Dissertations / Theses on the topic 'Radio sources (astronomy)'
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1
Mauch, Thomas. "Radio Sources in the Local Universe." University of Sydney, 2006. http://hdl.handle.net/2123/928.
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Doctor of PhilosophyThis thesis presents a census of radio sources selected from the NRAO (National Radio Astronomy Observatory) VLA (Very Large Array) Sky Survey (NVSS) and Sydney University Molonglo Sky Survey (SUMSS) catalogues which have also been observed in the first data release of the 6 degree Field Galaxy Survey (6dFGS), a galaxy redshift survey of the local universe. Radio detections were found for 4,506 galaxies in the 6dFGS near-infrared-selected primary sample, a radio detection rate of 16%. A further 1,196 radio sources were observed by 6dF which were missing from the 6dFGS primary sample either because their host galaxies were too blue in colour or they appeared stellar on optical plates. The full sample comprises the largest and most homogeneous set of spectra and redshifts of radio sources in the local universe ever obtained. Results from the study of these objects form an accurate benchmark from which their cosmic evolution may be understood. 6dF spectra of galaxies have been used to determine the physical cause of radio emission from each object as either star formation or an active galactic nucleus powered by a super-massive black hole. These two classes of radio source have been characterised via a determination of the local radio luminosity function at 1.4 GHz; plotting the variation in their space density with luminosity. The star-formation density of the universe at the present epoch has been determined, the value of which which turns out to be in excellent agreement with previously published values. Fractional luminosity functions have also been determined showing that more massive galaxies have higher star-formation rates and are more likely to host a radio-loud AGN. The large-scale structure of star-forming galaxies and radio-loud AGN in the local universe has been studied by determining their clustering properties via the two-point correlation function. Radio-loud AGN are found to cluster more strongly than star-forming galaxies confirming that these objects are biased tracers of the underlying matter distribution. Both star-forming galaxies and AGNs cluster similarly to the underlying host galaxy population in which they reside. This thesis also describes the 843 MHz SUMSS catalogue, made by fitting elliptical Gaussians to sources in images. The catalogue contains radio sources to a limiting peak brightness of 6 mJy/beam at declination less than -50 degrees and 10 mJy/beam at declination greater than -50 degrees. Image artefacts have been classified using a novel technique involving a decision tree, which correctly identifies and rejects spurious sources in over 96% of cases and has ensured the catalogue is more than 95% complete and 90% reliable over most of its flux density range.
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Mauch, Thomas. "Radio Sources in the Local Universe." Thesis, The University of Sydney, 2005. http://hdl.handle.net/2123/928.
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This thesis presents a census of radio sources selected from the NRAO (National Radio Astronomy Observatory) VLA (Very Large Array) Sky Survey (NVSS) and Sydney University Molonglo Sky Survey (SUMSS) catalogues which have also been observed in the first data release of the 6 degree Field Galaxy Survey (6dFGS), a galaxy redshift survey of the local universe. Radio detections were found for 4,506 galaxies in the 6dFGS near-infrared-selected primary sample, a radio detection rate of 16%. A further 1,196 radio sources were observed by 6dF which were missing from the 6dFGS primary sample either because their host galaxies were too blue in colour or they appeared stellar on optical plates. The full sample comprises the largest and most homogeneous set of spectra and redshifts of radio sources in the local universe ever obtained. Results from the study of these objects form an accurate benchmark from which their cosmic evolution may be understood. 6dF spectra of galaxies have been used to determine the physical cause of radio emission from each object as either star formation or an active galactic nucleus powered by a super-massive black hole. These two classes of radio source have been characterised via a determination of the local radio luminosity function at 1.4 GHz; plotting the variation in their space density with luminosity. The star-formation density of the universe at the present epoch has been determined, the value of which which turns out to be in excellent agreement with previously published values. Fractional luminosity functions have also been determined showing that more massive galaxies have higher star-formation rates and are more likely to host a radio-loud AGN. The large-scale structure of star-forming galaxies and radio-loud AGN in the local universe has been studied by determining their clustering properties via the two-point correlation function. Radio-loud AGN are found to cluster more strongly than star-forming galaxies confirming that these objects are biased tracers of the underlying matter distribution. Both star-forming galaxies and AGNs cluster similarly to the underlying host galaxy population in which they reside. This thesis also describes the 843 MHz SUMSS catalogue, made by fitting elliptical Gaussians to sources in images. The catalogue contains radio sources to a limiting peak brightness of 6 mJy/beam at declination less than -50 degrees and 10 mJy/beam at declination greater than -50 degrees. Image artefacts have been classified using a novel technique involving a decision tree, which correctly identifies and rejects spurious sources in over 96% of cases and has ensured the catalogue is more than 95% complete and 90% reliable over most of its flux density range.
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Maciel, Tamela. "Radio source evolution." Thesis, University of Cambridge, 2015. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.708637.
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McGilchrist, Mark Mungo. "The first 7C survey of radio sources." Thesis, University of Cambridge, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.303245.
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Wadadekar, Y. G. "Optical studies of VLA FIRST Survey sources /." Pune, India : Pune University, 2000. http://69.63.217.22/elibsql06_N10017_Documents/00000000033045.pdf.
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Lacy, Mark David. "Samples of radio sources selected at 38 MHz." Thesis, University of Cambridge, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.259734.
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Afonso, JoseÌ Manuel Lourenço Coutinho. "On the star-forming properties of faint radio sources." Thesis, Imperial College London, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.271584.
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Labiano, Álvaro. "Host galaxies and environments of compact extragalactic radio sources /." Groningen : Rijksuniversiteit Groningen, 2006. http://dissertations.ub.rug.nl/faculties/science/2006/a.labiano.ortega/.
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Woermann, Beate. "Radio observation of the Gum Nebula Region." Thesis, Rhodes University, 1997. http://hdl.handle.net/10962/d1005271.
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This thesis describes the results of an investigation of the physical properties of the Gum Nebula. For this investigation a radio continuum map of the region was made at 2326 MHz and resolution ⅓° with the HartRAO antenna. This map was used to generate spectral index images and an infrared to radio flux density ratio (IRR) image. The latter image shows that the IRR of the nebula is in the range 20 to 250, identifying it as an old SNR. Several spectral index images of this region were generated using two different methods, one based on the isolation of the nebula from its background radiation, the other based on TT-plots (Turtle et al., 1962). The two methods yield similar results, which show that the nebula has a thermal shell with a non-thermal region in its interior. Below the galactic plane the thermal region dominates and above the plane the nonthermal region. These results suggest a model of an old SNR with an H II region shell. Spectral line observations of hydrogen recombination lines and hydroxyl (OH) were made with the HartRAO and the Mopra telescopes. The detection of hydrogen recombination lines at four positions in the thermal regions of the nebula give electron temperatures and emission measures in the ranges 4000 to 6000 K and 220 to 460 pc.cm⁻⁶ respectively. The turbulent velocities are of the order of 20 km/s. A search for shocked OH lines at 1667 MHz and 1720 MHz in the Gum Nebula gave results that were negative, but numerous unshocked 1667 MHz OH lines were detected. The latter were used in a test for an expansion of the nebula. The most plausible fit to the data gives an expansion centre at l = 260.5°, b = -2.5° and at a distance of 0.7 kpc from us. The front face angular radius and expansion velocity are 10.5° and 16 km/s respectively. The back face angular radius and expansion velocity are 8.50 and 7 km/s respectively.
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Rossitter, D. A. "The nature of faint radio sources in low frequency samples." Thesis, University of Cambridge, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.235304.
