jets. The dominant theory for the small size of these objects is that they are young radio sources which could grow into larger radio galaxies, but the currently small population of known CSOs makes it difficult to definitively determine whether or not this is the case. While a greater number of Gigahertz peaked sources can be identified by sifting through spectral surveys, this yields none of the dynamics of the sources, and also brings Quasars into the sample, which although interesting are peaked around 1 Gigahertz for very different reasons. We have used the 5 GHz VLBA Imaging and Polarization Survey (VIPS) to identify 103 CSO candidates morphologically, and are following up on these sources with multifrequency VLBA observations to confirm CSO identifications and to study their dynamics. The identification of candidates from within the survey will be discussed, as well as preliminary results from the follow-up observations (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Reionization bias in high-redshift quasar near-zonesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2008J. Stuart B. Wyithe ABSTRACT Absorption spectra of high-redshift quasars exhibit an increasingly thick Ly, forest, suggesting that the fraction of neutral hydrogen in the intergalactic medium (IGM) is increasing towards z, 6. However, the interpretation of these spectra is complicated by the fact that the Ly, optical depth is already large for neutral hydrogen fractions in excess of 10,4, and also because quasars are expected to reside in dense regions of the IGM. We present a model for the evolution of the ionization state of the IGM which is applicable to the dense, biased regions around high-redshift quasars as well as more typical regions in the IGM. We employ a cold dark matter based model in which the ionizing photons for reionization are produced by star formation in dark matter haloes spanning a wide range of masses, combined with numerical radiative transfer simulations which model the resulting opacity distribution in quasar absorption spectra. With an appropriate choice for the parameter which controls the star formation efficiency, our model is able to simultaneously reproduce the observed Ly, forest opacity at 4 < z < 6, the ionizing photon mean-free-path at z, 4 and the rapid evolution of highly ionized near-zone sizes around high-redshift quasars at 5.8 < z < 6.4. In our model, reionization extends over a wide redshift range, starting at z, 10 and completing as H ii regions overlap at z, 6,7. We find that within 5 physical Mpc of a high-redshift quasar, the evolution of the ionization state of the IGM precedes that in more typical regions by around 0.3 redshift units. More importantly, when combined with the rapid increase in the ionizing photon mean-free-path expected shortly after overlap, this offset results in an ionizing background near the quasar which exceeds the value in the rest of the IGM by a factor of ,2,3. We further find that in the post-overlap phase of reionization the size of the observed quasar near-zones is not directly sensitive to the neutral hydrogen fraction of the IGM. Instead, these sizes probe the level of the background ionization rate and the temperature of the surrounding IGM. The observed rapid evolution of the quasar near-zone sizes at 5.8 < z < 6.4 can thus be explained by the rapid evolution of the ionizing background, which in our model is caused by the completion of overlap at the end of reionization by 6 ,z, 7. [source] Environments of z > 5 quasars: searching for protoclusters at submillimetre wavelengthsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008R. S. Priddey ABSTRACT We present submillimetre (submm) continuum images of the fields of three luminous quasars at z > 5, obtained at 850 and 450 ,m using the Submillimetre Common-User Bolometer Array on the James Clerk Maxwell Telescope (JCMT). N -body simulations predict that such quasars evolve to become the central dominant galaxies of massive clusters at z= 0, but at z= 5,6 they are actively forming stars and surrounded by a rich protofilamentary structure of young galaxies. Our purpose in taking these images was to search for other luminous, star-forming galaxies in the vicinity of the signpost active galactic nuclei and thus associated with such a protocluster. Two of the quasar host galaxies are luminous submm galaxies (SMGs) in their own right, implying star formation rates ,103 M, yr,1. Despite the coarse 850-,m beam of the JCMT, our images show evidence of extended emission on a scale of ,100 kpc from at least one quasar , indicative of a partially resolved merger or a colossal host galaxy. In addition, at >3, significance we detect 12 (5) SMGs at 850 ,m (450 ,m) in the surrounding fields. Number counts of these SMGs are comparable with those detected in the fields of z, 4 radio galaxies, and both samples are, at the bright end, overabundant by a factor of ,4 relative to blank-field submm surveys. Whilst the redshift-sensitive 850 ,m/450 ,m and 850 ,m/1.4 GHz flux density ratios indicate that some of these SMGs are likely foreground objects, the