Radio Galaxies (radio + galaxy)

Distribution by Scientific Domains

Kinds of Radio Galaxies

  • high-redshift radio galaxy
  • young radio galaxy

  • Selected Abstracts

    The Granada workshop on High Redshift Radio Galaxies: An overview

    H. J. A. Röttgering
    Abstract The Granada workshop on High Redshift Radio Galaxies (HzRGs) gave an excellent overview of the progress that has been made in this field during the last 3 years. Here we briefly review some of the results, with an emphasis on what studies of HzRGs can teach us about the formation and evolution of massive galaxies, clusters and active galactic nuclei (AGN). Of great relevance for this workshop are scenarios that describe certain aspects of the evolution of radio galaxies, including (i) the sequence of events after merging of galaxies that ultimately lead to extended powerful radio sources and (ii) the mass assembly and virialization of the hosting massive galaxies and their associated (proto-)clusters. Furthermore, I briefly discuss two projects that are important for a further understanding of AGN and high redshift radio galaxies. First, using the MIDI instrument mounted on the VLT Interferometer, the dusty tori of nearby AGN can be studied in the range of 8,13 micron at high angular resolution. The first result on the nearby AGN NGC 1068 as presented by Jaffe et al. (2004) indicated the presence of a hot (T > 800 K), compact (,1 pc) component, possible identified with the base of the jet and a warm (270 K), well-resolved (3 × 4 pc) component associated with the alleged torus. Second, LOFAR is a new low frequency radio telescope that is currently being build in the Netherlands and is expected to be operational in 2008. With 50 stations spread over an area of 100 km in diameter, its resolution and sensitivity will be unprecedented in the frequency range 10,240 MHz. LOFAR will be a unique instrument that will impact a broad range of astrophysical topics varying from the epoch of reionisation, to gamma ray bursts and cosmic rays. Surveys with LOFAR will be of paramount importance for studies of HzRGs: It will enable (i) defining samples of radio galaxies with redshifts higher than 6, (ii) observations of starbursting galaxies in proto-clusters, and (iii) mapping out the low-frequency radio emission of virtually all northern radio-loud AGN in revolutionary detail. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Radio galaxies and type-2 quasars in the Spitzer Extragalactic First Look Survey

    M. Lacy
    Abstract We use mid-infrared colour-colour plots made with data from the Spitzer First Look Survey to explore the emission mechanisms of faint radio sources in that field. Using mid-infrared colours alone, we are able to see the transition from a starburst-dominated radio source population at sub-mJy levels to the radiogalaxy/quasar population at mJy levels. Both FR I and some FR II galaxies show little or no mid-infrared excess, suggesting they either lack dusty tori, or a strong continuum source to heat them, or both. Objects with hot, dusty tori, most likely radio-intermediate and radio-quiet AGN, make up a significant fraction of the faint radio source population. We also discuss the use of mid-infrared colour-colour plots to select radio quiet obscured AGN. Early results from imaging the host galaxies of these type-2 quasars at high spatial resolution with adaptive optics indicates that they, like type-1 quasars and radio galaxies, are often found to be hosted by giant elliptical galaxies. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    X-ray groups and clusters of galaxies in the Subaru,XMM Deep Field

    A. Finoguenov
    Abstract We present the results of a search for galaxy clusters in the Subaru,XMM Deep Field (SXDF). We reach a depth for a total cluster flux in the 0.5,2 keV band of 2 × 10,15 erg cm,2 s,1 over one of the widest XMM,Newton contiguous raster surveys, covering an area of 1.3 deg2. Cluster candidates are identified through a wavelet detection of extended X-ray emission. The red-sequence technique allows us to identify 57 cluster candidates. We report on the progress with the cluster spectroscopic follow-up and derive their properties based on the X-ray luminosity and cluster scaling relations. In addition, three sources are identified as X-ray counterparts of radio lobes, and in three further sources, an X-ray counterpart of the radio lobes provides a significant fraction of the total flux of the source. In the area covered by near-infrared data, our identification success rate achieves 86 per cent. We detect a number of radio galaxies within our groups, and for a luminosity-limited sample of radio galaxies we compute halo occupation statistics using a marked cluster mass function. We compare the cluster detection statistics in the SXDF with that in the literature and provide the modelling using the concordance cosmology combined with current knowledge of the X-ray cluster properties. The joint cluster log(N) , log(S) is overpredicted by the model, and an agreement can be achieved through a reduction of the concordance ,8 value by 5 per cent. Having considered the dn/dz and the X-ray luminosity function of clusters, we conclude that to pin down the origin of disagreement a much wider (50 deg2) survey is needed. [source]

