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ABSTRACT: From an optical spectroscopic survey of 3CR radio galaxies with z<0.3, we
discovered a new spectroscopic class of powerful radio-loud AGN. The defining
characteristics of these galaxies are that compared with radio galaxies of
similar radio luminosity they have: a [O III]\Hb ratio of ~0.5, indicative of
an extremely low level of gas excitation; a large deficit of [O III] emission
and radio core power. We interpret these objects as relic AGN, i.e. sources
that experienced a large drop in their level of nuclear activity, causing a
decrease in their nuclear and line luminosity. This class opens a novel
approach to investigating lifetimes and duty cycles of AGN.
01/2011;
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ABSTRACT: We present optical nuclear spectra for nine 3CR radio sources obtained with the Telescopio Nazionale Galileo, that complete our spectroscopic observations of the sample up to redshifts $<$ 0.3. We measure emission line luminosities and ratios, and derive a spectroscopic classification for these sources. Comment: Accepted for publication in A&A. We provide as additional material two tables presenting the main data for the whole sample, combining the results presented here with those of Paper I and Paper II
10/2010;
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ABSTRACT: We use the emission line measurements of 3CR radio sources with redshift < 0.3, to explore their spectroscopic properties. The 3CR sources show a bimodal distribution of Excitation Index, a new spectroscopic indicator that measures the relative intensity of low and high excitation lines. This unveils the presence of two main sub-populations of radio-loud AGN, High and Low Excitation Galaxies (HEG and LEG, respectively). All broad-line objects are HEG from the point of view of their narrow emission line ratios and all HEG are FRII radio-galaxies with log L(178) [erg/s] > 32.8. Conversely LEG cover the whole range of radio power encompassed by this 3CR subsample (30.7 < log L(178) < 35.4) and they are of both FRI and FRII type. The brightest LEG are all FRII. HEG and LEG obey to two (quasi) linear correlations between the optical line and extended radio luminosities, with HEG being brighter than LEG in the [OIII] line by a factor of ~10. HEG and LEG are offset also in a plane that compares the black hole mass and the ionizing nuclear luminosity. However, although HEG are associated with higher nuclear luminosities, we find LEG among the brightest radio sources of the sample and with a clear FRII morphology, indistinguishable from those seen in HEG. This suggests that LEG are not simply objects with a lower level of accretion. We speculate that the differences between LEG and HEG are related to a different mode of accretion: LEG are powered by hot gas, while HEG require the presence of cold accreting material. The high temperature of the accreting gas in LEG accounts for the lack of "cold" structures (i.e. molecular torus and Broad Line Region), for the reduced radiative output of the accretion disk, and for the lower gas excitation. [ABRIDGED] Comment: Accepted for publication in A&A
11/2009;
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Grant R. Tremblay,
Marco Chiaberge,
William B. Sparks,
Stefi A. Baum,
Mark G. Allen,
David J. Axon,
Alessandro Capetti,
David J. E. Floyd, F. Duccio Macchetto,
George K. Miley,
Jacob Noel-Storr,
Christopher P. O'Dea,
Eric S. Perlman,
and Alice C. Quillen
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ABSTRACT: We present 19 nearby (z < 0.3) 3CR radio galaxies imaged at low and high excitation as part of a Cycle 15 Hubble Space Telescope (HST) snapshot survey with the Advanced Camera for Surveys (ACS). These images consist of exposures of the Hα (6563 Å, plus [N II] contamination) and [O III]λ5007 emission lines using narrowband linear ramp filters adjusted according to the redshift of the target. To facilitate continuum subtraction, a single-pointing 60 s line-free exposure was taken with a mediumband filter appropriate for the target's redshift. We discuss the steps taken to reduce these images independently of the automated recalibration pipeline so as to use more recent ACS flat-field data as well as to better reject cosmic rays. We describe the method used to produce continuum-free (pure line-emission) images, and present these images along with qualitative descriptions of the narrow-line region morphologies we observe. We present Hα+[N II] and [O III] line fluxes from aperture photometry, finding the values to fall expectedly on the redshift-luminosity trend from a past HST/WFPC2 emission line study of a larger, generally higher redshift subset of the 3CR. We also find expected trends between emission line luminosity and total radio power, as well as a positive correlation between the size of the emission line region and redshift. We discuss the associated interpretation of these results, and conclude with a summary of future work enabled by this data set.
