The work in this thesis involves detailed multi-frequency radio continuum (from 325 MHz to 15 GHz) and Hi spectroscopic studies of a few represent tative nearby galaxies which are experiencing gas-loss from their disks due to different physical processes. These processes are starburst-driven superwind, active galactic nucleus (AGN) −driven nuclear outflow, ram pressure stripping and tidal interactions. Gas-loss could affect the evolution of individual galaxies with age as well as their evolution with cosmic epoch. We have made use of both the Giant Metrewave Radio Telescope (GMRT) and the Very Large Array (VLA) for our study. Stars and gases are the two major constituents of a galaxy. The properties of the gaseous medium in them change with galaxy-types, such as the presence of large amount of gas in the late type disk galaxies and little interstellar medium (ISM) in the ellipticals or early−type galaxies. Galaxies in groups and clusters interact with each other and with the low density gaseous medium surrounding them, resulting in a possible evolution of their structures and physical properties. Basic differences in their observed properties depend on their history of interactions with the environment and also on the history of their central activities. Tidal interaction among galaxies may result in the flow of gas into the central region of the galaxy. This can trigger a central starburst and/or feed a central super-massive black hole to trigger an AGN activity. These activities produce either starburst-driven superwind or AGN-driven nuclear outflows (accretion disk wind or bipolar radio jet/bubbles) and the galaxy may lose their metal-enriched central gas concentration to the intra-cluster (ICM) or intergalactic medium (IGM). There are suggestions that when large amount of gas is cleared out from the central region of an ultra-luminous infrared galaxy (ULIRG), the dust enshrouded hidden AGN may unveil itself as a bona fide quasi-stellar object (QSO). Galaxies also lose gas usually from the outer parts when they move through the intra-cluster medium (ICM) or intragroup medium (IGrM) due to ram pressure stripping. When the ram pressure is stronger than the pressure by which the gas is bound to the galaxy, most of the gas may be lost or displaced from the disk thus affecting the star formation and metal formation in the disk of the galaxy. Starburst galaxies and superwinds: The starformation rate or the supernovae rate in some gas rich galaxies are 10−1000 times higher than that of the normal galaxies. This process can consume large amount (1−30×109 M) of gas over a short time scale (107−8yr) in a small region (typically 1 kpc). During such bursts of starformation, the cumulative effect of many supernovae and stellar winds from the massive stars in the central region of a disk galaxy imparts huge amount of mechanical energy to the ISM of the galaxy which then creates a high pressure (4 orders of magnitude higher then the average ISM pressure in the Milky way), high temperature (106−7 K) bubble of gas. This high pressure and high temperature bubble of gas expands and flows outwards in the direction of the steepest pressure gradient. This outflowing hot gas carries cooler gas and dust from the ISM along with it. It carries out heavy metals cooked in the central region of a galaxy to the external environment including the ICM or IGM. The typical outflow rate is 10−100Myr−1 with typical outflow velocity of few 100−1500 km s−1and it persists over few to 10 million yr. The observed sizes of such outflows are 1−20 kpc. We have studied a remarkable starburst-superwind system, NGC1482. This early type galaxy has been discovered to have a bi-conical shaped soft X-ray outflow as well as Hα and [Nii] outflow. The low-frequency radiocontinuum flux density was used to estimate the supernova rate, which could be used to constrain the dynamics of the driving force. The high-frequency high-resolution VLA observations revealed the central starforming region which is at the base of the bi-conical structure and presumably driving this outflow. The Hi observations also have resulted in finding two blobs of Hi on opposite sides of the galaxy defining an axis perpendicular to the outflow axis and rotating about it. We have also detected a multi-component broad (∼ 250 km s−1) Hi-absorption spectrum against the central continuum source. The absorption spectrum is nearly 70 km s−1 asymmetric towards the blue side with a component blue-shifted by 120 km s−1. The observed absorption could be due to the Hi-clouds driven outward by the central superwind outflow. Active galaxies and nuclear outflows: The inflow of gas to the central region of a galaxy triggers the starformation as well as the AGN activity. Co-existence of both these phenomena in some cases are well known. The accretion disk of the AGN can produce a hot wind emitting in X-rays in addition to the radio continuum jet perpendicular to the accretion disk. It is known that the AGN jets, in particular Seyfert jets have no correlation with the rotation axis of the host disk-galaxy. In such a scenario a very complex geometry can arise. Radio jets may interact with the starburst-driven winds or winds from the accretion disk or with the clouds of ISM