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The behaviour of the radio source population at high redshift has been a major area of research in extragalactic radio astronomy during the past decade. In particular, the question of at what redshift the co-moving number density of radio sources reaches a peak, has been of considerable interest. Radio surveys are particularly good for yielding large numbers of distant radio galaxies and quasars but at the price of having the edshift of the individual sources very difficult to obtain. This thesis has examined the behaviour of faint radio sources at large redshift, in particular those objects showing the classical double structure as is commonly found in samples selected at low frequency. The first part of this thesis identifies the need for at least approximate redshift information for complete flux limited samples to properly address the behaviour of these sources at large redshift; known redshift indicators are shown to be inadequate for this purpose. A new method of obtaining the intrinsic radio luminosity and hence redshift for classical double radio sources from the bright 3C sample from the radio data alone, is introduced. The same relationship is shown to be applicable to similar sources found at larger redshifts when selected from fainter samples. The middle part of this work is concerned with the construction of two new faint radio samples selected at 151 MHz. In addition to VLA and 5km telescope mapping, a review of existing optical data together with new CCD imaging and some spectroscopy is given. Finally, by taking these two new samples together with the intermediate sample of Eales and the bright sample of LRL, the new method of deriving radio luminosity has been used to explore the shape of the evolution function and degree of linear size evolution required for double radio sources at a much larger redshift than has been previously possible.
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Laycock, S. C. "Very steep spectrum radio sources and clusters of galaxies." Thesis, University of Cambridge, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.233300.
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The topics covered in this dissertation are all areas of study involving observations at low radio frequencies. There are three main subject areas: a study of the twin-tailed radio galaxy 3C3.1; a study of both an old and a new sample of radio sources that exhibit very steep radio spectra at low frequency; the design and construction of a new radio telescope operating at low radio frequency together with the making of a new radio source survey. 3C3.1 has been studied by other authors but new high angular resolution, high sensitivity observations at low radio frequency have allowed further progress to be made in understanding the behaviour of this source. It has been thought that 3C3.1 type sources would be responsible for most (if not all) of the very steep spectrum radio sources. 3C3.1 is relatively close, hence easy to study. A model has been developed which explains the previously not understood brightness distribution along the long luminous jets. In order to quantify the predicted behaviour more precisely a set of simple numerical simulations was performed. Very steep spectrum radio sources are by their nature easier to detect at low radio frequencies. In the past, it has been shown that most, if not all, very steep spectrum sources are associated with clusters of galaxies. Both optical and further radio observations of a sample of sources prepared by the author, and a sample prepared by other workers were undertaken. The optical observations of high sensitivity have greatly strengthened the hypothesis that all of such sources are indeed associated with clusters of galaxies. The radio observations, both performed at high and low radio frequencies, have shown that such sources seem to have in general evolved from conventional sources with both 'tailed' and 'double' radio structure. A serious limitation for further work at low radio frequencies is the availibility of high sensitivity, high resolution instruments. A twenty five element interferometer with a one mile baseline operating at 38 MHz was designed and constructed. This allowed an appraisal of the operating conditions at such low frequency. A deep radio survey of the north pole was performed and a new sample of very steep spectrum constructed.
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West, Craig James, and cwest@astro swin edu au. "Development of disk-based baseband recorders and software correlators for radio astronomy." Swinburne University of Technology. Centre for Astrophysics and Supercomputing, 2004. http://adt.lib.swin.edu.au./public/adt-VSWT20050804.143155.
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This thesis details work undertaken in the field of radio astronomy instrumentation. Specific components of the data collection and processing systems used by radio astronomers have been implemented using non-traditional approaches. Traditionally, the correlation of radio astronomy data has taken place on dedicated,
specific hardware. This thesis deals with the implementation of equivalent correlators
using software running on generic clusters of personal computers - the software approach to radio astronomy. Toward this end a system has been developed that records the raw telescope output onto computer hard drives, allowing
easy access to the data on cluster supercomputers. Part of this thesis describes the design, construction, testing and utilisation of these data recording systems. The correlator software used to process these data
on supercomputers is also fully described, including extensive tests of the software
and a detailed comparison between its output and the output of an existing hardware correlator. The software correlator is shown to produce output that agrees extremely well with the hardware correlator, verifying its accuracy and performance. Finally, results of on-going scientific investigations that use the software correlators described in this thesis are outlined, illustrating the flexibility and usefulness of the software approach to radio astronomy.
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Hungwe, Faith. "Variability analysis of a sample of potential southern calibration sources." Thesis, Rhodes University, 2009. http://hdl.handle.net/10962/d1005281.
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A considerable number of Very Long Baseline Interferometry (VLBI) surveys have been conducted in the northern hemisphere and very few in the southern hemisphere mostly because of a lack of telescopes and therefore adequate baseline coverage. Thus there is a deficit of calibrator sources in the southern hemisphere. Further, some of the most interesting astronomical objects eg. the galactic centre and the nearest galaxies (the small and large Magellanic Clouds) lie in the southern hemisphere and these require high resolution studies. With a major expansion of radio astronomy observing capability on its way in the southern hemisphere (with the two SKA (Square Kilometre Array) precursors, meerKAT (Karoo Array Telescope) and ASKAP (Australian SKA Pathfinder), leading to the SKA itself) it is clear that interferometry and VLBI in the southern hemisphere need a dense network of calibration sources at different resolutions and a range of frequencies. This work seeks to help redress this problem by presenting an analysis of 31 southern sources to help fill the gaps in the southern hemisphere calibrator distribution. We have developed a multi-parameter method of classifying these sources as calibrators. From our sample of 31 sources, we have 2 class A sources (Excellent calibrators), 16 class B sources (Good calibrators), 9 class C sources (Poor calibrators) and 4 class D sources (Unsuitable calibrators).
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Higgins, Stephen William. "Numerical simulations of jet-cloud collisions and the structure of extragalactic radio sources." Thesis, Liverpool John Moores University, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.244791.
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Anderson, Craig Stuart. "Broadband Radio Polarimetry as a Probe of Magnetised Plasma Towards Powerful Radio Sources." Thesis, The University of Sydney, 2016. http://hdl.handle.net/2123/16014.
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This thesis considers whether “Faraday complexity” — information imprinted as a frequency-dependent signal on linearly polarised radiation by Faraday rotation as it propagates through magnetised cosmic plasmas — can be exploited as an effective probe of magneto-ionised structure in active galactic nuclei (AGNs), radio galaxies, and intervening material. A blind search for Faraday complexity amongst 563 radio sources over 1.3–2 GHz results in a signal-to-noise-limited detection rate of 12%. The Faraday-complex sources are found to preferentially lie behind magnetised, turbulent interfaces between neutral and ionised gas in the Galactic interstellar medium. A targeted survey of 36 objects over 1.3–10 GHz reveals striking Faraday complexity in all of the sample sources. The characteristics of the radio emission, including the spectral index of the radio emission and its linear extent, temporal variation in polarisation, and the Faraday-thickness of emission components, suggests that Faraday complexity arises in the AGNs themselves at these frequencies. The data supply strong constraints on the global magnetised structure of AGN and their jets. Observations of the nearby radio galaxy Fornax A over 1.3–3.1 GHz reveal complex frequency-dependent polarisation structure. Filaments of strong depolarisation are revealed to be real physical structures, likely caused by Kelvin-Helmholtz and/or Rayleigh-Taylor instabilities at the lobe / intergalactic medium interface. Faraday complexity also exists away from the depolarised filaments in the body of the lobe, and I argue that this must be caused by thermal plasma in the lobe that is well-separated from the synchrotron-emitting plasma. I conclude that broadband observations of Faraday complexity do indeed provide a singular probe of magnetised plasmas in diverse cosmic environments.
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Franzen, Thomas Matthew Owen. "Radio sources contaminating the CMB at 1 to 2 cm." Thesis, University of Cambridge, 2011. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.609365.
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Wheaton, Vivienne. "Investigation of radio emission from Supernova Remnant 1987A." Thesis, The University of Sydney, 2002. https://hdl.handle.net/2123/27884.
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This thesis investigates the recent radio emission from the remnant of Supernova
1987A in the Large Magellanic Cloud. Radio emission from this developing supernova
remnant was detected in June 1990, just over three years after the initial
outburst from the supernova explosion. Monitored with the Molonglo Observatory
Synthesis Telescope and the Australia Telescope Compact Array, the SNR has been
increasing in radio brightness to the present day. Observations from the Hubble
Space Telescope, ROSAT and Chandra confirm that the remnant is also brightening
in optical and X-ray wavelengths. The continued brightening has been understood
as the interaction of the expanding supernova shock front with density structures
from the progenitor’s stellar wind. The hydrodynamical processes involved in the
expansion of shock fronts in supernova remnants are modelled using self similar solutions.