    The ionization of the emission-line gas in young radio galaxies

    J. Holt
    ABSTRACT This paper is the second in a series in which we present intermediate-resolution, wide-wavelength coverage spectra for a complete sample of 14 compact radio sources, taken with the aim of investigating the impact of the nuclear activity on the circumnuclear interstellar medium (ISM) in the early stages of radio source evolution. In the first paper (Holt, Tadhunter & Morganti), we presented the kinematic results from the nuclear emission-line modelling and reported fast outflows in the circumnuclear gas. In this paper, we use the line fluxes to investigate the physical conditions and dominant ionization mechanisms of the emission-line gas. We find evidence for large electron densities and high reddening in the nuclear regions, particularly in the broader, blueshifted components. These results are consistent with the idea that the young, recently triggered radio sources still reside in dense and dusty cocoons deposited by the recent activity triggering event (merger/interaction). In addition, we find that the quiescent nuclear and extended narrow components are consistent with active galactic nucleus (AGN) photoionization, split between simple-slab AGN photoionization and mixed-medium photoionization models. For the nuclear broader and shifted components, the results are less clear. Whilst there are suggestions that the broader components may be closer to shock plus precursor models on the diagnostic diagrams, and that the electron temperatures and densities are high, we are unable to unambiguously distinguish the dominant ionization mechanism using the optical emission-line ratios. This is surprising given the strong evidence for jet,cloud interactions (broad emission lines, large outflow velocities and strong radio-optical alignments), which favours the idea that the warm gas has been accelerated in shocks driven by the radio lobes expanding through a dense cocoon of gas deposited during the triggering event. [source]

    A survey for redshifted molecular and atomic absorption lines , II.

    3 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]

    Deep spectroscopy of the FUV,optical emission lines from a sample of radio galaxies at z, 2.5: metallicity and ionization,

    A. 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 wavelengths

    R. 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 sample of mJy radio sources at 1.4 GHz in the Lynx and Hercules fields , I. Radio imaging, multicolour photometry and spectroscopy

    E. E. Rigby
    ABSTRACT With the goal of identifying high-redshift radio galaxies with Fanaroff,Riley class I (FR I) classification, here are presented high-resolution, wide-field radio observations, near-infrared and optical imaging and multi-object spectroscopy of two fields of the Leiden,Berkeley Deep Survey. These fields, Hercules.1 and Lynx.2, contain a complete sample of 81 radio sources with S1.4 GHz > 0.5 mJy within 0.6 deg2. This sample will form the basis for a study of the population and cosmic evolution of high-redshift, low-power, FR I radio sources which will be presented in Paper II. Currently, the host galaxy identification fraction is 86 per cent with 11 sources remaining unidentified at a level of r,, 25.2 mag (Hercules; four sources) or r,, 24.4 mag (Lynx; seven sources) or K, 20 mag. Spectroscopic redshifts have been determined for 49 per cent of the sample and photometric redshift estimates are presented for the remainder of the sample. [source]

    VIMOS-VLT spectroscopy of the giant Ly, nebulae associated with three z, 2.5 radio galaxies,

