Thesis (PhD) - Faculty of Information and Communication Technology, Faculty of Life and Social Sciences, Swinburne University of Technology, 2009. / Submitted in total fulfilment of the requirements for the degree of Doctor of Philosophy, Faculty of Information and Communication Technology, Swinburne University of Technology, 2009. Typescript. Bibliography: p. 165-186.

This dissertation describes the measurement of angular diameters of compact radio sources by the technique of interplanetary scintillation. The design, construction and testing of a four acre radio aerial functioning at a frequency of 81.5 MHz is described, and its operation during a survey of the sky between declinations -07° and +46° and right ascensions ten hours and sixteen hours. The calibration of the apparatus is explained and the method of analysis of the output from the receiving equipment. The theory of interplanetary scintillation has been adapted to this frequency and extended, especially for the case of radio sources at large solar elongations. More stringent limits have been set on the rate of change with distance from the sun of the size of the irregularities in the interplanetary medium. Some nine hundred radio sources have been studied in the survey, and one hundred and ninety-four have been found to contain structure of angular dimension less than one second of arc. Limits have been put on all the others. Fifty per cent of sources in the 3C catalogue have been found to show interplanetary scintillations. Angular diameters of eighty-five sources have been measured: these measured values are in good agreement with other existing measurements, and values are now available for a large number of sources in the 4C catalogue in the area covered by the survey. The radio source 3C 273 has been found to contain two small diameter components, and the more compact of these to be surprisingly strong. A more rigorous test of the correlation between spectral index of and the presence or absence of fine structure in a source has been carried out. A correlation between an enhancement of scintillation and a reduction in cosmic ray index has been noted. A description of the discovery of pulsed radio sources is given.

Long Baseline Interferometry is a well established technique that enables high angular resolution measurements to be made with a radio interferometer containing independent local oscillators and signal recorders. This Thesis addresses the problems encountered when this technique is applied to low frequency (81.5 MHz) astronomy, and specifically methods of improving its phase stability. The technique is used to reassess existing evidence for the existence of large-scale structure associated with the quasar 3C48, and to give a much better understanding of the structure and behaviour of the supernova remnant Cassiopeia A at low frequencies. Much of the instrumentation used for this work already existed, but in a form unsuitable for measuring phase. The first section of the Thesis shows how these data collection and digital correlator systems could be modified and extended to improve sensitivity and generate true interferometric phase. Two phase calibration schemes are also considered. Firstly, the ‘thin triangle’ method, which uses the closure phase generated by a three-element interferometer to determine the true astronomical phase of a source comprising a single, bright point source and an associated resolvable component. This includes an analysis of how best to determine closure phase when the signal-to-noise ratio on one or more of the baselines is low, and shows techniques based on the ‘triple-product’ method, proposed by Cornwell, to perform best. Secondly, a new method of instrumental stabilisation is introduced and demonstrated, in which the interferometer is calibrated by broadband signals from VHF public broadcast transmitters, which are processed along with the astronomical data. The technique is shown to be capable of removing practically all the instrumental phase drifts usually associated with a long baseline interferometer on baselines up to 100 km. A number of new observations are also presented, mostly made with the stabilised mobile interferometer described above. Firstly of the bright quasar 3C48 to investigate earlier reports of an associated steep-spectrum extended component. These show the importance of considering the effects of confusion in the earlier observations and prompt a critical reassessment of the existing evidence, which is shown to be flawed. Detailed observations of the supernova remnant Cassiopeia A are also presented, with a resolution and sensitivity previously unattained at metre wavelengths. Comparisons with 20 cm VLA data of the same epoch show the remnant to look remarkably similar at the two frequencies, though tentative spectral index measurements indicate that its younger features have slightly steeper spectra than the rest. Furthermore, sensitive observations made on a 128 km baseline show that Cassiopeia A does not contain the steep-spectrum compact component with a flux density of more than 5 Jy reported by many earlier authors. An even deeper search at 408 MHz, this time for pulsed emission from the remnant, puts an upper limit of 80 mJy on the flux density due to any associated pulsar with a period 4 > P > 0.02 sec.

