This thesis investigates techniques and algorithms for mitigating radio frequency interference (RFI) affecting radio astronomy observations. In the past radio astronomy has generally been performed in radio-quiet geographical locations and unused parts of the radio spectrum, including small protected frequency bands. The increasing use of the entire spectrum and global transmitters such as satellites are forcing the astronomy community to begin implementing active interference cancelling. The amount of harmful interference affecting observations will also increase as future instruments such as the Square Kilometre Array (SKA) are required to use larger bandwidths to reach up to 100 times the current sensitivity levels, and as spectral line observations require observing in bands licensed to other spectrum users. Particular attention is paid to interference cancellation algorithms which make use of reference beams. This has proven to be successful in removing interference from the contaminated astronomical data. Reference antenna cancellers are closely analysed, leading to filters and techniques that can offer improved RFI excision for some important applications. It is shown that pre- and post-correlation reference antenna cancellers give similar results, and an important aspect of the cancellers is the use of a second reference signal when the reference interference-to-noise ratio is low. These modified filters can theoretically offer infinite interference suppression in the voltage domain, equivalent to that of post-correlation interference cancellers, and their internal structure can offer an understanding of the residual RFI and added receiver noise components of a variety of reference antenna techniques. The effect of variable geometric delays is also considered and various filters are compared as a function of the geometric fringe rate.

Also issued as a Ph.D. thesis in the Dept. of Electrical Engineering, 1970. / Bibliography: p.107.

In this thesis, new effective and efficient implementation structures of three-dimensional (3D) spatio-temporal (ST) Finite Impulse Response (FIR) uniform and non-uniform cone and frustum filters using well-known filter banks are investigated. The performance of the proposed implementation structures for 3D ST FIR uniform and non-uniform cone and frustum filters are investigated for 3D broadband beamforming in radio astronomy applications. First, implementations of two 3D ST uniform FIR cone filters are investigated. The 3D cone filters are designed by cascading either the well-known uniform quadrature mirror – cosine-modulated (QM-CM) filter bank or directly designed filter banks (DDFBs), with 2D low-pass circularly-symmetric spatial filters. In addition, two 3D ST uniform FIR frustum filters are derived from the cone filters by implementing partial bands of the filter banks with corresponding 2D spatial filters. The performance of the proposed implementation structures for 3D ST uniform QM-CM and DDFBs cone and frustum filters are evaluated using broadband beamforming signals in radio astronomy applications. The performance of the QM-CM and DDFBs cone and frustum filters shows improvement in terms of Signal-to-Interference-plus-Noise ratio (SINRs) over existing 3D ST cone and frustum filters. In addition to their effective performance, these cone and frustum filters can be efficiently implemented with equivalent or less computational complexity compared to existing methods. Second, implementations of two 3D ST non-uniform cone and frustum filters are explored. These cone and frustum filters are obtained by cascading either QM-CM or DDFBs non-uniform filter banks, with 2D low-pass circularly-symmetric spatial filters. The motivation for the 3D ST non-uniform cone and frustum filters is to achieve better approximation at low temporal frequencies than using the uniform ones. The performance of the 3D ST non-uniform cone and frustum filters is evaluated and compared with the performance of the uniform 3D ST cone and frustum filters. Results indicate that the performance of the proposed 3D ST non-uniform QM-CM and DDFBs cone filters shows some improvement in selective filtering compared to the performance of 3D ST uniform cone filters. / Graduate / 0544 / hussamss@uvic.ca