    M. Villar-Martín
    ABSTRACT The morphological and spectroscopic properties of the giant (>60 kpc) Ly, nebulae associated with three radio galaxies at z, 2.5 (MRC 1558,003, 2025,218 and 0140,257) have been investigated using integral field spectroscopic data obtained with the Visible Multi-Object Spectrograph (VIMOS) on VLT. The morphologies are varied. The nebula of one source has a centrally peaked, rounded appearance. In the other two objects, it consists of two spatial components. The three nebulae are aligned with the radio axis within ,30°. The total Ly, luminosities are in the range (0.3,3.4) × 1044 erg s,1. The Ly, spectral profile shows strong variation through the nebulae, with full width at half-maximum (FWHM) values in the range ,400,1500 km s,1 and velocity shifts Voffset, 120,600 km s,1. We present an infall model that can successfully explain the morphology, size, surface brightness distribution and the velocity field of the Ly, nebula associated with MRC 1558,003. It can also explain why Ly, is redshifted relative to other emission lines and the FWHM values of the non-resonant He ii line. This adds further support to our previous conclusion that the quiescent giant nebulae associated with this and other high-redshift powerful radio galaxies are in infall. A problem for this model is the difficulty to reproduce the large Ly, FWHM values, which might be the consequence of a different mechanism. We have discovered a giant (,85 kpc) Ly, nebula associated with the radio galaxy MRC 0140,257 at z= 2.64. It shows strikingly relaxed kinematics (FWHM < 300 km s,1 and Voffset, 120 km s,1), unique among high- z (,2) radio galaxies. [source]

    Imaging and spectroscopy of ultrasteep spectrum radio sources,

    Carlos G. Bornancini
    ABSTRACT We present a sample of 40 ultrasteep spectrum (USS; ,,, 1.3, S,,,,) radio sources selected from the Westerbork in the Southern Hemisphere (WISH) catalogue. The USS sources have been imaged in K band at the Cerro Tololo Inter-American Observatory (CTIO) and with the Very Large Telescope (VLT) at Cerro Paranal. We also present VLT, Keck and William Herschel Telescope (WHT) optical spectroscopy of 14 targets selection from four different USS samples. For 12 sources, we have been able to determine the redshifts, including four new radio galaxies at z > 3. We find that most of our USS sources have predominantly small (<6 arcsec) radio sizes and faint magnitudes (K, 18). The mean K -band counterpart magnitude is . The expected redshift distribution estimated using the Hubble K,z diagram has a mean of , which is higher than the predicted redshift obtained for the Sydney University Molonglo Sky Survey,NRAO VLA Sky Survey (SUMSS,NVSS) sample and the expected redshift obtained in the 6C** survey. The compact USS sample analysed here may contain a higher fraction of galaxies which are high redshift and/or are heavily obscured by dust. Using the 74, 352 and 1400 MHz flux densities of a subsample, we construct a radio colour,colour diagram. We find that all but one of our USS sources have a strong tendency to flatten below 352 MHz. We also find that the highest redshift source from this paper (at z= 3.84) does not show evidence for spectral flattening down to 151 MHz. This suggests that very low frequency selected USS samples will likely be more efficient to find high redshift galaxies. [source]

    The first appearance of the red sequence of galaxies in proto-clusters at 2 ,z, 3

    Tadayuki Kodama
    ABSTRACT We explore the evolved galaxy population in the proto-clusters around four high- z radio galaxies at 2 ,z, 3 based on wide-field near-infrared (NIR) imaging. Three of the four fields are known proto-clusters as demonstrated by overdensities of line-emitting galaxies at the same redshifts as the radio galaxies found by narrow-band surveys and spectroscopic follow-up observations. We imaged the fields of three targets (PKS 1138,262, USS 0943,242 and MRC 0316,257) to a depth of Ks, 22 (Vega magnitude, 5,) over a 4 × 7 arcmin2 area centred on the radio galaxies with a new wide-field NIR camera, Multi-Object Infra-Red Camera and Spectrograph (MOIRCS), on the Subaru Telescope. Another target (USS 1558,003) was observed with Son of ISAAC on the New technology Telescope (NTT) to a depth of Ks= 20.5 (5,) over a 5 × 5 arcmin2 area. We apply colour cuts in J,Ks and/or JHKs in order to exclusively search for galaxies located at high redshifts: z > 2. To the 5, limiting magnitudes, we see a significant excess of NIR-selected galaxies by a factor of 2 to 3 compared to those found in the field of GOODS-South. The spatial distribution of these NIR-selected galaxies is not uniform and traces structures similar to those of emission-line galaxies, although the samples of NIR-selected galaxies and emitters show little overlap, from which we conclude that the former tend to be an evolved population with much higher stellar mass than the latter, young and active emitters. We focus on the NIR colour,magnitude sequence of the evolved population and find that the bright-end (Mstars > 1011 M,) of the red sequence is well populated by z, 2 but much less so in the z, 3 proto-clusters. This may imply that the bright-end of the colour,magnitude sequence first appeared between z= 3 and 2, an era coinciding with the appearance of sub-mm galaxies and the peak of the cosmic star formation rate. Our observations show that during the same epoch, massive galaxies are forming in high-density environments by vigorous star formation and assembly. [source]