of the galaxy accelerating and ionizing the gas. Sometimes the mass outflow rate may be ten times the mass accretion rate necessary to fuel the AGN, suggesting heavy mass loading of these AGN outflows. Depending on the energy of the outflow processes gas may be ejected out of the gravitational field of the galaxy. We have studied a composite galaxy, NGC6764 with an AGN and a very young starburst with two episodes of starformation, one 3−5 and another 15−50 Myr ago. The high-resolution high-frequency radio-continuum obser- vations reveal a radio core and roughly bi-conical radio emission oriented along the major axis of the galaxy with a feature which could be a jet. The lower-resolution images show bi-polar radio bubbles oriented along the minor-axis of the galaxy. These edge-brightened radio bubbles are asymmetric in size, extent (north-south) and luminosity. There is an east-west asymmetry in the spectral index of the bubbles. In a representative sample of dozen such radio bubbles in nearby galaxies we find that invariably all of these have an AGN. Similar to few other galaxies in the sample we also found that the radio continuum bubbles in NGC6764 are well correlated with the Hα filaments extending along the minor-axis of the galaxy. The CO(J=2-1) and CO(J=1-0) flux density ratio is also higher at the tip of the bubbles. Molecular gas plumes are seen extending along minor axis and have components blue-shifted by 140 km s−1. Our high-resolution Hi observations also show an absorption component at the systemic velocity with a weak component blue shifted by 120 km s−1. We have discussed the possibility of the radio plasma ejected from the AGN being carried outwards along the minor-axis by the superwind created by the young circumnuclear starburst. That bubble of hot gas from the superwind mixed with the relativistic plasma from the AGN is interacting with the cooler Hi and molecular gas of the ISM and driving it outwards. This interaction which is possibly in an early phase of expansion is also giving rise to the outflowing Hα filaments in this interesting composite galaxy. Cluster galaxies and stripping processes: When a galaxy moves through the hot and dense ICM with velocities 1000 km s−1, the ram pressure exerted by the ICM can strip the loosely bound and more tenuous gas of the galaxy. As a result of this the dense molecular gas or the stars in the galaxy remain almost unaffected but the tenuous gas moving out of the galaxy’s gravitational field could reach the ICM. The fate of such stripped gas is not well constrained. Recently very long tails with sizes of 50−125 kpc have been discovered. Some of these are magnetised, some ionised, some neutral and some are million degree hot. These tails may cool and eventually form galaxies or may evaporate and mix with the ICM. They enrich the ICM with metals and magnetic fields. In some cases galaxies are known to have become as high as 90 % deficient in Hi in comparison to the corresponding field galaxy of same type, size and luminosity. In the cluster/group environment tidal interactions with other group/cluster members or the cluster potential well could also take place affecting the observed properties of the galaxy. In addition, tidal interactions could also facilitate the removal of gas by ram pressure due to the ICM or IGrM. NGC4438 which we have studied in detail is an archetypal example of a galaxy which has been severely affected by the cluster environment. This late-type galaxy in the central region of the Virgo cluster is known to have interacted with the northern companion NGC4435. We have unambiguously resolved the radio nucleus from the lobes of radio continuum emission, and have shown it to have an inverted spectrum confirming it to be the nucleus. The lobes are almost perpendicular to the central molecular or stellar disk which is seen nearly edge-on. Projected onto the plane of the sky the lobeaxis is roughly parallel to the direction of the ram pressure wind. The lobes are very asymmetric in its extent, size and luminosity. The lobes are shelllike in structure and are interacting strongly with the asymmetric ISM. In the region of interaction both Hα and soft X-ray emission shells are seen. We explore possible reasons for the asymmetry in the lobes which is unlikely to be only due to the asymmetry in the density of the ISM on opposite sides of the galaxy. On a larger scale we have imaged the diffuse lower-frequency radio-continuum emission 5 kpc away from the central region seen on the western side of the disk of the galaxy. This extended emission has flatter spectral index at higher frequencies which suggests it to be a mixture of thermal and non-thermal components. In this region Hα, soft X-ray, Hi, molecular gas and relativistic plasma (i.e. all phases of the ISM) have been detected. We have found a linear structure on the western side near the same region with mass of nearly 200 million M We have imaged the Hi−emission from the stellar disk for the first time. The Hi -velocity field shows that the extra-planar gas could be rotating slower then the disk as seen in cases of ram pressure stripping. At lower resolution we detect more Hi from the halo of the galaxy. The iso-velocity contours appear to curve towards