Such solutions are used in constructing scaling models to describe radio
emission from supernova remnants. Assumptions need to be made about the way
physical parameters are scaled to fit observed emission. Variations of the “minishell”
scaling models (Chevalier 1996) are applied to the observed radio emission
from SNR 1987A in order to generate plausible scalings of the magnetic field and
electron energy distribution which combine to create the observed synchrotron emission.
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Mguda, Zolile Martin. "Bent tail radio sources as tracers of galaxy clusters at high redshift and SMBH mass estimates." Doctoral thesis, Faculty of Science, 2021. http://hdl.handle.net/11427/33807.
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Bent tail radio sources (BTRSs) are radio galaxies which have jets that show a characteristic C‐shape that is believed to be due to ram pressure caused by the motion of the galaxy through the ambient medium. They are generally found in galaxy clusters in the local Universe. They have already been used in observations as tracers of galaxy clusters at redshifts of up to z _ 1. They have, however, been shown to be numerous in galaxy groups as well. The ability to find high redshift galaxy clusters is important in cosmology because they are important cosmological probes. According to the _ CDM model, galaxy clusters form around redshift of z _ 2 and finding clusters of halo mass greater than 1014 M_ at redshift greater than z = 2:5 would disprove the current concordance model. Finding galaxy clusters at those redshifts is more feasible with the new generation of radio telescopes and the upcoming square kilometer array (SKA). In this work we look at some SMBH mass measurements, which are crucial in the determination of the correlations between the SMBH mass and some galaxy characteristics including jet length and luminosity. The high redshift SMBH mass measurement methods are calibrated using local Universe correlations. This makes SMBH mass measurement an important aspect in the study of high redshift radio galaxies and hence BTRSs. We use cosmological simulations from the MareNostrum Universe simulation to look at the efficacy of using BTRSs as tracers of clusters assuming the ram pressure is the cause of the jet bending. This is the first step in predicting the possible number of BTRSs that we may observe with the SKA. We find that SMBH masses can be measured up to redshift of z = 4:5 using the virial mass estimator method. The BTRSs are equally likely to be found in galaxy clusters and galaxy groups in the local Universe. This means that around 50% of the BTRSs that we are likely to find at high redshift will be in galaxy clusters. However, finding a pair of BTRSs in close proximity is a sign of a galaxy cluster environment. These results are still dependent on the resolution of degeneracies in our understanding of the duty cycles of AGN radio jets, projection effects of the radio jets, the environmental dependence of radio‐loudness in galaxies and other open questions.
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McAlpine, Kim. "The SKA's the limit : on the nature of faint radio sources." Thesis, Rhodes University, 2012. http://hdl.handle.net/10962/d1007271.
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From abstract: Within the next few years a large number of new and vastly more sensitive radio astronomy facilities are scheduled to come online. These new facilities will map large areas of the sky to unprecedented depths and transform radio astronomy into the leading technique for investigating the complex processes which govern the formation and evolution of galaxies. This thesis combines multi-wavelength techniques, highly relevant to future deep radio surveys, to study the evolution and properties of faint radio sources.TeX
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Poulsen, Andrew Joseph. "Real-time Adaptive Cancellation of Satellite Interference in Radio Astronomy." Diss., CLICK HERE for online access, 2003. http://contentdm.lib.byu.edu/ETD/image/etd238.pdf.
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Anderson, Martin William Bruce. "A radio survey of selected fields from the ROSAT All Sky Survey /." View thesis, 2002. http://library.uws.edu.au/adt-NUWS/public/adt-NUWS20030704.133327/index.html.
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Draper, Aden R. "Understanding the connection between active galactic nuclei and host star formation through multi-wavelength population synthesis modeling." Diss., Georgia Institute of Technology, 2012. http://hdl.handle.net/1853/45748.
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Supermassive black holes, black holes with masses <106 Msun, are found at the centers of all massive galaxies. These massive black holes grew from smaller seed black holes through accretion events. Accreting black holes are very bright in the radio through very hard X-ray spectral regimes. Due to the location of these accreting black holes at the centers of galaxies, they are referred to as active galactic nuclei (AGN). It is understood that AGN are an important phase of galaxy evolution; however, the role of AGN in massive galaxy formation is very poorly constrained. Here, the unique tool of multi-wavelength population synthesis modeling is used to study the average properties of AGN and their host galaxies with a focus on host galaxy star formation and the role of black hole growth in galaxy evolution. Knowledge of the AGN population from deep X-ray surveys is combined with theoretical AGN spectral energy distributions to predict various observables of the AGN population in wavelength regions from the far infrared to very hard X-rays. Comparison of the model predictions to observations constrains the model input parameters and allows for the determination of average properties of the AGN population. Particular attention is paid to a special class of AGN known as Compton thick AGN. These AGN are deeply embedded in gas and dust such that the column density obscuring the line of sight to the central engine of the AGN exceeds 1/σT ~ 10²⁴ cm⁻², where σT is the Thomson cross-section of the electron---a column density comparable to that of the human chest. Theoretical and simulational evidence suggest that these Compton thick AGN may be recently triggered, rapidly accreting AGN, making them of special interest to researchers. I found that Compton thick AGN are likely to contribute ~20% of the peak of the cosmic X-ray background (XRB) at ~30 keV and demonstrated that a significant portion of Compton thick AGN may be accreting very rapidly. Moreover, Compton thick AGN do not appear to follow the orientation based unified model of AGN. According to the unified model, AGN exhibit a range of obscuration levels due to a dusty 'torus' which, depending on the orientation of the torus to the observer's line of sight, may obscure the central engine of the AGN. Upon further investigation into the stellar populations of AGN host galaxies, it appears that the unified model holds in general at z < 1, but not at z > 1. I found that this is likely due to the dominant triggering mechanism of AGN switching from major mergers at z > 1.5 to secular processes by z ~ 1.
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Galluzzi, Vincenzo <1987>. "Multi-frequency polarimetric study of a complete sample of extragalactic radio sources: radio source populations and cosmological perspectives." Doctoral thesis, Alma Mater Studiorum - Università di Bologna, 2018. http://amsdottorato.unibo.it/8650/1/Tesi_PhD_Galluzzi_Vincenzo.pdf.
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The high-frequency (> 20 GHz), bright flux density (> 200 mJy) radio population is dominated by blazars, i.e. compact Doppler-boosted objects, whose emission arises from knot-like synchrotron structures along the relativistic jet. Their polarization properties was so far poorly constrained at high frequency and results in literature are typically biased.
Extending the characterization of polarization properties of radio sources to high frequencies provides invaluable information about magnetic fields and plasma in the inner and unresolved regions of relativistic jets. Furthermore, extragalactic radio sources are an important contaminant for the angular power spectrum of CMB (Cosmic Microwave Background) at scales smaller than 30 arcmin, up to 100 GHz: their polarimetry is crucial to search for primordial B-modes, the footprint of inflation.
We present the analysis of high sensitivity (rms ~ 0.6 mJy) multi-frequency and multi-epoch polarimetric observations of a complete sample of 104 compact extragalactic radio sources drawn from the faint (> 200 mJy at 20 GHz in total intensity) Planck-ATCA Coeval Observations (PACO) catalogue, performed with the Australia Telescope Compact Array (ATCA) at 7 frequencies, between 2.1 and 38 GHz. We found that polarization spectra of single sources cannot be simply inferred from total intensity ones, as different synchrotron components dominate the different emissions. We distinguish six spectral categories finding different behaviours in polarization fractions and position angles. Multi-epoch variability analysis (from 1.5 up to 10 yr time lags) is also presented.
ALMA (Atacama Large Millimetre and sub-millimetre Array) observations extends the analysis up to 100 GHz for a (complete) sub-sample of 32 objects. We still find synchrotron signal at these frequencies with no sign of thermal or dust emissions.