counts suggest that ,60 per cent lie in the same large-scale structures as the quasars. [source] A homogeneous sample of sub-damped Lyman systems , IV.MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2007Global metallicity evolution ABSTRACT An accurate method to measure the abundance of high-redshift galaxies involves the observation of absorbers along the line of sight towards a background quasar. Here, we present abundance measurements of 13 z, 3 sub-damped Lyman , (sub-DLA) systems (quasar absorbers with H i column density in the range 19 < log N(H i) < 20.3 cm,2) based on high-resolution observations with the VLT UVES spectrograph. These observations more than double the amount of metallicity information for sub-DLAs available at z > 3. These new data, combined with other sub-DLA measurements from the literature, confirm the stronger evolution of metallicity with redshift for sub-DLAs than for the classical damped Lyman , absorbers. In addition, these observations are used to compute for the first time, using photoionization modelling in a sample of sub-DLAs, the fraction of gas that is ionized. Based on these results, we calculate that sub-DLAs contribute no more than 6 per cent of the expected amount of metals at z, 2.5. We therefore conclude that, even if sub-DLAs are found to be more metal-rich than classical DLAs, their contribution is insufficient to solve the so-called ,missing-metals' problem. [source] 3C 254: MERLIN observations of a highly asymmetric quasarMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006P. Thomasson ABSTRACT Multifrequency, high-resolution radio observations of the quasar 3C 254 using the Multi-Element Radio-linked Interferometer Network (MERLIN) are presented. The quasar has a highly asymmetric radio structure, with the eastern component of the double-lobed structure being much closer to the nucleus and significantly less polarized than the western one. However, the two lobes are more symmetric in their total flux densities. The observations show the detailed structure of the hotspots which are very different on opposite sides of the radio core, reveal no radio jet and suggest that the oppositely directed jets may be intrinsically asymmetric. [source] Simulations of strong gravitational lensing with substructureMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006Adam Amara ABSTRACT Galactic-sized gravitational lenses are simulated by combining a cosmological N -body simulation and models for the baryonic component of the galaxy. The lens caustics, critical curves, image locations and magnification ratios are calculated by ray shooting on an adaptive grid. When the source is near a cusp in a smooth lens' caustic, the sum of the magnifications of the three closest images should be close to zero. It is found that in the observed cases this sum is generally too large to be consistent with the simulations, implying that there is not enough substructure in the simulations. This suggests that other factors play an important role. These may include limited numerical resolution, lensing by structure outside the halo, selection bias and the possibility that a randomly selected galaxy halo may be more irregular, for example, due to recent mergers, than the isolated halo used in this study. It is also shown that, with the level of substructure computed from the N -body simulations, the image magnifications of the Einstein cross-type lenses are very weak functions of source size up to 1 kpc. This is also true for the magnification ratios of widely separated images in the fold and cusp,caustic lenses. This means that selected magnification ratios for the different emission regions of a lensed quasar should agree with each other, barring microlensing by stars. The source size dependence of the magnification ratio between the closest pair of images is more sensitive to substructure. [source] A new sample of broad absorption-line quasars exhibiting the ghost of Lyman ,MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006Matthew North ABSTRACT We have searched the broad absorption-line quasar (BAL QSO) sample presented recently by Reichard et al. for objects exhibiting the so-called ,ghost of Lyman ,'. This ghost manifests as a hump near ,5900 km s,1 in the troughs of the broad absorption lines and provides strong evidence for the importance of line driving in powering the outflows from BAL QSOs. Of the 224 sample BAL QSOs selected from the Sloan Digital Sky Survey Early Data Release, 198 satisfy our redshift constraints and 58 show clear evidence of multiple-trough (MT) structure in the C iv 1550 Ĺ line. A composite spectrum constructed from this MT sample already shows evidence for a ghost feature. Narrowing our classification scheme further, we define a set of 36 objects that individually show evidence of a ghost feature, and then apply further cuts to arrive at a final ,best sample' that contains our seven strongest ghost candidates. A further five objects show evidence for a ghost feature that is almost strong enough to merit inclusion in our best sample. Despite its limited size, our best sample more than