the axis of rotation or direction of the ram pressure wind, as you go away from the mid plane. We discuss whether this might be due due to the interaction of NGC4438 with NGC4435. We have discovered a 50 kpc long faint tail of Hi having a mass of 140 million solar mass to the north-west of the NGC4438−NGC4435 system. This Hi−tail partially coincides with an extremely faint (µv> 28) stellar tail, which has been seen in the deep optical imaging of intra-cluster light. Such tails have not been predicted by the simulations of interaction between NGC4435 and NGC4438. Hence it seems to be a remnant of some past event in the evolution of this interesting system. To further study the effects of ram pressure stripping and tidal interaction in galaxies in a group, we have studied the group Ho 124. We found that the radio continuum bridge of tidal interaction between NGC2820 and NGC2814 has a very steep (α=−1.8) spectrum possibly due to the older relativistic plasma left in it. The Hi of NGC2820 has sharp truncation on the southeastern side parallel to the edge on disk, while it has a unipolar huge loop on the north-west. NGC2814 has both an Hi and radio continuum tail different from the connecting bridge with sharp truncation again on the side opposite to the tail. Although there is reasonable radio continuum emission from the disk of NGC2820, there is no detectable emission corresponding to the huge one sided Hi loop. The velocity field of the Hi-loop trails that of the underlying stellar disk. Also in the galaxy NGC2805, a member of the same group, we find the Hi to have accumulated on the northern side while there is a bow-shock shaped starformation arc on the southern side of the disk. All these features namely starformation arc, sharp cut off in the Hi-disk, Hi-loop and Hiand radio continuum tails are signatures of ram pressure stripping. Ram pressure stripping in groups is relatively rare but this could get assistance from tidal interactions which help loosen the gravitational bound of the stellar disk on the tenuous ISM. A more spectacular case of ram pressure stripping is seen in the cluster Abell 1367. We have studied a region of the cluster A1367 where three of its galaxies namely CGCG 09773, CGCG 09779 and CGCG 09787 exhibit amazingly long (50−75 kpc) tails of radio continuum and optical emission lines (Hα) pointing roughly away from the cluster centre. They also show arcs of starformation on the side facing the ram pressure of the cluster medium. In our Histudy we found that all three of them have higher mass of Hi on the down-stream side. Two of the galaxies (CGCG 09773 and CGCG 09779) exhibit sharper gradients in Hiintensity on the side of the tail or on the down-stream side. However the Hi emission in all the three galaxies extends to much smaller distances than the radio-continuum and Hαtails, and are possibly still bound to the gravitational pull of the respective galaxies. These results are in good agreement with the hydrodynamical simulations of ram pressure stripping in cluster medium. In this study we have found a number of interesting results on a few nearby galaxies where different gas-loss processes have modified the morphology and kinematics of the ISM and/or the stellar distribution of the respective parent galaxies. We have found evidence of blue-shifted Hi absorption lines driven outwards by the starburst-driven superwinds and/or AGN-driven nuclear outlows. The synchrotron plasma outflowing from an AGN in a composite galaxy has been suggested to be interacting with the superwind which also drives other components of the ISM outwards. In groups or clusters of galaxies we have discovered an Hiloop, Hitails, regions of compressed Hi, trailing velocity fields, slow-rotating extra-planar gas, displaced ISM and asymmetries in various radio continuum or Hifeatures as evidences of ram pressure stripping mechanism affecting the member galaxies. The results obtained from this study illustrates the manifestations of gas loss proceeses in galaxies existing in different environments, and should provide valuable insights for future investigations with larger statistical samples towards a more complete understanding of gas loss processes in galaxies and their implications on galaxy evolution

The Square Kilometer Array radio astronomy telescope will achieve the majority of its extremely large aperture area with thousands of parabolic dishes, each illuminated by a wideband antenna feed, and this thesis introduces a new such antenna. The wide bandwidth of this new antenna is achieved with the development of a directional log periodic antenna. Scaling the log periodic elements into three-dimensional space is the present method used for directional log periodic antennas. We propose confining these often complex log periodic elements into a single plane, while the ground "plane" takes on a three dimensional form, permitting low-cost implementations without requiring the introduction of a complicated scaffolding to support the log periodic elements in 3-D. This low-cost solution would scale well in the implementation of the thousands of antenna feeds that the Square Kilometer Array demands. We also introduce a previously unreported LP design: the log periodic log spiral antenna.