    Extragalactic MeV ,-ray emission from cocoons of young radio galaxies

    M. Kino
    ABSTRACT Strong ,-ray emission from cocoons of young radio galaxies is predicted for the first time. Considering the process of adiabatic injection of the shock dissipation energy and mass of the relativistic jet in active nuclei into the cocoon, while assuming thermalizing electron plasma interactions, we find that the thermal electron temperature of the cocoon is typically predicted to be of the order of , MeV, and is determined only by the bulk Lorentz factor of the relativistic jet. Together with the time-dependent dynamics of the cocoon expansion, we find that young cocoons can yield thermal bremsstrahlung emissions at energies ,MeV. [source]

    Hot and cold gas accretion and feedback in radio-loud active galaxies

    M. J. Hardcastle
    ABSTRACT We have recently shown that X-ray observations of the population of ,low-excitation' radio galaxies, which includes most low-power, Fanaroff,Riley class I sources as well as some more powerful Fanaroff,Riley class II objects, are consistent with a model in which the active nuclei of these objects are not radiatively efficient at any waveband. In another recent paper, Allen et al. have shown that Bondi accretion of the hot, X-ray emitting phase of the intergalactic medium (IGM) is sufficient to power the jets of several nearby, low-power radio galaxies at the centres of clusters. In this paper, we combine these ideas and suggest that accretion of the hot phase of the IGM is sufficient to power all low-excitation radio sources, while high-excitation sources are powered by accretion of cold gas that is in general unrelated to the hot IGM. This model explains a number of properties of the radio-loud active galaxy population, and has important implications for the energy input of radio-loud active galactic nuclei into the hot phase of the IGM: the energy supply of powerful high-excitation sources does not have a direct connection to the hot phase. [source]

    Ly, excess in high-redshift radio galaxies: a signature of star formation,

    M. Villar-Martín
    ABSTRACT About 54 per cent of radio galaxies at z, 3 and 8 per cent of radio galaxies at 2 ,z < 3 show unusually strong Ly, emission, compared with the general population of high-redshift (z, 2) radio galaxies. These Ly,-excess objects (LAEs) show Ly,/He ii values consistent with or above standard photoionization model predictions. We reject with confidence several scenarios to explain the unusual strength of Ly, in these objects: shocks, low nebular metallicities, high gas densities and absorption/scattering effects. We show that the most successful explanation is the presence of a young stellar population which provides the extra supply of ionizing photons required to explain the Ly, excess in at least the most extreme LAEs (probably in all of them). This interpretation is strongly supported by the tentative trend found by other authors for z, 3 radio galaxies to show lower ultraviolet rest-frame polarization levels, or the dramatic increase in the detection rate at submm wavelengths of z > 2.5 radio galaxies. The enhanced star formation activity in LAEs could be a consequence of a recent merger which has triggered both the star formation and the active galactic nucleus/radio activities. The measurement of unusually high Ly, ratios in the extended gas of some high-redshift radio galaxies suggests that star formation activity occurs in spatial scales of tens of kpc. We argue that, although the fraction of LAEs may be incompletely determined, both at 2 ,z < 3 and at z, 3, the much larger fraction of LAEs found at z, 3 is a genuine redshift evolution and not due to selection effects. Therefore, our results suggest that the radio galaxy phenomenon is more often associated with a massive starburst at z > 3 than at z < 3. [source]