doubles the number of known BAL QSOs with clear ghost signatures and should make an excellent basis for detailed follow-up studies. [source] XMM,Newton observations of warm absorbers in PG quasarsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2004C. E. Ashton ABSTRACT We present XMM,Newton European Photon Imaging Camera observations of warm absorbers in the quasars PG 1114+445 and PG 1309+355, both of which exhibit evidence for absorption by warm material in the line of sight. We find the absorption in PG 1114+445 to be in two phases, a ,hot' phase with a log ionization parameter, ,, of 2.57, and a ,cooler' phase with log ,= 0.83; an unresolved transition array (UTA) of M-shell iron is observed in the cooler phase. The absorption in this quasar is similar to that observed in the Seyfert 1 NGC 3783. The absorption in PG 1309+355 consists of a single phase, with log ,= 1.87. The absorbing gas lies at distances of 1018,1022 cm from the continuum radiation sources in these active galactic nuclei (AGN), which suggests that it could originate in a wind emanating from a molecular torus. We derive distances assuming that these X-ray warm absorbers have the same velocity as the ultraviolet warm absorbers observed in these quasars. The distances to the warm absorbers from the central continuum source scale approximately with the square root of the AGN ionizing luminosity, a result consistent with the warm absorber originating as a torus wind. The kinetic luminosities of these outflowing absorbers represent insignificant fractions (<10,3) of the energy budgets of the AGN. [source] GLITP optical monitoring of QSO 0957+561: VR light curves and variabilityMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2003A. Ullán ABSTRACT The Gravitational Lenses International Time Project (GLITP) collaboration observed the first gravitational lens system (QSO 0957+561) from 2000 February 3 to March 31. The daily VR observations were made with the 2.56-m Nordic Optical Telescope at Roque de los Muchachos Observatory, La Palma, Spain. We have derived detailed and robust VR light curves of the two components Q0957+561A and Q0957+561B. In spite of the excellent sampling rate, we have not found evidence in favour of true daily variability. With respect to variability on time-scales of several weeks, we measure VR gradients of about ,0.8 mmag d,1 in Q0957+561A and +0.3 mmag d,1 in Q0957+561B. The gradients are very probably originated in the far source. Thus, adopting this reasonable hypothesis (intrinsic variability), we compare them to the expected gradients during the evolution of a compact supernova remnant at the redshift of the source quasar. The starburst scenario is roughly consistent with some former events, but the new gradients do not seem to be caused by supernova remnant activity. [source] Extended X-ray emission in the high-redshift quasar GB 1508+5714 at z= 4.3MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2003W. Yuan ABSTRACT We report the discovery of extended X-ray emission around the powerful high-redshift quasar GB 1508+5714 at z= 4.3, revealed in a long Chandra ACIS observation. The emission feature is 3,4 arcsec away from the quasar core, which corresponds to a projected distance of about 25 kpc. The X-ray spectrum is best fitted with a power law of photon index 1.92 ± 0.35 (90 per cent confidence limit). The X-ray flux and luminosity reach 9.2 × 10,15 erg cm,2 s,1 (0.5,8 keV) and 1.6 × 1045 erg s,1 (2.7,42.4 keV rest frame, ,,= 0.73, ,m= 0.27, H0= 71 km s,1 Mpc,1), which is about 2 per cent of the total X-ray emission of the quasar. We interpret the X-ray emission as inverse Compton scattering of cosmic microwave background photons. The scattering relativistic electron population could either be a quasi-static diffuse cloud fed by the jet, or an outer extension of the jet with a high bulk Lorentz factor. We argue that the lack of an obvious detection of radio emission from the extended component could be a consequence of Compton losses on the electron population, or of a low magnetic field. Extended X-ray emission produced by inverse Compton scattering may be common around high-redshift radio galaxies and quasars, demonstrating that significant power is injected into their surroundings by powerful jets. [source] The cluster environments of radio-loud quasars at 0.6 < z < 1.1MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2003J. M. Barr ABSTRACT We have carried out multicolour imaging of a complete sample of radio-loud quasars at 0.6 < z < 1.1 and find groups or clusters of galaxies in the fields of at least eight and possibly 13 of the 21 sources. There is no evidence for an evolution in the richness of the environments of radio-loud quasars from other low-redshift studies to z, 0.9. The quasars associated with groups and clusters in our sample do not necessarily reside in the centre of the galaxy distribution, which rarely displays a spherical geometry. Clustering is preferentially associated with small or asymmetric steep-spectrum radio sources. The quasars with the largest projected angular size are, in nearly all cases, found