Studies of carbon's evolutionary path have been conducted via millimeter and submillimeter observations of circumstellar envelopes (CSEs), planetary nebulae (PNe), molecular clouds and comets. The 12C/13C isotope ratio was measured in Galactic molecular clouds using the CN isotopologs. A gradient of 12CN/13CN was determined to be 12C/13C = 6.01 DGC +12.28, where DGC is distance from the Galactic center. The results of CN are in agreement with those of CO and H2CO indicating a true ratio not influenced by fractionation effects or isotope-selective photodissociation. The 12C/13C isotope ratios in the envelopes of various types of stars were also measured from both CO and CN isotopologs. Such objects as carbon and oxygen-rich asymptotic giant branch (AGB) stars, supergiants, planetary nebulae, and S-type stars were observed. Results from this study indicate 12C/13C values for supergiants ~ 10 and AGB stars 12C/13C ~ 20- 76. Theory would suggest a lower ratio for objects undergoing third dredge-up, though this is seemingly not the case. Multiple carbon-bearing species including CO, HCN, HNC, CN, CS, and HCO+ have also been observed towards the oxygen-rich supergiant, VY CMa. This object has recently revealed a unique chemistry where carbon is not solely contained in CO, and may play a more important role in the chemical network of oxygen-rich circumstellar envelopes. Additionally, observations of species with carbon- carbon bonds, such as CCH and c-C3H2, have been conducted towards evolved planetary nebulae, such as the Helix and Ring nebulae. There is a close agreement in the inventories of species found in PNe and diffuse clouds, suggesting a potential molecular precursor to the interstellar medium. Observations of carbon-bearing species (H2CO and CO) in comets C/1995 O1 (Hale-Bopp), C/2001 Q4 (NEAT), and C/2002 T7 (LINEAR) have been conducted. Formaldehyde is known to have an extended distribution in these objects, likely arising from silicate-organic grains. Evidence of cometary fragmentation was also obtained for H2CO in comet T7 LINEAR as well as for HNC and HCO+ in Hale-Bopp. Such events could contribute to planetary distribution of organics.

In this thesis we make progress toward establishing the observational method of 21 cm intensity mapping as a sensitive and efficient method for mapping the large-scale structure of the Universe. In Part I we undertake theoretical studies to better understand the potential of intensity mapping. This includes forecasting the ability of intensity mapping experiments to constrain alternative explanations to dark energy for the Universe's accelerated expansion. We also considered how 21 cm observations of the neutral gas in the early Universe (after recombination but before reionization) could be used to detect primordial gravity waves, thus providing a window into cosmological inflation. Finally we showed that scientifically interesting measurements could in principle be performed using intensity mapping in the near term, using existing telescopes in pilot surveys or prototypes for larger dedicated surveys. Part II describes observational efforts to perform some of the first measurements using 21 cm intensity mapping. We develop a general data analysis pipeline for analyzing intensity mapping data from single dish radio telescopes. We then apply the pipeline to observations using the Green Bank Telescope. By cross-correlating the intensity mapping survey with a traditional galaxy redshift survey we put a lower bound on the amplitude of the 21 cm signal. The auto-correlation provides an upper bound on the signal amplitude and we thus constrain the signal from both above and below. This pilot survey represents a pioneering effort in establishing 21 cm intensity mapping as a probe of the Universe.

In this thesis we make progress toward establishing the observational method of 21 cm intensity mapping as a sensitive and efficient method for mapping the large-scale structure of the Universe. In Part I we undertake theoretical studies to better understand the potential of intensity mapping. This includes forecasting the ability of intensity mapping experiments to constrain alternative explanations to dark energy for the Universe's accelerated expansion. We also considered how 21 cm observations of the neutral gas in the early Universe (after recombination but before reionization) could be used to detect primordial gravity waves, thus providing a window into cosmological inflation. Finally we showed that scientifically interesting measurements could in principle be performed using intensity mapping in the near term, using existing telescopes in pilot surveys or prototypes for larger dedicated surveys. Part II describes observational efforts to perform some of the first measurements using 21 cm intensity mapping. We develop a general data analysis pipeline for analyzing intensity mapping data from single dish radio telescopes. We then apply the pipeline to observations using the Green Bank Telescope. By cross-correlating the intensity mapping survey with a traditional galaxy redshift survey we put a lower bound on the amplitude of the 21 cm signal. The auto-correlation provides an upper bound on the signal amplitude and we thus constrain the signal from both above and below. This pilot survey represents a pioneering effort in establishing 21 cm intensity mapping as a probe of the Universe.