We compute number counts in polarization at 20 and 100 GHz to a deeper level than available so far and provide forecasts for forthcoming CMB experiments.
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Brady, Emer [Verfasser], and Hans-Walter [Akademischer Betreuer] Rix. "Radio Sources in the COSMOS Field: Star Forming Properties of High Luminosity Radio Sources / Emer Brady ; Betreuer: Hans-Walter Rix." Heidelberg : Universitätsbibliothek Heidelberg, 2016. http://d-nb.info/1180737245/34.
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Ricci, Roberto. "High-frequency properties of extragalactic radio sources." Doctoral thesis, SISSA, 2004. http://hdl.handle.net/20.500.11767/3995.
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Tinti, Sara. "The demography of the youngest radio sources." Doctoral thesis, SISSA, 2005. http://hdl.handle.net/20.500.11767/4034.
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In the framework of the evolutionary model for radio sources mentioned above and described in detail in Chapter 2, the well defined anti-correlation between the radio turnover frequency, Vp, and the projected angular size, O (O'Dea & Baum 1997; Fanti et al. 2002) means that the youngest objects have the highest turnover frequencies and that the
peak frequency is expected to move towards lower frequencies as the source expands and the energy density decreases. In principle, the highest the turnover frequency, the youngest the radio source is.
The aim of this thesis is to search and study \extreme" GPS sources, with spectral very close to their birth. These objects, however, are likely to be very rare because their expansion rate, and therefore the rate of decrease of their peak frequency, is thought to be very rapid. The shape of the radio spectrum and the turnover frequency have been used as selection tools for this class of objects. A complete sample of bright candidate GPS sources peaking above ' 5 GHz has been obtained by Dallacasa et al. (2000), who termed such sources \High Frequency Peakers" (HFPs). Only a fraction of their objects are expected to be truly newborn sources since the sample is contaminated by beamed,
at-spectrum quasars (see also Snellen et al. 1999, Stanghellini et al. 2003) caught during a flaring phase of a strongly self-absorbed component that dominates the radio spectrum. Since the process that characterizes both
flaring blazars and young radio sources is the expansion of an emitting blob in the ISM, i.e. the lobes in the case of young sources, a knot of the jet that dominates the radio spectrum for blazars, it is quite diffcult to distinguish the two phenomena with a single observation. Howeever, their time evolution has quite different timescales: GPS/CSS sources are thought to expand at mildly relativistic velocities with marginal or no influence on the radio spectrum while large Doppler factors in blazars boost the variability amplitudes in the spectrum and decrease the corrisponding observed timescales. In order to disentangle the two classes in our sample we have carried out various observational projects (described in Chapter 3) to:
- study the spectral shape evolution and the variability properties of the sample;
- determine the polarisation properties;
- look for extended emission that may indicate that we are really dealing with an evolved-old source but can also be the relic of previous large scale radio activity in a re-born radio source;
- study the milliarcsecond morphology of the candidates.
The information on variability of HFP candidates, on their morphology at different resolutions and the polarimetric study can help us to define criteria to discriminate between the two classes of objects. In Chapter 4 we discuss various criteria and the consistency of their indications on the source nature.
Since it has been proved that the incidence of blazar objects in a spectral selected sample increases with increasing selection frequency and, as a consequence, with increasing turnover frequency (Stanghellini 2003), this kind of analysis is crucial to select truly young radio sources. Clearly, a serious blazar contamination of GPS samples may lead astray analyses of their statistical properties and implies that evolutionary models based on them need to be reconsidered. To effectively explore the luminosity and peak frequency evolution of GPS sources, we need samples that provide a wide coverage of the turnover frequency and flux density (Vp-Sp) plane. In Chapter 5 we give an account of the data sets we used. We also analyze the effect of the different selection criteria adopted to define the samples. We have finally studied an evolutionary model in the framework of the self-similar evolution scenario by Fanti et al. (1995) and Begelman (1996) comparing the redshift and peak frequency distribution yielded by the model with the observed ones. A summary of the main results are given in Chapter 6.
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Labiano, Ortega Álvaro. "Host galaxies and environments of compact extragalactic radio sources." [S.l. : [Groningen : s.n.] ; University Library Groningen] [Host], 2006. http://irs.ub.rug.nl/ppn/29144461X.
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Bonavera, Laura. "Spectra of extragalactic radio sources after Planck." Doctoral thesis, SISSA, 2011. http://hdl.handle.net/20.500.11767/4626.
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Orienti, Monica <1979>. "On the nature of High Frequency Peaker radio sources." Doctoral thesis, Alma Mater Studiorum - Università di Bologna, 2007. http://amsdottorato.unibo.it/363/1/tesi_Orienti.pdf.
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Orienti, Monica <1979>. "On the nature of High Frequency Peaker radio sources." Doctoral thesis, Alma Mater Studiorum - Università di Bologna, 2007. http://amsdottorato.unibo.it/363/.
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Whittam, Imogen Helen. "Exploring the faint source population at 15.7 GHz." Thesis, University of Cambridge, 2014. https://www.repository.cam.ac.uk/handle/1810/246264.
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A sample of 296 faint (> 0.5 mJy) radio sources is selected from an area of the Tenth Cambridge (10C) survey at 15.7 GHz in the Lockman Hole. The 10C survey is complete to 0.5 mJy at 15.7 GHz and has a resolution of 30 arcsec. By matching this catalogue to several lower frequency surveys (e.g. including a deep GMRT survey at 610 MHz, a WSRT survey at 1.4 GHz, NVSS, FIRST and WENSS) I have investigated the radio spectral properties of the sources in this sample; all but 30 of the 10C sources are matched to a source in one or more of these surveys. I have found a significant increase in the proportion of flat spectrum sources at flux densities below 1 mJy – the median spectral index between 15.7 GHz and 610 MHz changes from 0.75 for flux densities greater than 1.5 mJy to 0.08 for flux densities less than 0.8 mJy. Thus a population of faint, flat spectrum sources is emerging at flux densities greater than approximately 1 mJy. The spectral index distribution of this sample of sources selected at 15.7 GHz is compared to those of two samples selected at 1.4 GHz from FIRST and NVSS. I find that there is a significant flat spectrum population present in the 10C sample which is missing from the samples selected at 1.4 GHz. The 10C sample is compared to a sample of sources selected from the SKADS Simulated Sky by Wilman et al.; this simulation fails to reproduce the observed spectral index distribution and significantly under predicts the number of sources in the faintest flux density bin. I conclude that it is likely that the observed faint, flat spectrum sources are a result of the cores of FRI sources becoming dominant at high frequencies, rather than the emergence of a new population of starforming galaxies. I have used recent Very Long Baseline Interferometry (VLBI) observations by Middleberg et al. with a resolution of 10 mas to investigate the properties of these faint 10C sources in the Lockman Hole and find that 33 out of the 51 10C sources in the VLBI field (65 percent) are detected by the VLBI observations. The high brightness temperature of these VLBI-detected sources rules out the possibility that this faint, high frequency population is dominated by starbursting or starforming sources and indicates that they must be Active Galactic Nuclei. The sources in the Lockman Hole 10C sample are matched to optical, infrared and Xray data available in the field. A complete sample of 96 sources with high-resolution radio information available is defined; multi-wavelength counterparts are identified for 80 out of the 96 sources in this sample, for which is it possible to derive photometric redshifts. The radioto- optical ratios of these sources show that the 10C sample is almost completely dominated by radio galaxies. 59/80 sources have luminosities greater than the FRI/FRII dividing luminosity. The nature of these radio galaxies is investigated, using the multi-wavelength data to split the sources into high-excitation and low-excitation radio galaxies (HERGs and LERGs respectively). This shows that 34 sources are probably HERGs and 33 are probably LERGs, with 29 which could not be classified at this stage. The properties of these HERGs and LERGs are compared and I find that the HERGs tend to be found at higher redshifts, have flatter spectra, higher flux densities and smaller linear sizes. This study is extended to lower flux densities using new, very deep, observations made with the Arcminute Microkelvin Imager in two fields. I use these observations to extend the 15.7-GHz source count down to 0.1 mJy, a factor of five deeper than the 10C count. These new deeper counts are consistent with the extrapolation of the fit to the 10C count, and do not show any evidence for an upturn. There is therefore no evidence for a new population (e.g. of starforming sources) contributing to the 15.7 GHz source count above 0.1 mJy, and suggesting that the faint, high-frequency population continues to be dominated by radio galaxies. Recent models of the high-frequency source counts under-predict the number of sources observed by a factor of two, consistent with the fact that these models fail to include the dominance of the cores and the faintness of the extended structures of these sources.