in non-clustered environments. Radio-based selection (including source size) of high-redshift groups and clusters can be a very efficient method of detecting rich environments at these redshifts. We find that in optical searches for galaxy overdensities above z, 0.6, multiple filters must be used. If the single-filter counting statistics used by groups at lower redshift are applied to our data, uncertainties are too large to make accurate quantifications of cluster richness. This means that genuine clustering of galaxies about quasars will be missed and, in ,10 per cent of cases, putative clusters turn out to be false detections. The statistics are further diluted by the fact that galaxy overdensities are generally not centred on the quasar. [source] A search for high-redshift molecular absorption lines towards millimetre-loud, optically faint quasarsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2003M. T. Murphy ABSTRACT We describe initial results of a search for redshifted molecular absorption towards four millimetre-loud, optically faint quasars. A wide frequency bandwidth of up to 23 GHz per quasar was scanned using the Swedish,ESO Submillimetre Telescope at La Silla. Using a search list of commonly detected molecules, we obtained nearly complete redshift coverage up to zabs= 5. The sensitivity of our data is adequate to have revealed absorption systems with characteristics similar to those seen in the four known redshifted millimetre-band absorption systems, but none were found. Our frequency-scan technique nevertheless demonstrates the value of wide-band correlator instruments for searches such as these. We suggest that a somewhat larger sample of similar observations should lead to the discovery of new millimetre-band absorption systems. [source] Sunyaev,Zel'dovich effect from quasar-driven blast wavesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2002P. Platania Abstract Quasar-driven winds are currently the best candidates for accounting for the pre-heating of the intergalactic medium in clusters. Such winds, occurring during early phases of the evolution of spheroidal galaxies, shock-heat the interstellar gas, thus inducing a detectable Sunyaev,Zel'dovich effect. We estimate the amplitude and the angular scale of such an effect as well as its counts as a function of the Comptonization parameter y. The contamination arising from radio emission by the quasar itself is also discussed. The corresponding mean Compton distortion of the cosmic microwave background spectrum is found to be well below the COBE/FIRAS upper limit. [source] Possible evidence for a variable fine-structure constant from QSO absorption lines: systematic errorsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2001M.T. Murphy Comparison of quasar (QSO) absorption spectra with laboratory spectra allows us to probe possible variations in the fundamental constants over cosmological time-scales. In a companion paper we present an analysis of Keck/HIRES spectra and report possible evidence suggesting that the fine-structure constant, ,, may have been smaller in the past: over the redshift range . In this paper we describe a comprehensive investigation into possible systematic effects. Most of these do not significantly influence our results. When we correct for those which do produce a significant systematic effect in the data, the deviation of from zero becomes more significant. We are led increasingly to the interpretation that , was slightly smaller in the past. [source] Further constraints on variation of the fine-structure constant from alkali-doublet QSO absorption linesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2001M.T. Murphy Comparison of quasar (QSO) absorption-line spectra with laboratory spectra provides a precise probe for variability of the fine-structure constant, ,, over cosmological time-scales. We constrain variation in , in 21 Keck/HIRES Si iv absorption systems using the alkali-doublet (AD) method in which changes in , are related to changes in the doublet spacing. The precision obtained with the AD method has been increased by a factor of 3: . We also analyse potential systematic errors in this result. Finally, we compare the AD method with the many-multiplet method, which has achieved an order of magnitude greater precision, and we discuss the future of the AD method. [source] SCUBA observations of Hawaii 167MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2000Geraint F. Lewis We present the first submillimetre observations of the z=2.36 broad absorption line system Hawaii 167. Our observations confirm the hypothesis that Hawaii 167 contains a massive quantity of dust, the optical depth of which is sufficient to extinguish completely our ultraviolet view of a central, buried quasar. The submillimetre luminosity and associated dust mass of Hawaii 167 are similar to those of the ultraluminous class of infrared galaxies, supporting the existence of an evolutionary link between these and the active galaxy population. Hawaii 167 appears to be a young quasar that is emerging from its dusty cocoon. [source] The merger-driven evolution