Thesis (MScEng)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: This thesis describes the design and construction of a two-element digital correlation interferometer operating at 1.42 GHz. This instrument consists of two 92cm off set-parabolic reflectors arranged in an east-west baseline, with a maximum length of 24m. The reflectors are out-fi tted with helical beam antennas as primary feeds, and are further equipped with low-noise front-ends. A wideband dual-conversion superheterodyne receiver processes the astronomical signals before digitisation. The receiver is implemented as far as possible using a ffordable off -the-shelf technologies. The cross-correlation between the two antenna signals is measured using a 256MHz bandwidth digital FX correlator, and is implemented on a Reconfi gurable Open Architecture for Computing Hardware (ROACH) board. The preliminary observations made with this interferometer, suggests that it is possible to detect the sun, and some other much weaker sources such as Taurus A. The design of an interferometer in general is presented, from the formulation of the underlying instrumental requirements, to making meridian drift scan observations. The interferometer developed may serve as a demonstrator for other engineering students to gain a working knowledge of radio interferometry, which should prove invaluable when addressing the challenges the Square Kilometre Array (SKA) is faced with. / AFRIKAANSE OPSOMMING: In hierdie tesis word die ontwerp en konstruksie van 'n twee-element digitale korrelasie interferometer wat by 1.42 GHz ontvang beskryf. Die instrument bestaan uit twee 92 cm afsetparaboliese re ektorantennas met 'n oos-wes basislyn, met maksimum lengte 24 m. Heliese antennas word gebruik as primêre voer vir die reflektors, en verder maak die voorent gebruik van laeruis versterkers . Die wyeband superheterodineontvanger verwerk die astronomiese seine in twee stappe voordat dit digitaliseer word. Bekostigbare kommersiëele komponente word gebruik in die vervaardiging van die ontvanger. Die kruiskorrelasie tussen die twee antenna seine word digitaal gemeet deur 'n FX korreleerder met 'n 256MHz bandwydte op 'n ROACH bord. Die resultate toon aan dat dit moontlik is om die son waar te neem, sowel as sommige ander swakker bronne, soos Taurus A. Al die kwessies insluitende die instrumentale vereistes betrokke by die ontwerp van 'n interferometer word bespreek. Verder word die gebruik van interferometers om meridiaan meetings te doen bespreek. Die interferometer dien as 'n praktiese demonstrasie van radio interferometrie vir ingenieurstudente. Hierdie demonstrasie is voordelig vir studente om die uitdagings van die SKA te bestudeer.