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Sol, Hélène. "Jets et sources radio extragalactiques." Paris 7, 1987. http://www.theses.fr/1987PA077163.
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Travaux sur les proprietes radiatives et dynamiques des jets associes a des galaxies ou a des quasars. L'etude du jet optique de la galaxie active pks 0521-36 est abordee en detail. On decrit ensuite le passage d'un jet vlbi de quelques pc a un jet observe a grande echelle sur plusieurs dizaines de kpc. On traite, sur ce sujet, des mecanismes physiques qui entrent en jeu pour interpreter ce phenomene. En derniere partie on pose le probleme de la determination de la constante de hubble en termes de mirages gravitationnels et celui des jets d'echelle stellaire
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Elmer, Michael James. "Improved Methods for Phased Array Feed Beamforming in Single Dish Radio Astronomy." BYU ScholarsArchive, 2012. https://scholarsarchive.byu.edu/etd/3340.
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Among the research topics needing to be addressed to further the development of phased array feeds (PAFs) for radio astronomical use are challenges associated with calibration, beamforming, and imaging for single dish observations. This dissertation addresses these concerns by providing analysis and solutions that provide a clearer understanding of the effort required to implement PAFs for complex scientific research. It is shown that calibration data are relatively stable over a period of five days and may still be adequate after 70 days. A calibration update system is presented with the potential to refresh old calibrators. Direction-dependent variations have a much greater affect on calibration stability than temporal variations. There is an inherent trade-off in beamformer design between achieving high sensitivity and maintaining beam pattern stability. A hybrid beamformer design is introduced which uses a numerical optimizer to balance the trade-off between these two conflicting goals to provide the greatest sensitivity for a desired amount of pattern control. Relative beam variations that occur when electronically steering beams in the field of view must be reduced in order for a PAF to be useful for source detection and imaging. A dual constraint beamformer is presented that has the ability to simultaneously achieve a uniform main beam gain and specified noise response across all beams. This alone does not reduce the beam variations but it eliminates one aspect of the problem. Incorporating spillover noise control through the use of rim calibrators is shown to reduce the variations between beams. Combining the dual constraint and rim constraint beamformers offers a beamforming option that provides both of these benefits.
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Cau, Massimo <1968>. "New detections and statistics of diffuse radio sources in galaxy clusters." Doctoral thesis, Alma Mater Studiorum - Università di Bologna, 2019. http://amsdottorato.unibo.it/9066/1/Cau_Massimo_tesi.pdf.
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Galaxy clusters are the largest gravitationally bound structures of the Universe, formed from density fluctuations and grown hierarchically through the extreme process of merging and mass accretion. They represent very interesting tools to study the cosmology and the evolution of large scale structures. Synchrotron non-thermal emission detected in the forms of radio halos, relics and minihalos according to their morphology, size and location is linked to the different dynamical state of the hosting clusters. Halos and relics are detected in clusters characterized by a strong merger activity and a dynamical disturbed state, while minihalos are present only in relaxed cool-core clusters.
In spite of many encouraging results obtained up to now, the occurrence and the luminosity function of diffuse radio sources with the redshift are still unknown: present data are strongly limited to nearby clusters (z < 0.2 − 0.3).
The aim of this PhD thesis is mainly to investigate the evolutionary history of non-thermal properties of galaxy clusters and to determine whether the correlations observed at low redshift evolve with time. To attempt this ambitious goal we selected an homogeneous sample of 44 massive and high X-ray luminous galaxy clusters in the redshift range
0.3 ≤ z < 0.7, extracted from the Ebeling MAssive Cluster Survey (MACS). We have undertaken an observational campaign on this sample with the JVLA in L-band, C and D configurations.
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Huerta, Escudero Eliu Antonio. "Source modelling of extreme and intermediate mass ratio inspirals." Thesis, University of Cambridge, 2011. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.609770.
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Herzog, Andreas [Verfasser], Dominik J. [Akademischer Betreuer] Bomans, and Marcus [Akademischer Betreuer] Brüggen. "The broadband spectra of infrared-faint radio sources / Andreas Herzog. Gutachter: Dominik J. Bomans ; Marcus Brüggen." Bochum : Ruhr-Universität Bochum, 2016. http://d-nb.info/1089006764/34.
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Brady, James Michael. "Real-Time Beamformer Development and Analysis of Weak Signal Detection with Interference Mitigation for Phased-Array Feed Radio Astronomy." BYU ScholarsArchive, 2016. https://scholarsarchive.byu.edu/etd/5644.
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In recent years, the Brigham Young University (BYU) Radio Astronomy Systems group has developed phased-array feeds and the data acquisition processing systems necessary to perform radio astronomy observations. This thesis describes the development and testing of a real-time digital beamforming system that reduces both the time required to process phased-array feed data and the disk space used to record this data compared to post-processing beamforming systems. A real-data experiment is also discussed in this thesis, which focuses on some of the data post-processing required for one of BYU's data acquisition systems.Radio-frequency interference mitigation techniques for phased-array feed radio astronomy have been studied for several years, but the effect that these techniques have on weak-signal detection is not well understood. This thesis provides analysis of a simulated weak-source observation for the Green Bank 20-meter telescope and BYU 19 element phasedarray feed with radio-frequency interference present. Interference mitigation techniques are shown to reduce the detectability of weak sources compared with the no interference case, but it is also shown that a weak source can be detected that would otherwise be masked by interference.
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Decoene, Valentin. "Sources and detection of high-energy cosmic events." Thesis, Sorbonne université, 2020. http://www.theses.fr/2020SORUS028.
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Ces dernières années, de nouvelles classes de sources (sursaut radio rapide, fusion d’étoiles à neutron, etc…) ont été découvertes grâce aux progrès de l’astronomie photonique à toutes les longueurs d’ondes. Avec l’avènement de l’astronomie multi-messager, nous avons été capable de les scruter à travers les rayons cosmiques, les neutrinos, les photons et les ondes gravitationnelles. La première partie de cette thèse est dédiée à l’étude des neutrinos de hautes et ultra hautes énergies. Depuis la découverte en 2013 par IceCube d’un flux diffus de neutrinos, deux questions centrales sont apparues : quelles sont les sources de ce flux diffus, et pouvons-nous détecter des neutrinos à plus haute énergie encore (>PeV). Afin d’aider à répondre à ces questions, nous présentons en premier un modèle d’émission de neutrinos de hautes énergies résultant de la fusion d’étoiles à neutron. Nous examinons ensuite la possibilité de radio détecter les gerbes atmosphériques induites par les neutrinos d’ultra hautes énergies, à travers une étude sur l’optimisation d’un tel détecteur, et une analyse détaillée des caractéristiques du signal et de sa reconstruction. La deuxième partie de cette thèse est dédiée à l’étude des sursauts radio rapides (FRB). Ces pulses radio, brefs, cohérents et nombreux, n’ont pas été identifiés encore et de nombreuses inconnues expérimentales persistent. En premier, nous proposons un modèle de source expliquant les taux de sursauts observés. Enfin, nous présentons un programme observationnel, conduit par l’instrument NenuFAR, situé à la Station de Radio Astronomy de Nançay, dédié à l’observation des FRB aux basses fréquences (100MHz)In the last decade, new classes of sources (fast radio burst, binary neutron star mergers, ...) have been discovered thanks to progress in photon astronomy at all wavelengths. With the advent of multimessenger astronomy, we were able to scrutinize them with cosmic rays, neutrinos, photons and gravitational waves. The first part of this thesis is dedicated to the study of the high-energy and ultra-high energy neutrinos. Since the discovery in 2013 by IceCube of a diffuse neutrino flux, two major questions have been raised: what are the sources of this diffuse flux, and can we detect neutrinos at higher energies (> PeV). To help answering these questions, we first present a model of high-energy neutrino emissions from neutron star mergers. We then examine the radio detection of extensive air showers induced by ultra-high-energy neutrinos, with a study of a detector optimisation, and a detailed analysis of the signal characteristics and develops a reconstruction procedure. The second part of this thesis is dedicated to the study of the fast radio bursts. These brief, coherent and numerous radio pulses, have not been identified yet and many experimental unknowns remain. We first propose a source model focusing on the question of the pulses rates. Finally, we present an observational program conducted with the NenuFAR instrument, located in the Nançay Radio Astronomy Station, dedicated to the detection and the characterisation of FRBs at low frequency (<100MHz)
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Ludovici, Dominic Alesio. "Radio wavelength studies of the Galactic Center source N3, spectroscopic instrumentation for robotic telescope systems, and developing active learning activities for astronomy laboratory courses." Diss., University of Iowa, 2017. https://ir.uiowa.edu/etd/5557.