    Chemical enrichment of the intracluster medium by FR II radio sources

    D. Heath
    ABSTRACT We present 2D axisymmetric hydrodynamic simulations investigating the long-term effect of Fanaroff,Riley type II radio galaxies on the metal distribution of the surrounding intracluster medium (ICM). A light jet is injected into a cooling flow atmosphere for 10,30 Myr. We then follow the subsequent evolution for 3 Gyr on a spherical grid spanning 3 Mpc in radius. A series of passive tracer particles were placed in an annulus about the cluster core to simulate metal carrying clouds in order to calculate the metallicity (Z) as a function of time and radial distance from the cluster centre. The jet has a significant effect on the ICM over the entire 3-Gyr period. By the end of the simulations, the jets produced metallicities of ,10 per cent of the initial metallicity of the cluster core throughout much of the cluster. The jets transport the metals not only in mixing regions, but also through upwelling ICM behind the jet, enriching the cluster over both long and short distances. [source]

    The dust-eliminated shape of quasar spectra in the near-infrared: a hidden part of the big blue bump

    Makoto Kishimoto
    ABSTRACT The near-infrared shape of the big blue bump component in quasar spectra has till now been essentially unknown. It usually cannot be observed directly, due to the strong hot dust emission that dominates quasar spectra longward of ,1 ,m. However, this is quite an important part of the spectrum theoretically. At least bare-disc models provide quite a robust prediction for the overall continuum shape in the near-infrared. Self-gravity should become important in the outer, near-infrared-emitting regions of the putative disc, possibly leaving a signature of disc truncation in the near-infrared. We propose here that this important part of the spectrum can be revealed for the first time by observing polarized flux from normal quasars. At least in some polarized quasars, the emission lines are all unpolarized and so the polarized flux should originate interior to the broad-line region, and therefore also interior to the dust-emitting region. This can then be used to eliminate the dust emission. We present the results of near-infrared polarimetry for three such quasars (Ton202, 4C 37.43 and B2 1208+32). The data for Ton202 have the highest signal-to-noise ratio, and the near-infrared polarized flux in this case is measured to have quite a blue shape, F,,,+0.42±0.29, intriguingly consistent with the simple multitemperature blackbody, bare-disc prediction of ,+1/3. All these data, although still with quite low signal-to-noise ratio for the other two objects, demonstrate the unique potential of the technique with future better data. We also present similar data for other quasars and radio galaxies, and briefly discuss the nature of the polarization. [source]

    A transition in the accretion properties of radio-loud active nuclei

    Danilo Marchesini
    ABSTRACT We present evidence for the presence of a transition in the accretion properties of radio-loud sources. For a sample of radio galaxies and radio-loud quasars, selected based on their extended radio properties, the accretion rate is estimated from the black hole mass and nuclear luminosity. The inferred distribution is bimodal, with a paucity of sources at accretion rates, in Eddington units, of the order of ,10,2, assuming a radiative efficiency of 10 per cent , and possibly spanning 1,2 orders of magnitude. Selection biases are unlikely to be responsible for such behaviour. We discuss possible physical explanations, including a fast transition to low accretion rates, a change in the accretion mode/actual accretion rate/radiative efficiency, the lack of stable disc solutions at intermediate accretion rates or the inefficiency of the jet formation processes in geometrically thin flows. This transition might be analogous to spectral states (and jet) transitions in black hole binary systems. [source]

    A physical classification scheme for blazars

    Hermine Landt
    ABSTRACT Blazars are currently separated into BL Lacertae objects (BL Lacs) and flat spectrum radio quasars based on the strength of their emission lines. This is performed rather arbitrarily by defining a diagonal line in the Ca H&K break value,equivalent width plane, following Marchă et al. We readdress this problem and put the classification scheme for blazars on firm physical grounds. We study ,100 blazars and radio galaxies from the Deep X-ray Radio Blazar Survey (DXRBS) and 2-Jy radio survey and find a significant bimodality for the narrow emission line [O iii],5007. This suggests the presence of two physically distinct classes of radio-loud active galactic nuclei (AGN). We show that all radio-loud AGN, blazars and radio galaxies, can be effectively separated into weak- and strong-lined sources using the [O iii],5007,[O ii],3727 equivalent width plane. This plane allows one to disentangle orientation effects from intrinsic variations in radio-loud AGN. Based on DXRBS, the strongly beamed sources of the new class of weak-lined radio-loud AGN are made up of BL Lacs at the ,75 per cent level, whereas those of the strong-lined radio-loud AGN include mostly (,97 per cent) quasars. [source]