A galaxy’s evolution is quite sensitive to the impact of external influences. In this thesis, the impact of external environment from both large and small scale effects is investigated, along with a study of how the HI gas fraction of a galaxy can modulate a galaxy’s response to perturbations by galaxy–galaxy interactions. This thesis makes use of the statistical power of the Sloan Digital Sky Survey Data Release 7 (SDSS DR7) to assemble a large spectroscopic sample of galaxies, select samples of interest, and select control samples of galaxies matched to each galaxy within the sample of interest in mass, redshift, and (if applicable) local density. It is possible to trace a galaxy’s internal gas motions which mark its disturbance by using the metrics of star formation rate (SFR) and gas-phase metallicity. To investigate the influence of large scale environment, a sample of star forming galaxies in a locally dense environment, but relatively isolated from larger scale structure, is constructed. This sample is further divided into groups which are truly isolated from any large scale structure (no cluster potential within 1 Mpc), and those which, in spite of their relative local isolation, are embedded within a larger cluster structure (within 1 Mpc of a cluster). As the local galaxy density is identical between isolated and embedded group structures, a fair comparison between the star forming properties of the galaxies within those group structures can be made. Star forming galaxies whose groups are embedded within a larger structure are found to show statistically lower SFRs than those galaxies whose groups are truly isolated from any larger cluster potential. The impact of local galaxy–galaxy interactions is subsequently considered. Using a sample of star-forming galaxies in pairs from the SDSS DR7, the enhancement in SFRs and the suppression of metallicities is traced as a function of projected separation (rp). The metallicity dilution as a function of rp is presented for the first time. Galaxies in pairs are found to have SFRs and metallicity values which are offset from a carefully selected control sample to separations of at least 80 kpc/h. Using a suite of simulations developed for the purposes of comparison with these observational results, a new interpretive framework is developed for enhancements as a function of rp. To investigate the role that gas fraction plays in moderating the strength of interaction triggered starbursts, new data is obtained from the Jansky Very Large Array (VLA). The VLA data supplements the existing SDSS data with HI gas masses for a subsample of resolvable galaxy pairs at small rp(in kpc/h). HI masses are obtained and gas fractions are calculated for a sample of 34 paired galaxies. A positive correlation is detected at > 2σ between the gas fraction of a galaxy and the SFR enhancement of that galaxy. The work presented in this thesis has expanded the understanding of physical variables, both internal and external, which can change the star forming properties of a galaxy through an examination of tracers of internal gas flows in those galaxies. / Graduate / 0606 / jillian.scudder@gmail.com

A galaxy’s evolution is quite sensitive to the impact of external influences. In this thesis, the impact of external environment from both large and small scale effects is investigated, along with a study of how the HI gas fraction of a galaxy can modulate a galaxy’s response to perturbations by galaxy–galaxy interactions. This thesis makes use of the statistical power of the Sloan Digital Sky Survey Data Release 7 (SDSS DR7) to assemble a large spectroscopic sample of galaxies, select samples of interest, and select control samples of galaxies matched to each galaxy within the sample of interest in mass, redshift, and (if applicable) local density. It is possible to trace a galaxy’s internal gas motions which mark its disturbance by using the metrics of star formation rate (SFR) and gas-phase metallicity. To investigate the influence of large scale environment, a sample of star forming galaxies in a locally dense environment, but relatively isolated from larger scale structure, is constructed. This sample is further divided into groups which are truly isolated from any large scale structure (no cluster potential within 1 Mpc), and those which, in spite of their relative local isolation, are embedded within a larger cluster structure (within 1 Mpc of a cluster). As the local galaxy density is identical between isolated and embedded group structures, a fair comparison between the star forming properties of the galaxies within those group structures can be made. Star forming galaxies whose groups are embedded within a larger structure are found to show statistically lower SFRs than those galaxies whose groups are truly isolated from any larger cluster potential. The impact of local galaxy–galaxy interactions is subsequently considered. Using a sample of star-forming galaxies in pairs from the SDSS DR7, the enhancement in SFRs and the suppression of metallicities is traced as a function of projected separation (rp). The metallicity dilution as a function of rp is presented for the first time. Galaxies in pairs are found to have SFRs and metallicity values which are offset from a carefully selected control sample to separations of at least 80 kpc/h. Using a suite of simulations developed for the purposes of comparison with these observational results, a new interpretive framework is developed for enhancements as a function of rp. To investigate the role that gas fraction plays in moderating the strength of interaction triggered starbursts, new data is obtained from the Jansky Very Large Array (VLA). The VLA data supplements the existing SDSS data with HI gas masses for a subsample of resolvable galaxy pairs at small rp(in kpc/h). HI masses are obtained and gas fractions are calculated for a sample of 34 paired galaxies. A positive correlation is detected at > 2σ between the gas fraction of a galaxy and the SFR enhancement of that galaxy. The work presented in this thesis has expanded the understanding of physical variables, both internal and external, which can change the star forming properties of a galaxy through an examination of tracers of internal gas flows in those galaxies. / Graduate / 0606 / jillian.scudder@gmail.com