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The mysterious radio source N3 appears to be located within the vicinity of the Radio Arc region of the Galactic Center. To investigate the nature of this source, we have conducted radio observations with the VLA and the VLBA. Continuum observations between 2 and 50 GHz reveal that N3 is an extremely compact and bright source with a non-thermal spectrum. Molecular line observations with the VLA reveal a compact molecular cloud adjacent to N3 in projection. The properties of this cloud are consistent with other galactic center clouds. We are able to rule out several hypotheses for the nature of N3, though a micro-blazar origin cannot be ruled out.
Robotic Telescope systems are now seeing widespread deployment as both teaching and research instruments. While these systems have traditionally been able to produce high quality images, these systems have lacked the capability to conduct spectroscopic observations. To enable spectroscopic observations on the Iowa Robotic Observatory, we have developed a low cost (~ $500), low resolution (R ~ 300) spectrometer which mounts inside a modified filter wheel and a moderate cost (~ $5000), medium resolution (R ~ 8000) fiber-fed spectrometer. Software has been developed to operate both instruments robotically and calibration pipelines are being developed to automate calibration of the data.
The University of Iowa offers several introductory astronomy laboratory courses taken by many hundreds of students each semester. To improve student learning in these laboratory courses, we have worked to integrate active learning into laboratory activities. We present the pedagogical approaches used to develop and update the laboratory activities and present an inventory of the current laboratory exercises. Using the inventory, we make observations of the strengths and weaknesses of the current exercises and provide suggestions for future refinement of the astronomy laboratory curriculum.
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Nair, Dhanya G. [Verfasser], Andreas [Gutachter] Eckart, and J. Anton [Gutachter] Zensus. "Global Millimeter VLBI Array Survey of Ultracompact Extragalactic Radio Sources at 86 GHz / Dhanya G. Nair ; Gutachter: Andreas Eckart, J. Anton Zensus." Köln : Universitäts- und Stadtbibliothek Köln, 2018. http://d-nb.info/1180601548/34.
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Herrera, Ruiz Carmen Noelia [Verfasser], Ralf-Jürgen [Gutachter] Dettmar, and Eduardo [Gutachter] Ros. "The incidence and evolution of active galactic nuclei in 2865 COSMOS radio sources / Carmen Noelia Herrera Ruiz ; Gutachter: Ralf-Jürgen Dettmar, Eduardo Ros." Bochum : Ruhr-Universität Bochum, 2017. http://d-nb.info/1137380268/34.
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PAONESSA, FABIO. "VHF/UHF Antenna Array Measurements with a UAV-based Test Source." Doctoral thesis, Politecnico di Torino, 2017. http://hdl.handle.net/11583/2687180.
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Low-frequency arrays represent the new generation of radio astronomical facilities capable to expand the frontiers of the Universe observation from the Earth. The most important example is the Square Kilometre Array (SKA), which will become the world’s largest radio telescope upon its completion in the 2020s. Several arrays and thousands of antennas operating from 50 MHz to 20 GHz will compose the SKA, producing a total collecting area of several square kilometers. Besides the outstanding performance in terms of resolution and sensitivity, the advantage of antenna arrays is the great flexibility. In fact, the signals of all the array elements can be digitally combined and treated in different ways, e.g. from only one large group to multiple smaller ones. Moreover, a digital beam-forming system controls the observation direction that can be rapidly steered across the sky.
These complex instruments require advanced procedure to test their front-end electronics. In addition, the electromagnetic characterization and the array calibration are challenging tasks especially at lower frequencies. The strong interaction between the array elements, and with the surrounding environment as well, produces effects that can be difficult to predict during the design stage. Therefore, advanced in-situ measurement procedures are required to validate the facility. In the context of the Aperture Array Verification Program (AAVP) of the SKA telescope, a test-source mounted on an Unmanned Aerial Vehicle (UAV) has been conceived in Italy within a collaboration between the Institute of Electronics, Computer and Telecommunication Engineering (IEIIT) of the Italian National Research Council (CNR), the Department of Environment, Land and Infrastructure Engineering (DIATI) of the Polytechnic of Turin and the Italian National Institute for Astrophysics (INAF).
In this thesis work, the concept of a UAV-based VHF/UHF far-field test-source to perform in-situ antenna array measurements has been developed into a real system that has been used in several measurement campaigns around the world on some of the SKA pathfinders. Several relevant aspects such as measurement strategies, data processing, system design, modeling, integration and test have been addressed in great detail.
A detailed data analysis has been performed after all the above mentioned campaigns. The obtained results demonstrate that the UAV-system provides good accuracy and effectiveness to characterize the radiation pattern and the polarization properties of the single antennas, the embedded elements and the whole array. The provided data are already being used by the radio astronomical scientific community to improve both the data set of the existing telescopes and the design of the new ones. The UAV has been also used as calibration source for the digital beam-forming system, providing a direct solution with a higher signal-to-noise ratio with respect to the exploitation of astronomical sources. The versatility and the transportability of the system open up a wide variety of possibilities for antenna measurement in many other applications.
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Lima, Alexsandro Pereira. "Modelos anal?ticos para r?dio fontes extragal?cticas." Universidade Federal do Rio Grande do Norte, 2007. http://repositorio.ufrn.br:8080/jspui/handle/123456789/16526.