Fifty years after the discovery of the first pulsating neutron star, the field of pulsar science has grown into a multidisciplinary research field, working to address a wide range of problems in astrophysics - from stellar evolution models to high precision tests of General Relativity to analysing the detailed structure of the Interstellar Medium in the Milky Way. Over 2500 Galactic pulsars have been discovered. The next generation telescopes, such as the Square Kilometre Array, promise to discover the complete observable Milky Way population, of several tens of thousands, over the next decade. These point sources in the sky have extreme properties, with matter densities comparable to that of an atomic nucleus, and surface magnetic fields a trillion times stronger than Earth's magnetic field. Observationally, the most valuable property is their rotational stability - allowing us to anticipate and sum their beamed radio emission, as the pulsar spins around its axis, on millisecond to second timescales. The detected radio wave signals carry with them information of the ionised interstellar medium (IISM) paths they traveled along. The imprints reveal that the pulsar signals we detect travel along multiple paths. While the bulk of the emitted signal propagates along a straight line, we also receive delayed emission scattered through small angles, back into our line of sight. This scattering is caused by fluctuations in the free electron densities of the IISM. The impact of these inhomogeneities is exaggerated at low observing frequencies, where averaged pulsar profiles are observed to be broadened, and showcase exponential scattering tails characterised by a scattering timescale г. Simple theoretical models predict a power law dependence of г on frequency, with a spectral index α = 4. Despite these predictions, my analysis of pulsar data in this thesis, reveal a more complex frequency dependence on г. I investigate the scattering characteristics of a set of pulsars observed by the Low Frequency Array (LOFAR), at 110~MHz to 190~MHz. These data are ideal datasets for accurate studies of pulsar scattering, providing broad frequency bands at low frequencies. I find anomalously low power law spectral indices, α, describing the frequency dependence of г. These indices are likely due to anisotropic scattering mechanisms or small scattering clouds in the IISM. To conduct effective data analysis, I develop scattering fitting techniques by first analysing IISM effects on simulated pulsar data. I investigate the effects of two different types of scattering mechanisms, isotropic and anisotropic scattering, and consider each of their particular frequency-dependent impacts on pulsar data. The work on simulated data provides a robust fitting technique for extracting scattering parameters and a framework for the interpretation of the LOFAR data used in this study. The fitting technique simultaneously models scattering effects and standard frequency-dependent pulse profile evolution. I present results for 13 pulsars with simple pulse shapes, and find that г, associated with scattering by a single thin screen, has a power law dependence on frequency with α ranging from 1.50 to 4.0. My results show that extremely anisotropic scattering can cause low α measurements. The anomalous scattering properties can also be caused by the presence of small scattering clumps in the IISM, as opposed to the conventionally modelled large scattering screens. Evidence for both anisotropic scattering and small scattering clouds with high electron densities come from other areas of research. Indications of the anisotropic nature of the local IISM mostly come from high resolution pulsar scintillation analyses, while evidence for high density scattering clouds is often based on extreme scattering events measured through quasar observations. My results suggest that these anomalous scattering properties are more prevalent than formerly thought, prompting us to reconsider the physical conditions of the IISM, where traditionally high electron densities are reserved for H_II regions and anisotropy is not modelled. High quality, low frequency pulsar data, where anomalous propagation effects become measurable, are a valuable addition in assisting us to distinguish between the different physical mechanisms that can be at play. The more complex these IISM characteristics reveal themselves to be, the harder it will be to disentangle intrinsic profile emission from IISM propagation imprints. Successfully separating these effects, however, promises to improve our understanding of the intrinsic pulsar radio emission - a process that is still poorly understood.

>Magister Scientiae - MSc / I combine 610 MHz Giant Metrewave Radio Telescope (GMRT) data, 1.4 GHz Very Large Array (VLA) data and 1.4 GHz Westerbork Synthesis Radio Telescope (WSRT) observations, encompassing a ∼ 4 square degree field (sq. deg. field) centred on the verification strip of the Spitzer First Look Survey (FLS) field (RA = 17h18m00s, Dec = 59◦30′30′′), to study radio sources down to fluxes of about 0.1 mJy. The results of an analysis of a sample of 107 multi-component radio sources obtained by cross-correlating the VLA and GMRT catalogues are shown. The spectral index analysis shows that the majority of multi-component sources are steep-spectrum sources. Nevertheless the spread in the spectral distribution is wide, with a significant number of ultrasteep,flat or inverted sources. By cross-correlating 107 multi-component radio sources with the optical catalogues of Marleau et al. (2007) and Papovich et al. (2006), 23 objects were identified and spectroscopically classified as galaxies. Some of them are classified as star-forming or star-burst galaxies, perhaps indicating that AGN and starformation activity are ongoing in the same galaxy. The measured redshifts span the range 0 < z < 1.8 and peak at z ∼ 0.2. According to their radio power (P), 6 of the identified objects are in the range of FR II sources (P1.4GHz > 1024.5W/Hz) while 17 are in the range of FR I sources (P1.4GHz < 1024.5W/Hz). Most of the sources having P1.4GHz < 1024.5W/Hz are compact and few are extended and peculiar, while all sources in the range of P1.4GHz > 1024.5W/Hz are extended. Further optical followup is recommended to allow a more complete census of the sub-mJy population and more information on AGN feedback from such sources.