    A binary system of tailed radio galaxies

    I. Klamer
    ABSTRACT We present a detailed study of a binary system of tailed radio galaxies which, along with 3C 75, is the only such binary known to exist. The binary is located in a region of low galaxy density at the periphery of a poor cluster Abell S345, but lies close to the massive Horologium,Reticulum supercluster. The radio sources have bent-tail morphologies and show considerable meandering and wiggling along the jets, which are collimated throughout their lengths. This work presents observations of the large-scale-structure environment of the binary tailed radio sources with a view to examining the influence of large-scale flows on the morphology and dynamics of the associated radio tails. We argue that the orbital motions of the host galaxies together with tidal accelerations toward the supercluster have resulted in the complex structure seen in these radio tails. [source]

    Extended X-ray emission in the high-redshift quasar GB 1508+5714 at z= 4.3

    W. 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]

    Detection of type 2 quasars in the radio galaxies B3 0731+438 and 3C 257

    P. M. Derry
    ABSTRACT We present XMM,Newton observations and spectral fitting of two high-redshift, [O iii]-luminous, narrow-line radio galaxies, B3 0731+438 and 3C 257. Their X-ray continua are well fitted by a partial covering model with intrinsically unabsorbed and absorbed power-law components. The spectral models indicate that both objects harbour highly obscured nuclei, with NH, 0.5,2 × 1023 cm,2. Correcting for this absorption we find large intrinsic luminosities in the range LX, 0.2,1 × 1045 erg s,1. Thus, both sources are type 2 quasars. [source]

    Giant low surface brightness haloes in distant radio galaxies: USS0828+193

    M. Villar-Martín
    Abstract We present results on the spectroscopic study of the ionized gas in the high-redshift radio galaxy USS0828+193 at z= 2.57. Thanks to the high signal-to-noise (S/N) ratio of the emission lines in the Keck spectrum, we have been able to perform a detailed kinematic study by means of the spectral decomposition of the emission line profiles. This study reveals the existence of two types of material in this object: (i) a low surface brightness component with apparent quiescent kinematics consistent with gravitational motions and (ii) a perturbed component with rather extreme kinematics. The quiescent halo extends across the entire object for ,80 kpc. It is enriched with heavy elements and apparently ionized by the continuum from the active nucleus. The properties of the quiescent halo and its origin are discussed in this paper. We propose that it could be part of a structure that surrounds the entire object, although its nature is not clear (a rotating disc? low surface brightness satellites? a cooling flow nebula? material ejected in galactic winds? other?). [source]

    Emission-line outflows in PKS1549,79: the effects of the early stages of radio-source evolution?

    C. Tadhunter
    We present new spectroscopic observations of the southern radio galaxy . Despite the flat-spectrum character of the radio emission from this source, our optical spectra show no sign of the broad permitted lines and non-stellar continuum characteristic of quasar nuclei and broad-line radio galaxies. However, the high-ionization forbidden lines, including [O iii],,5007, 4959, are unusually broad for a narrow-line radio galaxy , and are blueshifted by 600 km s,1 relative to the low-ionization lines such as [O ii],,3726,3729. The [O ii] lines are also considerably narrower than the [O iii] lines, and have a redshift consistent with that of the recently detected H i 21-cm absorption-line system. Whereas the kinematics of the [O iii] emission lines are consistent with outflow in an inner narrow-line region, the properties of the [O ii] emission lines suggest that they are emitted by a more extended and quiescent gaseous component. We argue that, given the radio properties of the source, our line of sight is likely to be lying close to the direction of bulk outflow of the radio jets. In this case it is probable that the quasar nucleus is entirely obscured at optical wavelengths by the material responsible for the H i absorption-line system. The unusually broad [O iii] emission lines suggest that the radio source is intrinsically compact. Overall, our data are consistent the idea that is a radio source in an early stage of evolution. [source]