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The great majority of analytical models for extragalactic radio sources suppose self-similarity and can be classified into three types: I, II and III. We have developed a model that represents a generalization of most models found in the literature and showed that these three types are particular cases. The model assumes that the area of the head of the jet varies with the jet size according to a power law and the jet luminosity is a function of time. As it is usually done, the basic hypothesis is that there is an equilibrium between the pressure exerted both by the head of the jet and the cocoon walls and the ram pressure of the ambient medium. The equilibrium equations and energy conservation equation allow us to express the size and width of the source and the pressure in the cocoon as a power law and find the respective exponents. All these assumptions can be used to calculate the evolution of the source size, width and radio luminosity. This can then be compared with the observed width-size relation for radio lobes and the power-size (P-D) diagram of both compact (GPS and CSS) and extended sources from the 3CR catalogue. In this work we introduce two important improvement as compared with a previous work: (1)We have put together a larger sample of both compact and extended radio sources
A grande maioria dos modelos anal?ticos de r?dio fontes extragal?cticas utilizam a hip?tese de auto-similaridade, onde os modelos podem ser classificados em tr?s tipos: I, II e III. N?s desenvolvemos um modelo anal?tico que representa uma generaliza??o dos modelos existentes na literatura e mostramos que os tr?s tipos s?o casos particulares desse modelo. O modelo assume que a ?rea da cabe?a do jato varia com seu tamanho de acordo com uma lei de pot?ncia e que a luminosidade do jato ? uma fun??o do tempo. A hip?tese b?sica comumente usada ? a de que a cabe?a do jato e o casulo se expandem em equil?brio de press?o com o meio ambiente. As equa??es de equil?brio e conserva??o da energia permitem que expressemos o tamanho e largura da fonte e a press?o no casulo como uma lei de pot?ncia e encontremos seus respectivos expoentes. Todas essas suposi??es podem ser usadas para calcular a evolu??o do tamanho e comprimento da fonte e de sua luminosidade r?dio. Podemos comparar esses resultados com as rela??es observadas da largura-tamanho para r?dio l?bulos e diagrama pot?ncia-tamanho (P-D) de fontes compactas (GPS e CSS) e de fontes extensas do cat?logo 3CR. Neste trabalho introduzimos duas importantes melhorias em rela??o `a trabalhos anteriores: (1) Coletamos uma grande amostra de r?dio fontes compactas e extensas com raios de hot spots conhecidos de forma a estabelecer a rela??o entre a ?rea da cabe?a e o tamanho da ponte; (2) O n?mero de fontes em nossa amostra com a largura da ponte medida aumentou consideravelmente em rela??o a trabalhos anteriores. Isso nos permitiu aumentar a gama de tamanhos de fontes em nossa an?lise. Essa compara??o nos possibilita determinar os v?rios par?metros do modelo e entendermos os processos f?sicos envolvidos no fen?meno da evolu??o das r?dio fontes extragal?cticas
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Mancini, Pires Adriana. "Population study of radio-quiet and thermally emitting isolated neutron stars." Strasbourg, 2009. https://publication-theses.unistra.fr/public/theses_doctorat/2009/MANCINI_PIRES_Adriana_2009.pdf.
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The main objective of the thesis is to study the properties of the Galactic population of radio-quiet and thermally emitting isolated neutron stars (INSs). This is done by studying further the existing neutron star sample of nearby seven sources, known as the “Magnificent Seven” (M7), as well as by searching for new candidates and constraining possible populations. During the thesis, we investigated the proper motions of three of the faintest M7 in X-rays with the satellite Chandra. This work allowed us to constrain the neutron star displacement in two cases as well as to accurately determine the high proper motion of a third source, for the first time in X-rays with a significance approaching 10 standard deviations (Motch, Pires, Haberl, & Schwope, 2007, Ap&SS, 308, 217; Motch, Pires, Haberl, Schwope, & Zavlin, 2009, A&A 497, 423). The search of new INS candidates in the serendipitous catalogue of the XMM-Newton Observatory, with more than 120; 000 X-ray sources, had as well the aim to constrain the spatial density of thermally emitting sources located beyond the solar vicinity. This work allowed the long awaited discovery of a new thermally emitting INS with properties similar to those of the seven nearby sources discovered by ROSAT (Pires, Motch, Turolla, Treves, & Popov, 2009, A&A 498, 233). Moreover, deep optical observations with SOAR and the ESO-VLT have been obtained during the thesis work in order to optically identify a handful of INS candidates that have been selected among more than 72; 000 sources (Pires, Motch, & Janot-Pacheco, 2009, A&A, 504, 185). Finally, population synthesis of Galactic thermally emitting INSs allows constraining the global properties of this population based on the whole sample of XMM-Newton observations. By estimating the density of similar sources at more remote distances in the Milky Way, the final objective is to determine whether the spatial density derived from the group of seven nearby sources is a local anomaly caused by the Sun’s current location near regions of active stellar formation of the Gould BeltLa présente thèse de doctorat porte sur la population d'étoiles a neutrons isolées thermiques dénuées d'émission radio dans la Galaxie. Les mouvements propres de trois étoiles à neutrons ont été étudiés avec le satellite Chandra. Ce travail a permis de contraindre le déplacement dans deux cas et a mis en évidence pour la première fois dans le domaine des rayons X le mouvement propre d'une troisième source (Motch, Pires et al. 2007, Ap&SS, 308, 217; Motch, Pires el al. 2009, A&A 497, 423). La recherche de nouveaux candidats dans le catalogue du satellite XMM~Newton, avec plus de 120 mille sources, a eu également comme but de contraindre la densité spatiale des sources X thermiques situées à grandes distances. Ce travail a mené a la découverte très attendue d'une nouvelle étoile à neutrons isolée (Pires, Motch et al. 2009, A&A 498, 233). En outre, des observations optiques profondes ont été utilisées pour identifier l'échantillon des candidats qui ont été sélectionnées parmi plus de 72 milles sources (Pires, Motch and Janot-Pacheco, 2009, A&A, 504, 185). Finalement, le travail de modélisation permet de contraindre les propriétés globales de cette population a partir du relevé constitue par l'ensemble des observations failes par XMM-Newton. Ce travail a pour but de déterminer si la densité spatiale déduite du groupe de sept étoiles connues est une anomalie causée par notre position actuelle proche des zones actives de formation d'étoiles de la ceinture de Gould en estimant la densité des sources similaires dans la Voie Lactée à plus grande distance
O objetivo da tese é estudar as propriedades da população Galáctica de estrelas de nêutrons isoladas com emissão térmica em raios X mas silenciosas em rádio. Isto é feito investigando-se a amostra existente de sete fontes próximas, conhecidas como “Magnificent Seven” (M7), assim como através da procura por novos candidatos e restringindo possíveis cenários e populações. Durante a tese, nós investigamos os movimentos próprios de três das mais fracas fontes em raios X com o satélite Chandra. Este trabalho nos permitiu restringir o deslocamento da estrela de nêutrons em dois casos assim como medir com grande precisão o alto valor de movimento próprio de uma terceira fonte, pela primeira vez em raios X com uma significância alcançando 10 desvios-padrão (Motch, Pires, Haberl, & Schwope, 2007, Ap&SS, 308, 217; Motch, Pires, Haberl, Schwope, & Zavlin, 2009, A&A 497, 423). A procura por novos candidatos a estrelas de nêutrons isoladas no catálogo de fontes do satélite XMM-Newton, com mais de 120; 000 fontes de raios X, teve igualmente como objetivo restringir a densidade espacial de fontes com emissão térmica situadas além da vizinhança solar. Este trabalho levou à aguardada descoberta de uma nova estrela de nêutrons isolada em processo de resfriamento, a qual exibe propriedades similares às sete fontes descobertas pelo ROSAT (Pires, Motch, Turolla, Treves, & Popov, 2009, A&A 498, 233). Mais ainda, observações óticas profundas com os telescópios SOAR e ESO-VLT foram obtidas durante a tese de maneira a identificar no óptico a amostra mais brilhante de candidatos a estrelas de nêutrons, os quais foram selecionados entre mais de 72; 000 fontes (Pires, Motch, & Janot-Pacheco, 2009, A&A, 504, 185). Finalmente, a síntese de população de estrelas de nêutrons isoladas Galácticas permite restringir as propriedades globais da população com base na amostra total de observações em raios X realizadas com o satélite XMM-Newton. Estimando-se a densidade de fontes similares a maiores distânciasna Via Láctea, o objetivo final é determinar se a densidade espacial derivada do grupo de sete estrelas próximas corresponde a uma anomalia local causada pela proximidade do Sol em relação a regiões de ativa formação estelar do Cinturão de Gould
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Rosso, Françoise. "Structures magnétohydrodynamiques en astrophysique : jets et filaments." Grenoble 1, 1994. http://www.theses.fr/1994GRE10032.
