2,132 research outputs found

    A high resolution view of the jet termination shock in a hot spot of the nearby radio galaxy Pictor A: implications for X-ray models of radio galaxy hot spots

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    Images made with the VLBA have resolved the region in a nearby radio galaxy, Pictor A, where the relativistic jet that originates at the nucleus terminates in an interaction with the intergalactic medium, a so-called radio galaxy hot spot. This image provides the highest spatial resolution view of such an object to date (16 pc), more than three times better than previous VLBI observations of similar objects. The north-west Pictor A hot spot is resolved into a complex set of compact components, seen to coincide with the bright part of the hot spot imaged at arcsecond-scale resolution with the VLA. In addition to a comparison with VLA data, we compare our VLBA results with data from the HST and Chandra telescopes, as well as new Spitzer data. The presence of pc-scale components in the hot spot, identifying regions containing strong shocks in the fluid flow, leads us to explore the suggestion that they represent sites of synchrotron X-ray production, contributing to the integrated X-ray flux of the hot spot, along with X-rays from synchrotron self-Compton scattering. This scenario provides a natural explanation for the radio morphology of the hot spot and its integrated X-ray emission, leading to very different predictions for the higher energy X-ray spectrum compared to previous studies. From the sizes of the individual pc-scale components and their angular spread, we estimate that the jet width at the hot spot is in the range 70 - 700 pc, which is comparable to similar estimates in PKS 2153-69, 3C 205, and 4C 41.17. The lower limit in this range arises from the suggestion that the jet may dither in its direction as it passes through hot spot backflow material close to the jet termination point, creating a "dentist drill" effect on the inside of a cavity 700 pc in diameter.Comment: Accepted by the Astronomical Journal. 35 pages, 6 figure

    The Far-infrared Continuum of Quasars

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    ISO provides a key new far-infrared window through which to observe the multi-wavelength spectral energy distributions (SEDs) of quasars and active galactic nuclei (AGN). It allows us, for the first time, to observe a substantial fraction of the quasar population in the far-IR, and to obtain simultaneous, multi-wavelength observations from 5--200 microns. With these data we can study the behavior of the IR continuum in comparison with expectations from competing thermal and non-thermal models. A key to determining which mechanism dominates, is the measurement of the peak wavelength of the emission and the shape of the far-IR--mm turnover. Turnovers which are steeper than frequency^2.5 indicate thermal dust emission in the far-IR. Preliminary results from our ISO data show broad, fairly smooth, IR continuum emission with far-IR turnovers generally too steep to be explained by non-thermal synchrotron emission. Assuming thermal emission throughout leads to a wide inferred temperature range of 50-1000 K. The hotter material, often called the AGN component, probably originates in dust close to and heated by the central source, e.g. the ubiquitous molecular torus. The cooler emission is too strong to be due purely to cool, host galaxy dust, and so indicates either the presence of a starburst in addition to the AGN or AGN-heated dust covering a wider range of temperatures than present in the standard, optically thick torus models.Comment: 4 pages, to be published in the proceedings of "The Universe as Seen by ISO," ed. M. Kessler. This and related papers can be found at http://hea-www.harvard.edu/~ehooper/ISOkp/ISOkp.htm

    Obscuration in extremely luminous quasars

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    The spectral energy distributions and infrared (IR) spectra of a sample of obscured AGNs selected in the mid-IR are modeled with recent clumpy torus models to investigate the nature of the sources, the properties of the obscuring matter, and dependencies on luminosity. The sample contains 21 obscured AGNs at z=1.3-3 discovered in the largest Spitzer surveys (SWIRE, NDWFS, & FLS) by means of their extremely red IR to optical colors. All sources show the 9.7micron silicate feature in absorption and have extreme mid-IR luminosities (L(6micron)~10^46 erg/s). The IR SEDs and spectra of 12 sources are well reproduced with a simple torus model, while the remaining 9 sources require foreground extinction from a cold dust component to reproduce both the depth of the silicate feature and the near-IR emission from hot dust. The best-fit torus models show a broad range of inclinations, with no preference for the edge-on torus expected in obscured AGNs. Based on the unobscured QSO mid-IR luminosity function, and on a color-selected sample of obscured and unobscured IR sources, we estimate the surface densities of obscured and unobscured QSOs at L(6micron)>10^12 Lsun, and z=1.3-3.0 to be about 17-22 deg^-2, and 11.7 deg^-2, respectively. Overall we find that ~35-41% of luminous QSOs are unobscured, 37-40% are obscured by the torus, and 23-25% are obscured by a cold absorber detached from the torus. These fractions constrain the torus half opening angle to be ~67 deg. This value is significantly larger than found for FIR selected samples of AGN at lower luminosity (~46 deg), supporting the receding torus scenario. A far-IR component is observed in 8 objects. The estimated far-IR luminosities associated with this component all exceed 3.3x10^12 Lsun, implying SFRs of 600-3000 Msun/yr. (Abridged)Comment: ApJ accepte