The Astrophysical Journal Supplement Series 07/2009; 183(2):278. · 13.46 Impact Factor
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William B Sparks,
James Hough,
Thomas A Germer,
Feng Chen,
Shiladitya DasSarma,
Priya DasSarma,
Frank T Robb,
Nadine Manset,
Ludmilla Kolokolova,
Neill Reid, F Duccio Macchetto,
William Martin
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ABSTRACT: The identification of a universal biosignature that could be sensed remotely is critical to the prospects for success in the search for life elsewhere in the universe. A candidate universal biosignature is homochirality, which is likely to be a generic property of all biochemical life. Because of the optical activity of chiral molecules, it has been hypothesized that this unique characteristic may provide a suitable remote sensing probe using circular polarization spectroscopy. Here, we report the detection of circular polarization in light scattered by photosynthetic microbes. We show that the circular polarization appears to arise from circular dichroism of the strong electronic transitions of photosynthetic absorption bands. We conclude that circular polarization spectroscopy could provide a powerful remote sensing technique for generic life searches.
Proceedings of the National Academy of Sciences 06/2009; 106(19):7816-21. · 9.68 Impact Factor
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ABSTRACT: We present a search for FRI radio galaxies between 1 < z < 2 in the COSMOS field. In absence of spectroscopic redshift measurements, the selection method is based on multiple steps which make use of both radio and optical constraints. The basic assumptions are that 1) the break in radio power between low-power FRIs and the more powerful FRIIs does not change with redshift, and 2) that the photometric properties of the host galaxies of low power radio galaxies in the distant universe are similar to those of FRIIs in the same redshift bin, as is the case for nearby radio galaxies. We describe the results of our search, which yields 37 low-power radio galaxy candidates that are possibly FRIs. We show that a large fraction of these low-luminosity radio galaxies display a compact radio morphology, that does not correspond to the FRI morphological classification. Furthermore, our objects are apparently associated with galaxies that show clear signs of interactions, at odds with the typical behavior observed in low-z FRI hosts. The compact radio morphology might imply that we are observing intrinsically small and possibly young objects, that will eventually evolve into the giant FRIs we observe in the local universe. One of the objects appears as point-like in HST images. This might belong to a population of FRI-QSOs, which however would represent a tiny minority of the overall population of high-z FRIs. As for the local FRIs, a large fraction of our objects are likely to be associated with groups or clusters, making them "beacons" for high redshift clusters of galaxies. Our search for candidate high-z FRIs we present in this paper constitutes a pilot study for objects to be observed with future high-resolution and high-sensitivity instruments (shortened) Comment: 44 pages, 11 figures, version accepted for publication in ApJ
02/2009;
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ABSTRACT: We present results of Hubble Space Telescope (HST) observations of the radio galaxy 3C 299. The broadband F702W (R) and F555W (V) images (WFPC2/PC) show an elliptical galaxy with a comet-like structure extending to the northeast in the radio jet direction. The [O III] λ5007 emission-line map shows a biconical structure centered on the nucleus, which overlaps the structure found in the broadband filters. The radio core coincides with the center of the biconical structure and the radio axes are aligned with the direction of the cones. These data show clear evidence of a strong interaction between the radio jet and the northeastern morphology of the galaxy. We show evidence that this northeast region is an extended narrow-line region (ENLR); the line-ratio diagnostics show that models involving gas shocked by the radio jet plus ionization from a precursor H II region—itself produced by the ionizing photons of the postshocked gas on the preshocked gas—provide a good match to the observations. We investigate the spatial behavior of the ionizing parameter U, by determining the [O III]/[O II] line ratio, which is sensitive to the change of the ionization parameter, and we trace its behavior over the ENLR along the radio jet direction. We find that [O III]/[O II] does not follow a simple dilution model, but rather it is approximately constant over a large range of distance from the nucleus, thus requiring a local source of ionization, which seems to be compatible with the shock models driven by the radio jet.