The design and construction of a digital autocorrelation spectometer for radio astronomy is described. It is a 16 channel device with an overall bandwidth of 2 MHz. Extensive use was made of integrated circuitry. The use of the spectrometer in a search for singly ionized molecular hydrogen (Hg₂⁺) is described. The search covered a frequency range 1404 to 1409 MHz for the radio sources NML Cygnus and the Omega Nebula down to a level of T[subscript b] 0.25 K. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Neutral hydrogen (HI) is a key ingredient in galaxy formation and an excellent tracer of galaxy evolution and the environmental drivers thereof. HI volume density varies significantly with environment, where denser environments (i.e. galaxy groups and clusters) are comparatively HI deficient, but it is not well understood how HI volume density varies over cosmic time. The investigation of the change in HI volume density with these variables, assists in constraining galaxy evolution models. Due to sensitivity limits, HI has not been studied in detail beyond z ~ 0.2, except for objects at the extreme high-end of the HI mass function. With the exceptional sensitivity of MeerKAT, we are now able to explore HI at higher redshifts (z < 0.5 with L-band receivers), and detect a large number of galaxies in a single observation ( > 10), due to MeerKAT’s large field-of-view. This project serves as an exploratory HI study with the new MeerKAT instrument, of a low-redshift, HI-rich galaxy group (z = 0.044), three intermediate-redshift galaxy clusters (0.3 < z < 0.4), and the utilisation of gravitational lensing to search for magnified HI emission behind these clusters to z < 0.5. We present the discovery of a galaxy group in the XMMLSS field, containing 11 HI galaxies, as part of the MIGHTEE Early Science observations. This shows the success of blind HI searches for the discovery of HI groups, and the capacity for the dynamical and morphological studies of resolved HI in groups with MeerKAT. The MeerKAT Galaxy Cluster Legacy Survey sample includes three of the Hubble Frontier Field (HFF) clusters, Abell 2744, Abell S1063 and Abell 370. We perform blind HI emission searches, and HI spectral stacking, a statistical technique used to amplify the signal of low S/N spectra, on these clusters. We present a 7 sigma stacked detection for blue galaxies in Abell S1063, and calculate HI mass detection limits for Abell 2744 and Abell 370. In the blind and targeted searches for HI behind the HFF clusters, we detect no evidence of highly magnified HI emission within the limits of the MeerKAT L-band. The low thermal noise levels achieved in this work, despite short integration times, highlights the enormous potential of future MeerKAT HI observations of dense environments and the intermediate-redshift Universe. / Dissertation (MSc)--University of Pretoria, 2020. / NRF/SARAO / Physics / MSc / Restricted

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Thesis (M.S.)--Brigham Young University. Dept. of Electrical and Computer Engineering, 2003. / Includes bibliographical references (p. 377-382).

A basket weaving technique for making radio continuum maps has been developed at the Hartebeesthoek Radio Astronomy Observatory (HartRAO). This data reduction technique significantly reduces scanning effects by using independent maps scanned in orthogonal directions. The observation and data analysis procedures that were developed are presented. The technique was used to map the supernova remnant MSH 15-52 at frequencies of 5000 MHz and 8500 MHz. The flux spectral index for this supernova remnant was found to be 0.83 ± 0.02 in this frequency range. Two regions (A and B) of the Galactic plane were observed at 8500 MHz with a resolution of 6'. Region A covered the 5°x5° area 47.5°< k 52.5°, Ibl < 2.5°, and region B was the 4.2°x3° area 320.4°< I <334.6°, Ibl < 1.5°. Far infrared observations at 60 !lm were used in conjunction with the radio maps to separate the thermal and non-thermal components of the radio emission. The technique can be used to map the Galactic plane at 8500 MHz using dual polarisation once the receiver at HartRAO has been upgraded. This would fulfil a need for a medium resolution, high frequency survey of the southern Galactic plane. / Adobe Acrobat Pro 9.4.6 / Adobe Acrobat 9.46 Paper Capture Plug-in