of warm infrared luminous galaxiesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2009Joshua D. Younger ABSTRACT We present a merger-driven evolutionary model for the production of luminous (LIRGs) and ultraluminous infrared galaxies (ULIRGs) with warm infrared (IR) colours. Our results show that simulations of gas-rich major mergers including star formation, black hole growth and feedback can produce warm (U)LIRGs. We also find that while the warm evolutionary phase is associated with increased active galactic nucleus (AGN) activity, star formation alone may be sufficient to produce warm IR colours. However, the transition can be suppressed entirely , even when there is a significant AGN contribution , when we assume a single-phase interstellar medium, which maximizes the attenuation. Finally, our evolutionary models are consistent with the 25-to-60 flux density ratio versus LHX/LIR relation for local LIRGs and ULIRGs, and predict the observed scatter in IR colour at fixed LHX/LIR. Therefore, our models suggest a cautionary note in the interpretation of warm IR colours: while associated with periods of active black hole growth, they are probably produced by a complex mix of star formation and AGN activity intermediate between the cold star formation dominated phase and the birth of a bright, unobscured quasar. [source] Parsec-scale jet in the distant gigahertz-peaked spectrum quasar PKS 0858,279ASTRONOMISCHE NACHRICHTEN, Issue 2-3 2009Y. Y. KovalevArticle first published online: 13 FEB 200 Abstract The high redshift GPS quasar PKS 0858,279 exhibits the following properties which make the source unusual. Our RATAN-600 monitoring of 1,22 GHz spectrum has detected broad-band radio variability with high amplitude and relatively short time scale. In the same time, the milliarcsecond scale structure observed in a snapshot VLBA survey turned out to be very resolved which is not expected from the fast flux density variations. We performed 1.4,22 GHz VLBA observations of this quasar in 2005,2007. It has revealed a core-jet morphology. A high Doppler factor , is suggested for the jet, its nature is discussed in this report on the basis of the multi-frequency VLBA and RATAN data collected. Synchrotron self-absorption was confirmed to be dominating at low frequencies, the magnetic field strength of the dominating jet feature is estimated of an order of 0.1, mG (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Radio bimodality: Spin, accretion mode, or both?ASTRONOMISCHE NACHRICHTEN, Issue 2-3 2009M. Sikora Abstract A new scenario is suggested to explain a large diversity of the AGN radio properties and their dependence on the galaxy morphology. The scenario is based on the assumption that the growth of supermassive BHs is dominated by the accretion only during the quasar (high accretion rate) phase, otherwise , by mergers with less massive black holes. Following that, BHs are expected to spin much faster in giant ellipticals than in disk galaxies. Within the frame of the spin paradigm this explains the observed relation of the radio-dichotomy with the galaxy morphology. Various theoretical and observational aspects of such a dichotomy are discussed. In particular, the issue of the intermittency and suppression of a jet production at high accretion rates is addressed and a scenario for production of powerful, extended radio sources is drafted (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Bang: and there goes a quasarASTRONOMY & GEOPHYSICS, Issue 4 2010Article first published online: 23 JUL 2010 What makes a quasar glow? Colliding galaxies, according to an international team of astronomers scanning a quasar sideways-on. They have found that a collision between galaxies seems to fire up a quasar, driving intense activity from the central engine. [source] Modification of the gamma-ray spectra by internal absorption in optically violently variable blazars: the example cases of 3C 273 and 3C 279MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2008J. Sitarek ABSTRACT Recent observations with the low-threshold Cherenkov telescopes proved that sub-TeV ,-rays are able to arrive from active galaxies at relatively large distances in spite of the expected severe absorption in the extragalactic background light (EBL). We calculate the ,-ray spectra at TeV energies from two example optically violently variable quasars, 3C 273 and 3C 279, assuming that ,-rays are injected in the inner parts of the jets launched by the accretion discs. It is assumed that ,-rays in the broad energy range (from MeV up to TeV) are produced in these blazars with a power-law spectrum with the spectral index as observed from these objects by the EGRET telescope at GeV energies. We take into account the internal absorption of these ,-rays by considering a number of models for the radiation field surrounding the jet. The classical picture of a relativistic blob in a jet for the injection of primary ,-rays is considered, with the injection rate of ,-rays as observed by the EGRET telescope in the GeV energy range. The results of calculations are compared with positive detection