The Astrophysical Journal 01/2009; 526(2):623. · 6.02 Impact Factor
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ABSTRACT: We present a homogeneous and 92 % complete dataset of optical nuclear spectra for the 113 3CR radio sources with redshifts < 0.3, obtained with the Telescopio Nazionale Galileo. For these sources we could obtain uniform and uninterrupted coverage of the key spectroscopic optical diagnostics. The observed sample, including powerful classical FR II radio-galaxies and FR I, together spanning four orders of magnitude in radio-luminosity, provides a broad representation of the spectroscopic properties of radio galaxies. In this first paper we present an atlas of the spectra obtained, provide measurements of the diagnostic emission line ratios, and identify active nuclei with broad line emission. These data will be used in follow-up papers to address the connection between the optical spectral characteristics and the multiwavelength properties of the sample. Comment: Accepted for publication in A&A. Includes 104 gif figures
01/2009;
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ABSTRACT: We present 1998 Hubble Space Telescope observations of M87 that yield the first single-epoch optical and radio-optical spectral index image of the jet at 015 resolution. We find αro ≈ 0.67, comparable to previous measurements, and αo ≈ 0.9 (Fν ν-α), slightly flatter than previous workers. Reasons for this discrepancy are discussed. These observations reveal a large variety of spectral slopes. Bright knots exhibit significantly flatter spectra than interknot regions. The flattest spectra (αo ~ 0.5-0.6, comparable to or flatter than αro) are found in the two inner jet knots (D-East and HST-1), which contain the fastest superluminal components. The flux maximum regions of other knots have αo ~ 0.7-0.9. The maps of αo and αro appear poorly correlated. In knots A, B, and C, αo and αro are essentially anticorrelated with one another. Near the flux maxima of two inner jet knots (HST-1 and F), changes in αro appear to lag changes in αo, but in two other knots (D and E), the opposite relationship is observed. This is further evidence that the radio and optical emissions of the M87 jet come from substantially different physical regions. The delays observed in the inner jet are consistent with localized particle acceleration in the knots, with tacc tcool for optically emitting electrons in knots HST-1 and F, and tacc ~ tcool for optically emitting electrons in knots D and E. Synchrotron models fit to the radio-optical data yield νB 1016 Hz for knots D, A, and B, and somewhat lower values, νB ~ 1015-1016 Hz, in other regions of the jet. If the X-ray emissions from knots A, B, and D are cospatial with the optical and radio emission, we can strongly rule out the "continuous injection" model, which overpredicts the X-ray emissions by large factors. Because of the short lifetimes of X-ray synchrotron-emitting particles, the X-ray emission likely traces sites of particle acceleration and fills volumes much smaller than the optical emission regions.
The Astrophysical Journal 12/2008; 551(1):206. · 6.02 Impact Factor
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ABSTRACT: We present a Hubble Space Telescope image of the FR II radio galaxy 3C 401, obtained at 1.6 μm with NICMOS, in which we identify the infrared counterpart of the brightest region of the radio jet. The jet has a complex radio structure and brightens where bending occurs, most likely as a result of relativistic beaming. We analyze archival data in the radio, optical, and X-ray bands, and we derive its spectral energy distribution. Unlike all of the previously known optical extragalactic jets, the jet in 3C 401 is not detected in the X-rays, even in a long 48 ks X-ray Chandra exposure, and the infrared emission dominates the overall spectral energy distribution (SED). We propose that the dominant radiation mechanism of this jet is synchrotron. The low X-ray emission is then caused by two different effects: (1) the lack of any strong external photon field and (2) the shape of the electron distribution. This affects the location of the synchrotron peak in the SED, resulting in a sharp cutoff at energies lower than the X-rays. Thus 3C 401 shows a new type of jet, with intermediate spectral properties between those of FR I galaxies, which are dominated by synchrotron emission up to X-ray energies, and FR II galaxies/QSOs, which show strong high-energy emission due to inverse Compton scattering of external photons. This might indicate the presence of a continuous "sequence" in the properties of large-scale jets, analogous to the "blazar sequence" already proposed for subparsec-scale jets.
