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We use Spitzer Space Telescop and Heinrich Hertz Telescope(HHT) observations to study the earliest stages of low--mass starformation. Using spatially resolved absorption features, termedshadows, we study the cold cloud cores where stars form.We study Barnard 335, a prototypical isolated Bok globule with anembedded Class 0 protostar. We discover an 8 micron shadow in theinner regions of the core; using this feature we measure the densecore structure and mass. Using HHT observations we detect a rotatingstructure, a flattened molecular core, with a diameter~ 10,000 AU. The flattened molecular core is likely to be thesame structure as that generating the 8 micron shadow, and isexpected from theoretical simulations. This structure has not beenrobustly detected in previous observations although there have beensome prior indications of its presence.We study dense starless core structure through longer wavelengthobservations of shadows; we present Spitzer observations of 8 micron,24 micron, and 70 micron\ shadows of 14 cores in total. Combined withHHT observations of 12CO 2--1 and 13CO 2--1, we derive core sizes,masses, study core structure, and investigate the collapse status ofeach core. Our study of starless core CB190 reveals that the core islikely to be stable against collapse if magnetic pressure is presentat a reasonable level in the core. Our study of the 70 micron shadowassociated with the starless core L429 reveals that this object isvery likely to be collapsing. Finally, we study a sample of 12starless cores selected to have prominent 24 micron shadows. We findthat about 2/3 of these sources are likely to be collapsing.Additionally, we find indications that 1/2 of the cores revealed to becollapse candidates show indications of having 70 micron shadows. Weconclude that all cores dense enough to produce 70 micron shadows arecollapse candidates, and that the presence of a shadow at 24 micronis an indicator that the core is likely (60% probability)to be collapsing.

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This thesis presents a detailed multi-wavelength study of radio sources. A major part of the thesis focuses on radio sources in the FIRST survey while the latter part of the thesis studies low redshift radio galaxies in X-ray selected galaxy clusters. In the first part of the thesis a cross correlation analysis of FIRST radio sources with optical data from the NDWFS and infrared data from the FLAMINGOS survey in the Boötes and Cetus fields was performed. Optical counterparts were found for 76% (688/900) of sources in one band or more i.e., Bw, R, I or K. Photometric redshifts for these sources have been computed using the Hyperz code. The red-shifts obtained are fairly consistent with those expected from the K−z relation for brighter radio sources. A total number of 57 counterparts have extremely red colour (R− K > 5). Photometric redshifts derived using Hyperz imply that these Extremely Red Object (ERO) counterparts to FIRST radio sources are mostly located in the range z ∼ 0.7 −2, with the bulk of the population at z ∼ 1. A total of 25 ERO counterparts to FIRST radio sources were identified in R, J and K bands. These objects were separated into passively-evolving and dusty star-forming galaxies using their R, J and K colours. The relatively blue J − K colour of these galaxies suggest that most (72%, 18/25) are elliptical galaxies rather than dusty starburst galaxies. Using data from the Chandra XBoötes survey, a total of 92 (10%) FIRST radio sources were identified above the X-ray flux limit, fX (0.5 − 7) keV = 8 × 10−15 erg s−1 cm−2, and of these 79 optical counterparts are in common to the radio-X-ray matches. The majority (68%) of the radio-X-ray matched population were found to have −1 < log fX/ fopt < +1 indicative of AGNs. There is a significant population (23%) with high X-ray-to-optical flux ratio (log fX/ fopt > 1), suggesting high redshift and/or dust obscured AGN. In addition, there is also a population of sources that are X-ray faint optically bright sources with log fX/ fopt < −1. Spectroscopic identifications were found for 22 of the 79 sources. These optical spectra were dominated by broad line AGNs and also included narrow emission line galaxies. It was found that many classes of objects contribute to the X-ray/radio emission including quasars, BL Lacs, starburst galaxies, normal galaxies and galaxies with both AGN and starburst activity. This thesis also investigated the clustering analysis of FIRST radio sources optically identified in the SDSS DR6 survey using the two point angular correlation function ω(θ). The matched sources were found to have a larger amplitude of clustering compared to the full catalogue of radio sources consistent with similar studies in the literature. The angular correlation function was measured for different magnitude limited and flux limited subsamples. It was found that the angular correlation function scales with the depth of the optical survey as expected, whereas the amplitude of the angular correlation function increases as the radio flux increases. The last part of this thesis is devoted to studying radio galaxies in galaxy clusters at high frequencies to explore their contamination to the Sunyaev-Zel’dovich effect signal in these clusters. A total of 139 galaxies at low redshift (z < 0.25) in X-ray selected clusters were observed at four frequencies, 4.9, 9, 22, and 43 GHz using the NRAO Very Large Array. It was found that more than half of the observed sources have steep microwave spectra with steep spectral index, α < −0.5, as generally expected. However, about 60% of the unresolved or barely resolved sources have flat or inverted spectra. Most of these sources show an upward turn in flux at ν > 22 GHz, implying a higher flux than would be expected from an extrapolation of the lower frequency flux measurements. Our results quantify the need for careful source subtraction in increasingly sensitive measurements of the Sunyaev-Zel’dovich effect in clusters of galaxies. / Thesis (PhD)-University of KwaZulu-Natal, Westville, 2009.

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