    The population of SNe/SNRs in the starburst galaxy Arp 220. A self-consistent analysis of 20 years of VLBI monitoring

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    The nearby ultra-luminous infrared galaxy (ULIRG) Arp 220 is an excellent laboratory for studies of extreme astrophysical environments. For 20 years, Very Long Baseline Interferometry (VLBI) has been used to monitor a population of compact sources thought to be supernovae (SNe), supernova remnants (SNRs) and possibly active galactic nuclei (AGNs). Using new and archival VLBI data spanning 20 years, we obtain 23 high-resolution radio images of Arp 220 at wavelengths from 18 cm to 2 cm. From model-fitting to the images we obtain estimates of flux densities and sizes of all detected sources. We detect radio continuum emission from 97 compact sources and present flux densities and sizes for all analysed observation epochs. We find evidence for a LD-relation within Arp 220, with larger sources being less luminous. We find a compact source LF n(L)Lβn(L)\propto L^\beta with β=2.19±0.15\beta=-2.19\pm0.15, similar to SNRs in normal galaxies. Based on simulations we argue that there are many relatively large and weak sources below our detection threshold. The observations can be explained by a mixed population of SNe and SNRs, where the former expand in a dense circumstellar medium (CSM) and the latter interact with the surrounding interstellar medium (ISM). Nine sources are likely luminous, type IIn SNe. This number of luminous SNe correspond to few percent of the total number of SNe in Arp 220 which is consistent with a total SN-rate of 4 yr1^{-1} as inferred from the total radio emission given a normal stellar initial mass function (IMF). Based on the fitted luminosity function, we argue that emission from all compact sources, also below our detection threshold, make up at most 20\% of the total radio emission at GHz frequencies.Comment: Accepted for publication in Astronomy and Astrophysic

    Large-scale structure in a new deep IRAS galaxy redshift survey

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    We present here the first results from two recently completed, fully sampled redshift surveys comprising 3703 IRAS Faint Source Survey (FSS) galaxies. An unbiased counts-in-cells analysis finds a clustering strength in broad agreement with other recent redshift surveys and at odds with the standard cold dark matter model. We combine our data with those from the QDOT and 1.2 Jy surveys, producing a single estimate of the IRAS galaxy clustering strength. We compare the data with the power spectrum derived from a mixed dark matter universe. Direct comparison of the clustering strength seen in the IRAS samples with that seen in the APM-Stromlo survey suggests b_O/b_I=1.20+/-0.05 assuming a linear, scale independent biasing. We also perform a cell by cell comparison of our FSS-z sample with galaxies from the first CfA slice, testing the viability of a linear-biasing scheme linking the two. We are able to rule out models in which the FSS-z galaxies identically trace the CfA galaxies on scales 5-20h^{-1}Mpc. On scales of 5 and 10h^{-1}Mpc no linear-biasing model can be found relating the two samples. We argue that this result is expected since the CfA sample includes more elliptical galaxies which have different clustering properties from spirals. On scales of 20h^{-1}Mpc no linear-biasing model with b_O/b_I < 1.70 is acceptable. When comparing the FSS-z galaxies to the CfA spirals, however, the two populations trace the same structures within our uncertaintie