The Astrophysical Journal 12/2008; 629(1):100. · 6.02 Impact Factor
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ABSTRACT: The presence of optical synchrotron jets in radio galaxies is relatively rare. Here we show that of the nearest five FR I 3CR radio galaxies showing optical jets, four show evidence for almost circular, presumably face-on, dust disks. This is strong support for the twofold idea that (1) jets emerge close to perpendicular to inner gas disks and (2) optical nonthermal synchrotron emission is seen only when the jet points toward the observer. The implied critical angle to the line of sight is approximately 30°-40°; i.e., if the angle of the jet to the line of sight is less than about 40° we see an optical jet. The corresponding relativistic γ factor is ≈1.5, which is consistent with current observations of jet proper motion that show a range up to γ ~ 6 for M87. The relatively low speeds implied by γ ≈ 1.5 may be due to a global deceleration of the jet as in unified theories or else to stratification within the jet. Unresolved nuclei are common in the optical. Their luminosities are also consistent with the beaming concept when compared to inclination inferred from the dust lanes. The disk sizes are typically several hundred parsecs to kiloparsec size. The galaxy with an optical jet that does not show a face-on disk, M87, instead has more complex radial dust and ionized gas filaments.
The Astrophysical Journal 12/2008; 542(2):667. · 6.02 Impact Factor
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ABSTRACT: The Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph ultraviolet image of the FR I radio galaxy 3C 270 shows the presence of a jetlike structure emerging from the position of the nucleus. This feature, which represents the first jetlike component ever detected in the UV in a radio galaxy with jets lying almost on the plane of the sky, has the same position angle as the jet in the radio and X-ray images. We propose two different scenarios for the origin of the emission: (1) nonthermal synchrotron from a mildly relativistic component of the jet and (2) scattered light from the nucleus, where a BL Lac source may be hosted. Either of these pictures would have important consequences for the active galactic nuclei unification schemes and for our knowledge of the jet structure. In the Chandra image, a faint counterjet is also present. From a comparative analysis of the HST images and Chandra X-ray spectrum, we find that the nucleus is only moderately obscured. The obscuring structure might well reside in the geometrically thin dark disk observed on large scales. This fits the scenario in which a standard geometrically and optically thick torus is not present in FR I radio galaxies.
The Astrophysical Journal 12/2008; 582(2):645. · 6.02 Impact Factor
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ABSTRACT: We analyze the near-infrared luminosity profiles and photometric parameters of the host galaxies of 3CR radio sources with z < 0.3, to investigate their physical nature. Our sample includes 82 galaxies, of which 22 (27%) are FR Is and 60 (73%) are FR IIs. Using near-infrared data taken both with NICMOS on board the Hubble Space Telescope and from the ground with the Telescopio Nazionale Galileo, we find that luminosity profiles are very well described by a single Sérsic law in 52% of the cases and that for the remaining objects (48%) it is necessary to include an exponential profile, which indicates the presence of a second disklike component. The average bulge-to-disklike components luminosity ratio for the galaxies is (b/e) ~ 1.1. The analysis of the photometric parameters of the subsamples indicates that FR Is and FR IIs show rather similar bulges in terms of effective surface magnitude, effective radius, and Sérsic index. On the other hand, the disklike components in FR I and FR II hosts show, on average, different properties. Central surface magnitudes are dimmer and scale lengths are greater by a factor of 2 in FR Is when compared to FR IIs. We also estimate the black hole mass associated with each galaxy using two different methods that claim tight correlations of the black hole mass (MBH) with the infrared bulge luminosity (Lbulge) and with the Sérsic index (n). Our data indicate that masses obtained through these two methods show a high dispersion and that MBH obtained through Lbulge are systematically higher (by a factor of ~3) than those obtained using n. This result may reflect the fact that for our sample galaxies we do not find any correlation between Lbulge and n.