No description available.

Understanding how diffuse molecular clouds at large scales (~10 pc) assemble mass into dense, star-forming cores at small scales (~ 0.1 pc) is crucial to building a holistic theory of star formation. While recent observations suggest that filaments play an important role in the mass assembly of dense cores, detailed gas kinematics studies are still lacking. My dissertation presents three innovative techniques that enable us to study star-forming filaments' complex gas kinematics in unprecedented detail: multi-component spectral fit, multi-dimensional filament identification, and membership assignment of velocity-coherent structures. Through these techniques, I analyzed star-forming filaments in the Perseus Molecular Cloud and unveiled unexpectedly complex velocity structures at scales where filaments are well resolved, to as low as the 0.01 pc scale. Moreover, the correlations I discovered between the various filament properties further suggest a scenario in which thermally supercritical filaments grow continuously via accretion from their surroundings while simultaneously forming cores through fragmentation along their lengths. / Graduate / 2022-01-08

This thesis investigates a new approach for dealing with the adverse effects of large-scale deformations in the main reflector of large Cassegrain antennas. In this method, the incident aperture distribution is imaged onto a tertiary focal plane. This is accomplished by using an optical imaging system consisting of a lens mounted behind the Cassegrain focus of the antenna. The lens forms a real image of the product of the incident aperture distribution and the pupil function of the antenna. The pupil function describes the profile of the main reflector of the antenna. If the incident aperture distribution is a plane wave, a real image of the pupil function of the main reflector will be produced at the focal plane of the image lens. Any imperfections in the main reflector will be imaged onto the tertiary focal plane but over a smaller area as defined by the magnification of the system. In principle, an active correcting element placed into the tertiary focal plane could compensate for these errors, thus preserving the maximum efficiency of the antenna. Experimental verification of this principle was carried out in the lab using a dielectric lens 152.4mm in diameter. Phase perturbations were simulated by placing dielectric shims in the incident aperture plane. The phase of these shims in most cases was measured to within 10 degrees in the image plane. This degree of accuracy is found to be quite adequate for correcting large-scale errors in the main reflector of the antenna.

Radio frequency interference has become a large problem to radio astronomers. This thesis proposes the idea that radio frequency interference can be mitigated using a phased array feed in conjunction with a large reflector. A phased array feed would allow radio astronomers to observe fainter signals than is currently possible, while at the same time enabling rapid sky surveys. A phased array feed was designed and simulated, and sensitivity optimization was performed on the array feed. It was shown that higher sensitivity can be achieved using a 7-element phased array feed than with a conventional waveguide feed. Simulations were ran using RFI mitigation algorithms on the array to show that interference cancellation can, in principle, be performed using a phased array feed. In addition to these simulations, improvements were made to a previously designed RF receiver so that radio astronomy observations could be made and interference mitigation algorithms tested on a receiver platform.

The Brigham Young University (BYU) Radio Astronomy group, in collaboration with Cornell University, the University of Massachusetts, and the National Radio Astronomy Observatory (NRAO), have in recent years developed and deployed PAF systems that demonstrated the advantages of PAFs for astronomy. However, these systems lacked the necessary bandwidth and acquisition times to be scientifically viable. This thesis outlines the development of a 20-MHz bandwidth system that can acquire for much longer periods of time and across much larger bandwidths than previous BYU systems. A report of the deployment of this system on the 305-meter reflector at the Arecibo Observatory in Puerto Rico is also summarized.The Commonwealth Scientific and Industrial Research Organisation (CSIRO) is currently constructing a PAF-equipped synthesis imaging array named the Australian Square Kilometre Array Pathfinder (ASKAP) that offers great promise for widening FOVs and enhancing RFI mitigation techniques. Previous work in RFI mitigation has demonstrated effective cancellation for synthesis imaging arrays under the assumption that the processing bandwidth is narrowband and correlator dump times are short. However, these assumptions do not necessarily reflect real-world instrument limitations. This thesis explores simulated adaptive array cancellation algorithm effectiveness as applied on the ASKAP instrument given realistic bandwidths and correlator dump times. The results demonstrate that active RFI mitigation performed across long baselines is largely ineffectual.