and the upper limits on the sub-TeV ,-ray fluxes from these two sources. It is concluded that, even with the Stecker EBL model, the level of ,-ray emission from 3C 279 is close to the recent measurements in the sub-TeV ,-ray energies, provided that the injected ,-ray spectrum extends from the GeV energies over the next two decades with this same spectral index. We also suggest that a flare with a time-scale of a few days from 3C 273 could be detected by the MAGIC II stereo telescopes. [source] A survey for redshifted molecular and atomic absorption lines , II.MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 20083 Parkes quarter-Jansky flat-spectrum sample, Associated H i, millimetre lines in the z ABSTRACT We present the results of a z, 2.9 survey for H i 21-cm and molecular absorption in the hosts of radio quasars using the Giant Metrewave Radio Telescope and the Tidbinbilla 70-m telescope. Although the atomic gas has been searched to limits capable of detecting most known absorption systems, no H i was detected in any of the 10 sources. Previously published searches, which are overwhelmingly at redshifts of z, 1, exhibit a 42 per cent detection rate (31 out of 73 sources), whereas the inclusion of our survey yields a 17 per cent detection rate (two out of 12 sources) at z > 2.5. We therefore believe that our high-redshift selection is responsible for our exclusive non-detections, and find that at ultraviolet (UV) luminosities of LUV, 1023 W Hz,1, 21-cm absorption has never been detected. We also find this to not only apply to our targets, but also those at low redshift exhibiting similar luminosities, giving zero detections out of a total of 16 sources over z= 0.24 to 3.8. This is in contrast to the LUV, 1023 W Hz,1 sources where there is a near 50 per cent detection rate of 21-cm absorption. The mix of 21-cm detections and non-detections is currently attributed to orientation effects, where according to unified schemes of active galactic nuclei, 21-cm absorption is more likely to occur in sources designated as radio galaxies (type 2 objects, where the nucleus is viewed through dense obscuring circumnuclear gas) than in quasars (type 1 objects, where we have a direct view to the nucleus). However, due to the exclusively high UV luminosities of our targets it is not clear whether orientation effects alone can wholly account for the distribution, although there exists the possibility that the large luminosities are indicative of a changing demographic of galaxy types. We also find that below luminosities of LUV, 1023 W Hz,1, both type 1 and type 2 objects have a 50 per cent likelihood of exhibiting 21-cm absorption. Finally, we do not detect molecular gas in any of the sources. The lack of H i absorption, combined with the results from Paper I, suggests these sources are not conducive to high molecular abundances. [source] Redshifted 21-cm observations of high-redshift quasar proximity zonesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008J. Stuart B. Wyithe ABSTRACT The introduction of low-frequency radio arrays is expected to revolutionize the study of the reionization epoch. Observation of the contrast in redshifted 21-cm emission between a large H ii region and the surrounding neutral intergalactic medium (IGM) will be the simplest and most easily interpreted signature. However, the highest redshift quasars known are thought to reside in an ionized IGM. Using a semi-analytic model we describe the redshifted 21-cm signal from the IGM surrounding quasars discovered using the i -drop-out technique (i.e. quasars at z, 6). We argue that while quasars at z < 6.5 seem to reside in the post-overlap IGM, they will still provide valuable probes of the late stages of the overlap era because the light-travel time across a quasar proximity zone should be comparable to the duration of overlap. For redshifted 21-cm observations within a 32-MHz bandpass, we find that the subtraction of a spectrally smooth foreground will not remove spectral features due to the proximity zone. These features could be used to measure the neutral hydrogen content of the IGM during the late stages of reionization. The density of quasars at z, 6 is now well constrained. We use the measured quasar luminosity function to estimate the prospects for discovery of high-redshift quasars in fields that will be observed by the Murchison Widefield Array. [source] Microlensing by cosmic stringsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008Konrad Kuijken ABSTRACT We consider the signature and detectability of gravitational microlensing of distant quasars by cosmic strings. Because of the simple image configuration such events will have a characteristic lightcurve, in which a source would appear to brighten by exactly a factor of 2, before reverting to its original apparent brightness. We calculate the optical depth and event rate, and conclude that current predictions and limits on the total length of strings on the sky imply optical depths of , 10,8 and event rates of fewer than one event per 109 sources per year. Disregarding those predictions but replacing them