A significant fraction of the star formation density between z = 1–3 has been traced to luminous and ultra-luminous infrared galaxies. Among the most extreme objects seen are those identified via surveys at submillimetre wavelengths; the high infrared luminosities (LIR = 10¹¹−¹³ L⊙) of these “submillimetre galaxies” (SMGs) arise due to the reprocessing of UV and optical light from massive, young stars by interstellar dust, and imply star formation rates SFR= 50–2000M⊙ yr−¹. Such SFRs, combined with the observed increase in number density of SMGs by a factor ∼ 20× out to z = 2 make them candidates to be the progenitors of the most massive “red and dead” elliptical galaxies which dominate the cores of clusters in the present day, yet limitations in the technical capabilities of radio and infrared telescopes have long hindered a detailed understanding of these galaxies, and in particular, the physics of the interstellar medium (ISM). Hence the following key questions remain unanswered: (i) how much molecular gas (H2) do SMGs contain, and what are the likely upper-limits on how long they can sustain their present level of activity?; (ii) do the prodigious luminosities of SMGs stem from a common origin, or does the selection criterion return a “mixed bag” of galaxies?; (iii) what are internal kinematics of SMGs, and to what extent do these influence the global and local star-formation (Schmidt-Kennicutt) law?; (iv) is star-formation in distant SMGs distributed across the entire galaxy, or does it occur in isolated clumps?; (v) what are the typical densities and temperatures of star-forming regions in SMGs, and do they adhere to the observed correlation between far-infrared and radio emission in star-forming galaxies? Recent upgrades to the Karl G. Jansky Very Large Array (VLA), the inauguration of the Atacama Large Millimeter Array (ALMA), and the superlative legacy of the Herschel Space Observatory have jointly provided the toolkit to study the gas and dust emission in distant galaxies in unprecedented detail, and thus to begin to address these fundamental questions. In this thesis, I present ∼120 hours of new VLA observations of ¹²CO J =1−0 emission – the best tracer of molecular gas – in a sample of four lensed SMGs. The combination of high angular resolution with the VLA and the magnifying effects of gravitational lensing allow the ISM properties of these z ∼ 2.5 star-forming galaxies to be seen for the first time on sub-galactic scales, implying gas masses of 10⁹−¹ºM⊙ (subject to the 12CO-luminosity-to-H2-mass conversion factor, αCO), and demonstrating the presence of an extended, low-excitation gas reservoir. In conjunction with observations of the excited gas phase from the Plateau de Bure Interferometer (PdBI), these new data point to variations in the densities and temperatures of H2 throughout each galaxy. The wide bandwidth and phenomenal sensitivity of the VLA yields the first detections of 115-GHz continuum emission at high redshift, which I use in conjunction with well-sampled dust spectra from Herschel and data at longer wavelengths from the VLA to decompose the radio spectra of two galaxies into contributions arising from thermal dust emission, optically-thin free-free emission from Hii regions, and non-thermal synchrotron emission. From these measurements, estimates of SFRradio are made, providing an independent check on SFRIR, and enabling the degeneracy between the heating of dust due to star formation and that due to hidden AGN activity to be broken. Via this spectral index de-convolution, I find L[AGN]/LIR fractions of 35% and 55% for the two SMGs, in broad agreement with previously published estimates for these sources based on their mid-IR spectral properties. In the exceptional case of SMMJ23152–0102, magnification by the foreground cluster is so extreme (∼ 32.5×) that the VLA synthesised beam traces regions of order ∼ 130 pc in the source plane, and identifies a series of cool, dense clumps (Tk = 30–70 k; log[n(H2)/cm−3]= 3.6–3.9) within the gas reservoir, which contain between them 10–60% of the total molecular gas of the system. These clumps are offset from the far-infrared/radio correlation, which I argue has implications for their ages.