The Astrophysical Journal 12/2008; 667(2):780. · 6.02 Impact Factor
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Grant R. Tremblay,
Alice C. Quillen,
David J. E. Floyd,
Jacob Noel-Storr,
Stefi A. Baum,
David Axon,
Christopher P. O'Dea,
Marco Chiaberge, F. Duccio Macchetto,
William B. Sparks,
George K. Miley,
Alessandro Capetti,
Juan P. Madrid,
and Eric Perlman
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ABSTRACT: Among radio galaxies containing nuclear dust disks, the bipolar jet axis is generally observed to be perpendicular to the disk major axis. The FR I radio source 3C 449 is an outlier to this statistical majority, as it possesses a nearly parallel jet/disk orientation on the sky. We examine the 600 pc dusty disk in this galaxy with images from the Hubble Space Telescope. We find that a 1.6 μm/0.7 μm color map of the disk exhibits a twist in its isocolor contours (isochromes). We model the color map by integrating galactic starlight through an absorptive disk and find that the anomalous twist in the isochromes can be reproduced in the model with a vertically thin, warped disk. The model predicts that the disk is nearly perpendicular to the jet axis within 100 pc of the nucleus. We discuss physical mechanisms capable of causing such a warp. We show that precessional models or a torque on the disk arising from a possible binary black hole in the AGN causes precession on a timescale that is too long to account for the predicted disk morphology. However, we estimate that the pressure in the X-ray-emitting interstellar medium is large enough to perturb the disk, and we argue that jet-driven anisotropy in the excited ISM may be the cause of the warp. In this way, the warped disk in 3C 449 may be a new manifestation of feedback from an active galactic nucleus.
The Astrophysical Journal 12/2008; 643(1):101. · 6.02 Impact Factor
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ABSTRACT: We have studied the nuclei of 28 radio galaxies from the 3CR sample in the UV band. Unresolved nuclei (central compact cores: CCCs) are observed in 10 of the 13 FR I galaxies, and in five of the 15 FR II galaxies. All sources that do not have a CCC in the optical do not have a CCC in the UV. Two FR I galaxies (3C 270 and 3C 296) have a CCC in the optical but do not show the UV counterpart. Both of them show large dusty disks observed almost edge-on, possibly implying that they play a role in obscuring the nuclear emission. We have measured optical-UV spectral indices αo,UV between ~0.6 and ~7.0 (Fν ν-α). Broad-line radio galaxies have the flattest spectra, and their values of αo,UV are also confined to a very narrow range. This is consistent with radiation produced in a geometrically thin, optically thick accretion disk. On the other hand, FR I nuclei, which are most plausibly originated by synchrotron emission from the inner relativistic jet, show a wide range of αo,UV. There is a clear trend with orientation in that sources observed almost edge-on or with clear signs of dust absorption have the steepest spectra. These observations imply that in FR I galaxies, obscuration can be present, but the obscuring material is not in a "standard" geometrically thick torus. The most striking difference between these absorbing structures and the classic active galactic nucleus "tori" resides in the lower optical depth of the FR I galaxy obscuring material.
The Astrophysical Journal 12/2008; 571(1):247. · 6.02 Impact Factor
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ABSTRACT: We obtained optical imaging polarimetry with the ACS/HRC aboard the HST of the 9 closest radio-galaxies in the 3C catalogue with an FR I morphology. The nuclear sources seen in direct HST images in these galaxies are found to be highly polarized with levels in the range ~2-11 % with a median value of 7 %. We discuss the different mechanisms that produce polarized emission and conclude that the only viable interpretation is a synchrotron origin for the optical nuclei. This idea is strengthened by the analogy with the polarization properties of BL Lac objects, providing also further support to the FRI/BL Lac unified model. This confirms previous suggestions that the dominant emission mechanism in low luminosity radio-loud AGN is related to non-thermal radiation produced by the base of their jets. In addition to the nuclear polarization (and to the large scale optical jets), polarization is detected co-spatially with the dusty circumnuclear disks, likely due to dichroic transmission; the polarization vectors are tangential to the disks as expected when the magnetic field responsible for the grains alignment is stretched by differential rotation. We explored the possibility to detect the polarimetric signature of a misaligned radiation beam in FR I, expected in our sources in the frame of the FR I/ BL Lac unification. We did not find this effect in any of the galaxies, but our the results are not conclusive on whether a misaligned beam is indeed present in FR I. Comment: 16 page, 10 figures, accepted for publication in A&A
06/2007;