with limits on the density of cosmic strings from the cosmic microwave background fluctuation spectrum, leaves only a small region of parameter space (in which the sky contains about 3 × 105 strings with deficit angle of the order of 0.3 milli-seconds) for which a microlensing survey of exposure 107 source years, spanning a 20,40-year period, might reveal the presence of cosmic strings. [source] On the variability of quasars: a link between the Eddington ratio and optical variability?MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2008Brian C. Wilhite ABSTRACT Repeat scans by the Sloan Digital Sky Survey (SDSS) of a 278-deg2 stripe along the celestial equator have yielded an average of over 10 observations each for nearly 8000 spectroscopically confirmed quasars. Over 2500 of these quasars are in the redshift range such that the C iv, 1549 emission line is visible in the SDSS spectrum. Utilizing the width of these C iv lines and the luminosity of the nearby continuum, we estimate black hole masses for these objects. In an effort to isolate the effects of black hole mass and luminosity on the photometric variability of our data set, we create several subsamples by binning in these two physical parameters. By comparing the ensemble structure functions of the quasars in these bins, we are able to reproduce the well-known anticorrelation between luminosity and variability, now showing that this anticorrelation is independent of the black hole mass. In addition, we find a correlation between variability and the mass of the central black hole. By combining these two relations, we identify the Eddington ratio as a possible driver of quasar variability, most likely due to differences in accretion efficiency. [source] Reionization bias in high-redshift quasar near-zonesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2008J. Stuart B. Wyithe ABSTRACT Absorption spectra of high-redshift quasars exhibit an increasingly thick Ly, forest, suggesting that the fraction of neutral hydrogen in the intergalactic medium (IGM) is increasing towards z, 6. However, the interpretation of these spectra is complicated by the fact that the Ly, optical depth is already large for neutral hydrogen fractions in excess of 10,4, and also because quasars are expected to reside in dense regions of the IGM. We present a model for the evolution of the ionization state of the IGM which is applicable to the dense, biased regions around high-redshift quasars as well as more typical regions in the IGM. We employ a cold dark matter based model in which the ionizing photons for reionization are produced by star formation in dark matter haloes spanning a wide range of masses, combined with numerical radiative transfer simulations which model the resulting opacity distribution in quasar absorption spectra. With an appropriate choice for the parameter which controls the star formation efficiency, our model is able to simultaneously reproduce the observed Ly, forest opacity at 4 < z < 6, the ionizing photon mean-free-path at z, 4 and the rapid evolution of highly ionized near-zone sizes around high-redshift quasars at 5.8 < z < 6.4. In our model, reionization extends over a wide redshift range, starting at z, 10 and completing as H ii regions overlap at z, 6,7. We find that within 5 physical Mpc of a high-redshift quasar, the evolution of the ionization state of the IGM precedes that in more typical regions by around 0.3 redshift units. More importantly, when combined with the rapid increase in the ionizing photon mean-free-path expected shortly after overlap, this offset results in an ionizing background near the quasar which exceeds the value in the rest of the IGM by a factor of ,2,3. We further find that in the post-overlap phase of reionization the size of the observed quasar near-zones is not directly sensitive to the neutral hydrogen fraction of the IGM. Instead, these sizes probe the level of the background ionization rate and the temperature of the surrounding IGM. The observed rapid evolution of the quasar near-zone sizes at 5.8 < z < 6.4 can thus be explained by the rapid evolution of the ionizing background, which in our model is caused by the completion of overlap at the end of reionization by 6 ,z, 7. [source] Deep spectroscopy of the FUV,optical emission lines from a sample of radio galaxies at z, 2.5: metallicity and ionization,MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008A. Humphrey ABSTRACT We present long-slit near-infrared (NIR) spectra, obtained using the Infrared Spectrometer And Array Camera (ISAAC) instrument at the Very Large Telescope, which sample the rest-frame optical emission lines from nine radio galaxies at z, 2.5. One-dimensional spectra have been extracted and, using broad-band photometry, have been cross-calibrated with spectra from the literature to produce line spectra spanning a rest wavelength of ,1200,7000 Ĺ. The resulting line spectra have a spectral coverage that is unprecedented for radio galaxies at any redshift. We have also produced a composite of the rest-frame ultraviolet (UV),optical line fluxes of powerful, z, 2.5 radio