This thesis presents a study of high-redshift obscured (type-2) quasars, selected at mid-infrared and radio wavelengths. This population had remained elusive, even to hard X-ray surveys, and in Chapter 2 I compare the selection of type-2 quasars in X-ray and mid-infrared surveys, as well as explaining the criteria I will use to search for these objects at z ~ 2, around the peak in the unobscured (type-1) quasar activity. Chapter 3, presents a sample of radio-intermediate type-2 quasars selected from the criteria de- scribed in Chapter 2. Optical spectroscopy shows indeed that at least half of the objects have the characteristic narrow emission lines, and lie around the expected redshift of z = 2. The other half of the objects are consistent with also being type-2 quasars at similar redshifts, although no emission lines are visible. In Chapter 4,1 discuss the possibility of two types of obscured quasars, some obscured by a dusty torus and some by a dusty host galaxy, to explain the lack of emission lines in half of the sample. I model the number of type-1 quasars expected to follow similar selection criteria and at the same redshifts as our type-2 quasars, and find that the obscured quasars outnumber the unobscured by a ~2-3:1 ratio. I conclude that most supermassive black hole growth is obscured by dust. When comparing this to predictions from unified schemes, I find that this result is only consistent with the schemes provided host-obscuration is indeed happening. The lower ratio of type-2 to type-1 quasars inferred from X-ray surveys (~1:1) suggests that some of the type-2 quasars in this sample might be Compton thick. Radio data taken at three frequencies, are presented in Chapter 5, to study the spectral properties and intrinsic luminosities of our sample. I show that some type-2 quasars have flat radio spectra, which is inconsistent with obscuration by the torus, but consistent with host-obscuration. Some gigahertz-peaked spectra, characteristic of young radio jets, are present, but the majority of the sources have very steep spectra. These steep spectral indices can be explained by active developed jets in which continuous injection of electrons is accompanied by inverse-Compton losses against the cosmic microwave background. In Chapter 6, I select a similar sample of type-2 quasars in a different field, where X-ray data are available. The selection criteria are kept identical, except for the radio flux density cut, which is lowered. This is expected to introduce significant numbers of starburst contaminants. To filter these out, and due to a lack of spectroscopy, I use a bayesian method to fit the spectral energy distributions, obtain photometric redshifts, and select between a quasar and a starburst model. I measure the X-ray properties for the resultant sample of type-2 quasars. The entire sample is found to be Compton-thick, and repeating the modelling of Chapter 4, I find that the population of Compton-thick quasars is at least comparable to the population of unobscured quasars, and probably larger.

This thesis investigates techniques and algorithms for mitigating radio frequency interference (RFI) affecting radio astronomy observations. In the past radio astronomy has generally been performed in radio-quiet geographical locations and unused parts of the radio spectrum, including small protected frequency bands. The increasing use of the entire spectrum and global transmitters such as satellites are forcing the astronomy community to begin implementing active interference cancelling. The amount of harmful interference affecting observations will also increase as future instruments such as the Square Kilometre Array (SKA) are required to use larger bandwidths to reach up to 100 times the current sensitivity levels, and as spectral line observations require observing in bands licensed to other spectrum users. Particular attention is paid to interference cancellation algorithms which make use of reference beams. This has proven to be successful in removing interference from the contaminated astronomical data. Reference antenna cancellers are closely analysed, leading to filters and techniques that can offer improved RFI excision for some important applications. It is shown that pre- and post-correlation reference antenna cancellers give similar results, and an important aspect of the cancellers is the use of a second reference signal when the reference interference-to-noise ratio is low. These modified filters can theoretically offer infinite interference suppression in the voltage domain, equivalent to that of post-correlation interference cancellers, and their internal structure can offer an understanding of the residual RFI and added receiver noise components of a variety of reference antenna techniques. The effect of variable geometric delays is also considered and various filters are compared as a function of the geometric fringe rate.