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ABSTRACT: We analyze the near-infrared luminosity profiles and photometric parameters of the host galaxies of 3CR radio sources with z<0.3, to investigate their physical nature. Our sample includes 82 galaxies, of which 22 (27%) are FR Is and 60 (73%) are FR IIs. Using near-infrared data taken both with NICMOS onboard the Hubble Space Telescope and from the ground with the Telescopio Nazionale Galileo, we find that luminosity profiles are very well described by a single Sersic law in 52% of the cases and for the remaining objects (48%) it is necessary to include an exponential profile, which could indicate the presence of a disk. The average bulge to disk luminosity ratio for the galaxies is (b/d) ~ 1.1. The analysis of the photometric parameters of the sub samples indicates that FR Is and FR IIs show rather similar bulges in terms of effective surface magnitude, effective radius, and Sersic index. On the other hand, disks in FR Is and FR IIs hosts show, on average, different properties. Central surface magnitudes are dimmer and scale lengths are greater by a factor of 2 in FR Is when compared to FR IIs. We also estimate the black hole mass associated with each galaxy using two different methods that claim tight correlations between the black hole mass (M_BH) with the infrared bulge luminosity (L_bulge) and with the Sersic index (n). Our data indicate that masses obtained through these two methods show a high dispersion and M_BH obtained through L_bulge are systematically higher (by a factor of ~3) than those obtained using n. This result may reflect the fact that for our sample galaxies we do not find any correlation between L_bulge and n. Comment: 19 pages, 4 tables, 25 figures. Sumitted to ApJ. Corrected typos
12/2006;
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ABSTRACT: We study a complete and distance-limited sample of 25 LINERs, 21 of which have been imaged with the Hubble Space Telescope. In nine objects we detect an unresolved nucleus. In order to study their physical properties, we compare the radio and optical properties of the nuclei of LINERs with those of other samples of local AGNs, namely Seyfert galaxies and low-luminosity radio galaxies (LLRG). Our results show that the LINERs population is not homogeneous, as there are two subclasses: i) the first class is similar to LLRG, as it extends the population of radio-loud nuclei to lower luminosities; ii) the second is similar to Seyferts, and extends the properties of radio-quiet nuclei towards the lowest luminosities. The objects are optimally discriminated in the plane formed by the black hole mass vs. nuclear radio-loudness: all radio-loud LINERs have M_BH/M_sun > 10^8, while Seyferts and radio-quiet LINERs have M_BH/M_sun < 10^8. The different nature of the various classes of local AGN are best understood when the fraction of the Eddington luminosity they irradiate L_o/L_Edd is plotted against the nuclear radio-loudness parameter: Seyferts are associated with relatively {\it high} radiative efficiencies L_o/L_Edd >~ 10^-4 (and high accretion rates onto low mass black holes); LLRG are associated with low radiative efficiencies (and low accretion rates onto high black hole masses); all LINERs have low radiative efficiency (and accretion rates), and can be radio-loud or radio quiet depending on their black hole mass. Comment: Accepted for publication in ApJ, final revised version includes further comments from the referee, 17 pages, 8 figure, 4 tables
01/2005;
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ABSTRACT: We identify the dominant emission processes in 3C 270 in the infrared to
be non-thermal, and emission from its dusty disk does not contribute
significantly to the total infrared output. This implies that such disks
can be hard to find through their infrared emission, and that any
circumnuclear torus, if at all present, is small and thin.
Proceedings of the International Astronomical Union 10/2004; 222:75-76.
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ABSTRACT: We extend the study of the nuclei of 3CR objects as seen with the HST to higher redshift FR II radio sources (0.4<z<0.6). Our results reflect what has been found for FR II of lower redshift. The position of the nuclei on the plane formed by optical and radio luminosities is related to their optical spectral classification: the nuclei of both high and low excitation galaxies lie on the correlation found for FRI sources, while broad-lined objects have a significant optical excess. The nuclear properties of these sources are better understood when considering the equivalent width of the [OIII] emission line with respect to their optical luminosities. Even in the range of redshift considered here, low excitation galaxies show peculiar nuclear properties, more similar to those observed in FR I. This confirms that not all FRII are unified with quasars. Our findings have important implications for the FRII-quasar unification scheme: by reconsidering the classification of all 3CR radio galaxies with z<1 in the light of their nuclear properties, we derive the opening angle of the obscuring torus for different redshift bins. We find that the covering factor of the obscuring structure decreases as redshift increases (theta ~ 44 deg for z<0.3 to theta ~ 56 deg for 0.5<z<1). We argue that this behavior may be interpreted in the framework of the receding torus model, in which the opening angle of the torus increases as the luminosity of the accretion disk around the central black hole increases. Comment: 9 pages, 5 figures, accepted for publication in Astronomy & Astrophysics
07/2004;