galaxies. We have investigated the relative strengths of Ly,, H,, H,, He ii,1640 and He ii,4687, and we find that Av can vary significantly from object to object. In addition, we have identified new line ratios to calculate electron temperature: [Ne v],1575/[Ne v],3426, [Ne iv],1602/[Ne iv],2423, O iii],1663/[O iii],5008 and [O ii],2471/[O ii],3728. We calculate an average O iii temperature of 14100+1000,600 K. We have modelled the rich emission line spectra, and we conclude that they are best explained by active galactic nucleus (AGN) photoionization with the ionization parameter U varying between objects. For shock models (with or without the precursor) to provide a satisfactory explanation for the data, an additional source of ionizing photons is required , presumably the ionizing radiation field of the AGN. Single slab photoionization models are unable to reproduce the high- and the low-ionization lines simultaneously: the higher ionization lines imply higher U than do the lower ionization lines. This problem may be alleviated either by combining two or more single slab photoionization models with different U, or by using mixed-medium models such as those of Binette, Wilson & Storchi-Bergmann. In either case, U must vary from object to object. On the basis of N v/N iv] and N iv]/C iv we argue that, while photoionization is the dominant ionization mechanism in the extended emission line regions (EELR), shocks make a fractional contribution (,10 per cent) to its ionization. The N v/N iv] and N iv]/C iv ratios in the broad-line region (BLR) of some quasars suggest that shock ionization may be important in the BLR also. We find that in the EELR of z, 2 radio galaxies the N/H abundance ratio is close to its solar value. We conclude that N/H and metallicity do not vary by more than a factor of 2 in our sample. These results are consistent with the idea that the massive ellipticals which become the hosts to powerful AGN are assembled very early in the history of the universe, and then evolve relatively passively up to the present day. [source] Environments of z > 5 quasars: searching for protoclusters at submillimetre wavelengthsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008R. S. Priddey ABSTRACT We present submillimetre (submm) continuum images of the fields of three luminous quasars at z > 5, obtained at 850 and 450 ,m using the Submillimetre Common-User Bolometer Array on the James Clerk Maxwell Telescope (JCMT). N -body simulations predict that such quasars evolve to become the central dominant galaxies of massive clusters at z= 0, but at z= 5,6 they are actively forming stars and surrounded by a rich protofilamentary structure of young galaxies. Our purpose in taking these images was to search for other luminous, star-forming galaxies in the vicinity of the signpost active galactic nuclei and thus associated with such a protocluster. Two of the quasar host galaxies are luminous submm galaxies (SMGs) in their own right, implying star formation rates ,103 M, yr,1. Despite the coarse 850-,m beam of the JCMT, our images show evidence of extended emission on a scale of ,100 kpc from at least one quasar , indicative of a partially resolved merger or a colossal host galaxy. In addition, at >3, significance we detect 12 (5) SMGs at 850 ,m (450 ,m) in the surrounding fields. Number counts of these SMGs are comparable with those detected in the fields of z, 4 radio galaxies, and both samples are, at the bright end, overabundant by a factor of ,4 relative to blank-field submm surveys. Whilst the redshift-sensitive 850 ,m/450 ,m and 850 ,m/1.4 GHz flux density ratios indicate that some of these SMGs are likely foreground objects, the counts suggest that ,60 per cent lie in the same large-scale structures as the quasars. [source] Black hole growth in hierarchical galaxy formationMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007Rowena K. Malbon ABSTRACT We incorporate a model for black hole growth during galaxy mergers into the semi-analytical galaxy formation model based on ,CDM proposed by Baugh et al. Our black hole model has one free parameter, which we set by matching the observed zero-point of the local correlation between black hole mass and bulge luminosity. We present predictions for the evolution with redshift of the relationships between black hole mass and bulge properties. Our simulations reproduce the evolution of the optical luminosity function of quasars. We study the demographics of the black hole population and address the issue of how black holes acquire their mass. We find that the direct accretion of cold gas during starbursts is an important growth mechanism for lower mass black holes and at high redshift. On the other hand, the re-assembly of pre-existing black hole mass into larger units via merging dominates the growth of more massive black holes at low redshift. This prediction could be tested by future gravitational wave experiments. As redshift decreases, progressively less massive black holes have the highest fractional growth rates, in line with recent claims of ,downsizing' in quasar activity. [source] | ||||||||||