    On the evolution of young radio-loud AGN

    I. A. G. Snellen
    This paper describes an investigation of the early evolution of extragalactic radio sources using samples of faint and bright gigahertz peaked spectrum (GPS) and compact steep spectrum (CSS) radio galaxies. Correlations found between their peak frequency, peak flux density and angular size provide strong evidence that synchrotron self-absorption is the cause of the spectral turnovers, and indicate that young radio sources evolve in a self-similar way. In addition, the data seem to suggest that the sources are in equipartition while they evolve. If GPS sources evolve to large size radio sources, their redshift dependent birth-functions should be the same. Therefore, since the lifetimes of radio sources are thought to be short compared to the Hubble time, the observed difference in redshift distribution between GPS and large size sources must be due to a difference in slope of their luminosity functions. We argue that this slope is strongly affected by the luminosity evolution of the individual sources. A scenario for the luminosity evolution is proposed in which GPS sources increase in luminosity and large-scale radio sources decrease in luminosity with time. This evolution scenario is expected for a ram-pressure confined radio source in a surrounding medium with a King profile density. In the inner parts of the King profile, the density of the medium is constant and the radio source builds up its luminosity, but after it grows large enough the density of the surrounding medium declines and the luminosity of the radio source decreases. A comparison of the local luminosity function (LLF) of GPS galaxies with that of extended sources is a good test for this evolution scenario. Unfortunately, only a handful of GPS sources are known at low redshift, and an LLF can only be derived, assuming that their cosmological number density evolution is similar to that of steep spectrum sources. The LLF derived in this way is shown to be in good agreement with the proposed evolution scenario. However, the uncertainties are large, and larger, homogeneously selected samples of GPS sources are needed. [source]

    The environments of FRII radio sources

    M. J. Hardcastle
    Using ROSAT observations, we estimate gas pressures in the X-ray-emitting medium surrounding 63 FRII radio galaxies and quasars. We compare these pressures with the internal pressures of the radio-emitting plasma estimated by assuming minimum energy or equipartition. In the majority of cases (including 12/13 sources with modelled, spatially resolved X-ray emission) radio sources appear to be underpressured with respect to the external medium, suggesting that simple minimum-energy arguments underestimate the internal energy density of the sources. We discuss possible departures from the minimum-energy condition and the consequences of our result for models of the dynamics of radio galaxies, in particular self-similar models. [source]

    Deep spectroscopy of distant 3CR radio galaxies: the data

    P. N. Best
    Deep long-slit spectroscopic data are presented for a sample of 14 3CR radio galaxies at redshift z,1, previously studied in detail using the Hubble Space Telescope, the Very Large Array, and the UK Infrared Telescope (UKIRT). Analysis of the [O ii] 3727 emission-line structures at ,5 Ĺ spectral resolution is carried out to derive the kinematic properties of the emission-line gas. In line with previous lower resolution studies, a wide variety of kinematics are seen, from gas consistent with a mean rotational motion through to complex structures with velocity dispersions exceeding 1000 km s ,1. The data confirm the presence of a high-velocity gas component in 3C 265 and detached emission-line systems in 3C 356 and 3C 441, and show for the first time that the emission-line gas in the central regions of 3C 324 is composed of two kinematically distinct components. Emission-line fluxes and the colour of the continuum emission are determined down to unprecedentedly low observed wavelengths, ,<3500 Ĺ, sufficiently short that any contribution of an evolved stellar population is negligible. An accompanying paper investigates the variation in the emission-line ratios and velocity structures within the sample, and draws conclusions as to the origin of the ionization and kinematics of these galaxies. [source]

    Ionization, shocks and evolution of the emission-line gas of distant 3CR radio galaxies