    Keck spectroscopy of z=1-3 ULIRGs from the Spitzer SWIRE survey

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    (Abridged) High-redshift ultra luminous infrared galaxies contribute the bulk of the cosmic IR background and are the best candidates for very massive galaxies in formation at z>1.5. We present Keck/LRIS optical spectroscopy of 35 z>1.4 luminous IR galaxies in the Spitzer Wide-area Infra-Red Extragalactic survey (SWIRE) northern fields (Lockman Hole, ELAIS-N1, ELAIS-N2). The primary targets belong to the ``IR-peak'' class of galaxies, having the 1.6 micron (restframe) stellar feature detected in the IRAC Spitzer channels.The spectral energy distributions of the main targets are thoroughly analyzed, by means of spectro-photometric synthesis and multi-component fits (stars + starburst dust + AGN torus). The IR-peak selection technique is confirmed to successfully select objects above z=1.4, though some of the observed sources lie at lower redshift than expected. Among the 16 galaxies with spectroscopic redshift, 62% host an AGN component, two thirds being type-1 and one third type-2 objects. The selection, limited to r'<24.5, is likely biased to optically-bright AGNs. The SEDs of non-AGN IR-peakers resemble those of starbursts (SFR=20-500 Msun/yr) hosted in massive (M>1e11 Msun) galaxies. The presence of an AGN component provides a plausible explanation for the spectroscopic/photometric redshift discrepancies, as the torus produces an apparent shift of the peak to longer wavelengths. These sources are analyzed in IRAC and optical-IR color spaces. In addition to the IR-peak galaxies, we present redshifts and spectral properties for 150 objects, out of a total of 301 sources on slits.Comment: Accepted for publications on Astronomy and Astrophysics (acceprance date March 8th, 2007). 33 pages. The quality of some figures have been degrade

    Infrared Properties of High Redshift and X-ray Selected AGN Samples

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    The NASA/ISO Key Project on active galactic nuclei (AGN) seeks to better understand the broad-band spectral energy distributions (SEDs) of these sources from radio to X-rays, with particular emphasis on infrared properties. The ISO sample includes a wide variety of AGN types and spans a large redshift range. Two subsamples are considered herein: 8 high-redshift (1 < z < 4.7) quasars; and 22 hard X-ray selected sources. The X-ray selected AGN show a wide range of IR continuum shapes, extending to cooler colors than the optical/radio sample of Elvis et al. (1994). Where a far-IR turnover is clearly observed, the slopes are < 2.5 in all but one case so that non-thermal emission remains a possibility. The highest redshift quasars show extremely strong, hot IR continua requiring ~ 100 solar masses of 500 - 1000 Kelvin dust with ~ 100 times weaker optical emission. Possible explanations for these unusual properties include: reflection of the optical light from material above/below a torus; strong obscuration of the optical continuum; or an intrinsic deficit of optical emission.Comment: 8 pages, 3 figures (2 color), to be published in the Springer Lecture Notes of Physics Series as part of the proceedings for "ISO Surveys of a Dusty Universe," a workshop held at Ringberg Castle, Germany, November 8 - 12, 1999. Requires latex style files for this series: cl2emult.cls, cropmark.sty, lnp.sty, sprmindx.sty, subeqnar.sty (included with submission

    Three quasars from a survey of strong 25-µm emitters

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    We have carried out a spectroscopic survey of 750 sources that are strong 25-μm emitters from the IRAS Faint Source data base. Many of these sources are previously unknown active galactic nuclei including new IRAS quasars, three of which we describe here: F21382−2659, Z06367−6845 and Z05558−5008. They are all radio and X-ray quiet, and compared to the known IRAS quasars they have similar 25-μm luminosities, L(25 μm), but lower values of L(25 μm)/L(B). Their F(25 μm)/F(60 μm) IRAS colours lie in the range 0.33 to 1.08, indicating the presence of relatively warm dust, presumably in a dusty torus surrounding the central source, and with temperatures similar to those of the known IRAS quasars. The quasar with the warmest dust, F21382−2659, exhibits broad (full width at half-maximum ~4000 km s^(−1)) asymmetric Balmer lines with Hγ having an opposite asymmetry to the other broad lines; also Hβ (only) is double-peaked. Fe ii is very weak in F21382−2659 but strong in the other two quasars, and the anticorrelation between Fe ii and [O iii] holds as anticipated. Two of the quasars are unpolarized: although F21382−2659 is optically polarized (2.1 per cent at 4950 Å), we argue that this provides little insight into the orientation of its dust torus relative to the line of sight
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