Accelerated coplanar facet radio synthesis imaging

Imaging in radio astronomy entails the Fourier inversion of the relation between the sampled spatial coherence of an electromagnetic field and the intensity of its emitting source. This inversion is normally computed by performing a convolutional resampling step and applying the Inverse Fast Fourier Transform, because this leads to computational savings. Unfortunately, the resulting planar approximation of the sky is only valid over small regions. When imaging over wider fields of view, and in particular using telescope arrays with long non-East-West components, significant distortions are introduced in the computed image. We propose a coplanar faceting algorithm, where the sky is split up into many smaller images. Each of these narrow-field images are further corrected using a phase-correcting tech- nique known as w-projection. This eliminates the projection error along the edges of the facets and ensures approximate coplanarity. The combination of faceting and w-projection approaches alleviates the memory constraints of previous w-projection implementations. We compared the scaling performance of both single and double precision resampled images in both an optimized multi-threaded CPU implementation and a GPU implementation that uses a memory-access- limiting work distribution strategy. We found that such a w-faceting approach scales slightly better than a traditional w-projection approach on GPUs. We also found that double precision resampling on GPUs is about 71% slower than its single precision counterpart, making double precision resampling on GPUs less power efficient than CPU-based double precision resampling. Lastly, we have seen that employing only single precision in the resampling summations produces significant error in continuum images for a MeerKAT-sized array over long observations, especially when employing the large convolution filters necessary to create large images

Getting the most out of C.O.AS.T

Chapter 1 is a review of developments in optical aperture synthesis and is not original work. Chapter 2 considers the techniques estimating of fringe visibilities from photon-noise- limited data and is mostly a review of previous work, although some of the analysis is my own. Chapters 3–6 are original except where otherwise stated. The delay line described in chapter 7 is based on the design developed by Connes [16], but the specific implementation is my own. Chapter 8 describes experiments which are a continuation of the work of Baldwin et al. [6] and Haniff et al. [37], but the specific work descibed here is my own. Chapter 9 is original except where otherwise stated

Complex molecules in galactic dust cores - biologically interesting molecules and dust chemistry

The astronomical study of molecules has been an essential research field since the development of radio astronomy. Presently nearly 120 molecules have been identified in interstellar and circumstellar environments. The complexity of molecular species, and particularly organic molecules, that can be synthesized in the interstellar medium (ISM) leads to one interesting and important subfield in interstellar molecular studies, namely, the search and study for molecules of possible biological interest. Observationally, complex and most saturated molecules are observed exclusively toward compact hot, dense regions, often called "hot cores", in molecular clouds. To account for the observed amount of saturated organic molecules, interstellar dust particles play an important role. It has often been suggested that solid state reactions on grain surfaces provide an efficient way to synthesis saturated organic molecules. The objective of this study is to obtain observational data on biologically interesting molecules and to study important complex interstellar molecules. Since hot molecular cores are inherently compact, interferometric observations are therefore an ideal approach to study these sources. All our observations were all made with the Berkeley-Illinois-MarylandAssociation (BIMA) ArrayOpe

The Nature of Compact Sources Selected by LOFAR

Radio-loud active galactic nuclei (RLAGN) produce jets on large scales heating their local environments and preventing the hot phase of baryonic matter from cooling hence slowing or halting galaxy formation and evolution; a model popularly known as AGN feedback. To understand this model, new views of low-luminosity compact RLAGN inhabiting massive galaxies have been obtained using the new LOw Frequency ARray (LOFAR) northern-sky radio survey LoTSS (LOFAR Two Metre Sky Survey) to study RLAGN in the local Universe at low frequencies (150 MHz). In this thesis, I investigate lowluminosity compact RLAGN selected from LoTSS first Data Release (DR1) in an attempt to reveal their radio morphologies and the physical scales at which they affect the inter-galactic medium (IGM) of the host galaxy on sub-kpc scales. We have conservatively selected 55 low-luminosity compact RLAGN based on LoTSS DR1 within redshift range 0:03 < z < 0:1. I show using high-frequency Jansky Very Large Array (VLA) observations that only 9 out of 55 objects show extended radio emission (1-3 kpc), 43 are compact at the limiting angular resolution of 0.35 arcsec (corresponding to projected maximum physical sizes of < 1 kpc), while 3 are undetected. The extended objects display a wide range of radio morphologies on smaller angular scales: doubles (3), two-sided jets (3), one-sided jets (2), and complex (1). I discuss their radio spectra which range from steep to flat and/or inverted radio spectra (-1:31 < α < 0:36) and span the range seen in various compact RLAGN such as compact symmetric objects (CSOs), compact steep spectrum (CSS) sources, and gigahertz peaked-spectrum (GPS) sources. Assuming synchrotron self-absorption (SSA) models for flat and/or inverted radio spectrum sources, I predict the physical sizes of 35 RLAGN to be between 2-53 pc. On their position on the power-linear size (P - D) diagram, we see that these objects populate the bottom left lower-end of the diagram; this position corresponds to the low-radio power CSOs. Finally, I compare their radio properties with those of the Fanaroff-Riley class 0 (‘FR0’) sources. Interestingly, some of the compact RLAGN presented in this thesis show extended radio emission in the second Data Release (DR2) images; these appear to be in fact the radio cores of low-luminosity yet extended Fanaroff-Riley (FRI/FRII) sources that could populate the right-lower end of the P - D diagram, limited by LOFAR’s surface brightness. Could we be underestimating the sizes of some of these compact objects? Continued VLBI observations of LOFAR sources could help us answer this question, which in turn will help us further understand AGN feedback.