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Le but de cette these est l'etude de deux exemples de structures magnetohydrodynamiques en astrophysique. La premiere partie est consacree aux jets stellaires et extragalactiques. L'originalite de ce travail est d'avoir introduit un principe variationnel duquel se deduisent les equations de base du probleme. La methode consiste a rechercher les solutions rendant le lagrangien extremum et franchissant regulierement les surfaces critiques. Deux types de comportements sont obtenus pour les jets: soit leurs vitesses demeurent subsoniques et ils s'evasent de facon parabolique, soit ils atteignent des vitesses supersoniques et prennent une forme cylindrique. Ces differentes evolutions permettent d'interpreter les deux familles de radiosources fr i et fr ii. Le modele rend egalement compte des vitesses terminales observees. Une etude analytique d'une instabilite thermique liee au rayonnement synchrotron d'electrons relativistes fait l'objet de la seconde partie. Les regions instables s'organisent en filaments brillants dont la largeur est fixee par la diffusion des electrons perpendiculairement aux lignes de champ magnetique. L'analyse lineaire fournit une taille dependant uniquement de l'intensite du champ, en accord avec les observations. Une ebauche de theorie non lineaire complete ces resultats par des informations sur les contrastes de densite, luminosite et champ magnetique dans les filaments
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Nercessian, Eric. "Formation de molécules interstellaires : observations millimétriques et modélisations de sources moléculaires : [thèse en partie soutenue sur un ensemble de travaux]." Phd thesis, Grenoble 1, 1987. http://www.theses.fr/1987GRE10107.
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Une simulation numerique resolvant les equations cinetiques de la chimie des especes moleculaires interstellaires et des observations millimetriques effectuees sur pom1 ont permis: - l'etude de la correlation **(13)co/av dans 2 nuages sombres (l 1506 et l 1529) du complexe du taureau; - la modelisation d'un nuage moleculaire moyennement dense; - l'etude de la chimie de l'azote dans les enveloppes circumstellaires oxygenees
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Lacasa, Fabien. "Non-Gaussianity and extragalactic foregrounds to the Cosmic Microwave Background." Phd thesis, Université Paris Sud - Paris XI, 2013. http://tel.archives-ouvertes.fr/tel-00955975.
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This PhD thesis, written in english, studies the non-Gaussianity (NG) of extragalactic foregrounds to the Cosmic Microwave Background (CMB), the latter being one of the golden observables of today's cosmology. In the last decade has emerged research for deviations of the CMB to the Gaussian law, as they would discriminate the models for the generation of primordial perturbations. However the CMB measurements, e.g. by the Planck satellite, are contaminated by several foregrounds. I studied in particular the extragalactic foregrounds which trace the large scale structure of the universe : radio and infrared point-sources and the thermal Sunyaev-Zel'dovich effect (tSZ). I hence describe the statistical tools to characterise a random field : the correlation functions, and their harmonic counterpart : the polyspectra. In particular the bispectrum is the lowest order indicator of NG, with the highest potential signal to noise ratio (SNR). I describe how it can be estimated on data, accounting for a potential mask (e.g. galactic), and propose a method to visualise the bispectrum, which is more adapted than the already existing ones. I then describe the covariance of a polyspectrum measurement, a method to generate non-Gaussian simulations, and how the statistic of a 3D field projects onto the sphere when integrating along the line-of-sight. I then describe the generation of density perturbations by the standard inflation model and their possible NG, how they yield the CMB anisotropies and grow to form the large scale structure of today's universe. To describe this large scale structure, I present the halo model and propose a diagrammatic method to compute the polyspectra of the galaxy density field and to have a simple and powerful representation of the involved terms. I then describe the foregrounds to the CMB, galactic as well as extragalactic. I briefly describe the physics of the thermal Sunyaev-Zel'dovich effect and how to describe its spatial distribution with the halo model. I then describe the extragalactic point-sources and present a prescription for the NG of clustered sources. For the Cosmic Infrared Background (CIB) I introduce a physical modeling with the halo model and the diagrammatic method. I compute numerically the 3D galaxy bispectrum and produce the first theoretical prediction of the CIB angular bispectrum. I show the contributions of the different terms and the temporal evolution of the galaxy bispectrum. For the CIB angular bispectrum, I show its different terms, its scale and configuration dependence, and how it varies with model parameters. By Fisher analysis, I show it allows very good constraints on these parameters, complementary to or better than those coming from the power spectrum. Finally, I describe my work on measuring NG. I first introduce an estimator for the amplitude of the CIB bispectrum, and show how to combine it with similar ones for radio sources and the CMB, for a joint constraint of the different sources of NG. I quantify the contamination of extragalactic point-sources to the estimation of primordial NG ; for Planck it is negligible for the central CMB frequencies. I then describe my measurement of the CIB bispectrum on Planck data ; it is very significantly detected at 217, 353 and 545 GHz with SNR ranging from 5.8 to 28.7. Its shape is consistent between frequencies, as well as the intrinsic amplitude of NG. Ultimately, I describe my measurement of the tSZ bispectrum, on simulations and on Compton parameter maps estimated by Planck, validating the robustness of the estimation thanks to realist foreground simulations. The tSZ bispectrum is very significantly detected with SNR~200. Its amplitude and its scale and configuration dependence are consistent with the projected map of detected clusters and tSZ simulations. Finally, this measurement allows to put a constraint on the cosmological parameters : sigma_8*(Omega_b/0.049)^0.35 = 0.74+/-0.04 in agreement with other tSZ statistics.
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BERRUYER, DESIROTTE NICOLE. "Contribution a l'etude des enveloppes circumstellaires : effet du couplage grains-gaz." Nice, 1987. http://www.theses.fr/1987NICE4157.
Full textAbstract:
L'enveloppe d'une etoile est particulierement importante a sa naissance, son origine etant proto-stellaire, et aux phases finales de son evolution ou la matiere ejectee par la matiere forme un cocon entourant l'etoile. La description de ces enveloppes circumstellaires depend donc de sa constitution et du rayonnement emis par l'objet stellaire central. On precise les criteres observationnels d'un objet en formation en fonction de sa masse; et pour les objets evolues, on decrit un modele de vent stellaire tenant compte des grains de poussieres et de la pression de radiation
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Gay, Pamela Lynn. "Using radio sources to find galaxy clusters." Thesis, 2002. http://wwwlib.umi.com/cr/utexas/fullcit?p3110613.
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King, EA. "Compact structure in southern peaked spectrum radio sources." Thesis, 1994. https://eprints.utas.edu.au/20283/6/King_whole_thesis.pdf.
Full textAbstract:
Data for a sample of forty three Southern radio sources is presented. The majority
of the sources have spectra that are steep at high frequencies and that peak, or
at least flatten markedly, below a few giga-hertz. The sources have been studied
through both single dish observations and VLBI.
A dual frequency (2.3 and 8.4 GHz) flux density monitoring program lasting
nearly two and a half years and comprising measurements of all sources at intervals
of between one and three weeks has shown that the incidence of variability is
systematically low in peaked spectrum sources. This observation is consistent
with the assertion that the energy source powering the luminous cores in this
type object is of a different nature to that driving the compact cores seen in
many powerful fl.at spectrum radio sources. In order that the observations could
be performed on a regular basis with minimum user intervention, an automated
observing and data acquisition system was implemented. To stabilise the receiver
gain, the system incorporates a versatile.software implementation of a noise adding
radiometer. ·
The recently established Southern Hemisphere VLBI Experiment (SHEVE)
array is described and its capabilities and limitations discussed. The operational
procedures and special difficulties inherent in the operation of the array are explained.
Images for nine sources made from observations with the SHEVE array are
presented. For seven of the sources, images are available at both 2.3 and 8.4 GHz.
The images have a mean dynamic range of 65: 1 and provide accurate measu!ements
of the detailed structure in these sources, including component sizes, flux densities
and spectral indices. All of the peaked spectrum sources imaged are doubles and
few show any evidence of significant extended structure. These data constitute
the first high resolution observations for six of the sources.
Observations with a single baseline interferometer have provided the first survey
of the compact structure in thirty eight of the sources in the sample. Simple
models representing the basic source structures have been fit to the data for all
objects detected in this survey. These models show that about two thirds of the
sources surveyed have more than one component at milli-arcsecond resolution. A
comparison of the models with the images is used to set limits on the reliability
of the model fitting results.
A database of all the source properties obtained, both through the observations
described, and compiled from the literature is presented. Numerous parameters
intrinsic to the sources are calculated using these data and are briefly discussed.