    P. N. Best
    An analysis of the kinematics and ionization state of the emission-line gas of a sample of 14 3CR radio galaxies with redshifts z,1 is carried out. The data used for these studies, deep long-slit spectroscopic exposures from the William Herschel Telescope, are presented in an accompanying paper. It is found that radio sources with small linear sizes (,150 kpc) have lower ionization states, higher emission-line fluxes and broader line widths than larger radio sources. An analysis of the low-redshift sample of Baum et al. demonstrates that radio galaxies at low redshift show similar evolution in their velocity structures and emission-line ratios from small to large radio sources. The emission-line ratios of small radio sources are in agreement with theoretical shock ionization predictions, and their velocity profiles are distorted. Together with the other emission-line properties, this indicates that shocks associated with the radio source dominate the kinematics and ionization of the emission-line gas during the period that the radio source is expanding through the interstellar medium. Gas clouds are accelerated by the shocks, giving rise to the irregular velocity structures observed, whilst shock compression of emission-line gas clouds and the presence of the ionizing photons associated with the shocks combine to lower the ionization state of the emission-line gas. By contrast, in larger sources the shock fronts have passed well beyond the emission-line regions; the emission-line gas of these larger radio sources has much more settled kinematical properties, indicative of rotation, and emission-line ratios consistent with the dominant source of ionizing photons being the active galactic nucleus. This strong evolution with radio size of the emission-line gas properties of powerful radio galaxies mirrors the radio size evolution seen in the nature of the optical,ultraviolet continuum emission of these sources, implying that the continuum alignment effect is likely to be related to the same radio source shocks. [source]

    The kinetic power of jets magnetically accelerated from advection-dominated accretion flows in radio galaxies

    Shuang-Liang Li
    ABSTRACT There is a significant non-linear correlation between the Eddington ratio (Lbol/LEdd) and the Eddington-scaled kinetic power (Lkin/LEdd) of jets in low luminosity active galactic nuclei (AGNs). It is believed that these low luminosity AGNs contain advection-dominated accretion flows (ADAFs). We adopt the ADAF model developed by Li & Cao, in which the global dynamics of ADAFs with magnetically driven outflows is derived numerically, to explore the relation between the bolometric luminosity and the kinetic power of jets. We find that the observed relation, , can be well reproduced by the model calculations with reasonable parameters for ADAFs with magnetically driven outflows. Our model calculations are always consistent with the slope of the correlation independent of the values of the parameters adopted. Compared with the observations, our results show that over 60 per cent of the accreted gas at the outer radius escapes from the accretion disc in a wind before the gas falls into the black holes. The observed correlation between the Eddington-scaled kinetic power and Bondi power can also be qualitatively reproduced by our model calculations. Our results show that the mechanical efficiency varies from 10,2 to10,3, which is roughly consistent with that required in AGN feedback simulations. [source]

    Galaxy-galaxy interactions as triggers of star formation

    N.A. Popescu
    Abstract Galaxy-galaxy interactions represent the main physical processes that trigger substantial star formation in galaxies. In order to study the galaxy interactions effects on color and morphology of the galaxies in the fields of two radio galaxies at z , 1, we combined optical-NIR photometry and HST/WFPC2 morphology data from the Stanford et al. (2002) catalog, with HST/WFPC2 archival images. Using a strong pair isolation criterion in terms of the apparent angular separation and rest-frame line-of-sight velocity difference, we obtained a sample of galaxy pairs in the studied fields. The fact that close interactions and mergers induce star-forming episodes is confirmed by the presence of morphological disturbances and bluer optical-NIR colors at many analyzed galaxies (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Does equipartition hold in HFPs?

    D. Dallacasa
    Abstract Equipartition of energy between relativistic particles and magnetic field is known to be consistent with observations in most of the large size (hundreds of kpc) radio galaxies. Here we discuss whether such a property holds in the smallest and youngest radio sources, the High Frequency Peakers (HFPs). A few sources have small components with a turnover frequency occurring at a few GHz and therefore a direct measure of the magnetic field intensity can be derived from the spectral peak assuming the spectral turnover is originated by synchrotron self-absorption. The field strengths computed in this way are generally in excellent agreement with the equipartition fields. However, a few exceptions have been found, in sources with an extra deficit of photons in the optically thick part of the spectrum as a consequence of free-free absorption (FFA) (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]