Philosophiae Doctor - PhD / In this thesis, we present a multi-wavelength analysis of star-forming galaxies to shed new light on the evolution of the far-IR-radio relations in intermediate redshift (0.3 < z < 0.6) galaxy clusters and galaxy groups. The far-infrared (far-IR) emission from galaxies is dominated by thermal dust emission. The radio emission at 1.4 GHz is predominantly produced by non-thermal synchrotron radiation. The underlying mechanisms, which drive the far-IR-radio correlation, are believed to arise from massive star formation. A number of studies have investigated the relationship as a function of redshift in the field and have found no evolution out to at least z _ 2, however few works have been done in galaxy clusters. In nearby clusters, the median logarithmic ratio of the far-IR to radio luminosity is qFIR = 2.07_0.74, which is lower than the value found in the field, and there is an indication of an enhancement of radio emission relative to the far-IR emission. Understanding the properties of the far-IR-radio correlation in a sample of distant and massive cluster and groups plays an important role in understanding the physical processes in these systems. We have derived total infrared luminosities for a sample of cluster, group, and field galaxies through an empirical relation based on Spitzer MIPS 24 _m photometry. The radio flux densities were measured from deep Very Large Array 1.4 GHz radio continuum observations. We have studied the properties of the far-IR-radio correlation of galaxies at intermediate redshift clusters by comparing the relationship of these galaxies to that of low redshift clusters. We have also examined the properties of the galaxies showing radio excess to determine the extent that galaxy type or environment may explain the radio excess in galaxy clusters. We find that the ratio of far-IR to radio luminosity for galaxies in an intermediate redshift cluster to be qFIR = 1.72_0.63. This value is comparable to that measured in low redshift clusters. A higher fraction of galaxies in clusters show an excess in their radio fluxes when compared to low redshift clusters, and corroborates previous evidence of a cluster enhancement of radio excess sources at this earlier epoch as well. We have also investigated the properties of the far-IR-radio correlation for a sample of galaxy groups in the COSMOS field. We find a lower percentage of radio-excess sources in groups as compared to clusters. This provides preliminary evidence that the number of radioexcess sources may depend on galaxy environment. We also find that a larger fraction of radio-excess sources in clusters are red sequence galaxies.

The WiggleZ Dark Energy survey is currently being carried out using the AAOmega instrument on the AAT. It is measuring redshifts for 240,000 emission line galaxies with high star-formation rates over 1,000 sq. degrees of sky. These galaxies are selected for spectroscopic observation from a combination of optical and ultraviolet imaging. The target selection criterion applied to these datasets is highly optimised to select high redshift emission line galaxies. The redshift distribution of these galaxies peaks at z

Dusty star forming galaxies (DSFGs), characterized by their far-infrared (far-IR) emission, undergo the largest starbursts in the Universe, contributing to the majority of the cosmic star formation rate density at z = 1−4. The Herschel Space Observatory for the first time was able observe the full far-IR dust emission for a large population of high-redshift DSFGs, thereby accurately measuring their star formation rates. With gravitational lensing, we are able to surpass the Herschel confusion limit and probe intrinsically less luminous and therefore more normal star-forming galaxies. With this goal in mind, we have conducted a large Herschel survey, the Herschel Lensing Survey, of the cores of almost 600 massive galaxy clusters, where the effects of gravitational lensing are the strongest. In this thesis, I present follow-up studies of gravitationally lensed Herschel-detected DSFGs by utilizing multi-wavelength data from optical to radio. Specifically, I characterize the star forming properties of gravitationally lensed DSFGs by using these three subsamples: (1) A gravitationally lensed DSFG galaxy at z = 0.6 in one of the most massive galaxy clusters, Abell S1063 (at z = 0.3), (2) One of the brightest sources in HLS, which is a system of two strongly gravitationally lensed galaxies, one at z = 2.0 (optically faint gravitational arc) and the other at z = 4.7 (triply-imaged galaxy), (3) A sample of the brightest sources in HLS at z = 1−4, in which we detect rest-frame optical nebular emission lines (e.g. Hα, Hβ, [OIII]λλ4959,5007) by utilizing near-IR spectroscopy. The main results from these studies are as follows: (1) In the cluster-lensed DSFG at z = 0.6, discovered in the core of Abell S1063, we identify a luminous (SFR = 10 M⊙/yr) giant (D~1 kpc) HII region similar to those typically found at higher redshift (z~2). We show that the HII region is embedded in a rotating disk and likely formed in isolation, rather than through galaxy interaction, which is observed in local galaxies. We can use this source as a nearby laboratory for star forming regions at z ~ 2, in which more detailed follow-up of this source can help us to understand their origin/properties. (2) We discovered that one of the brightest sources in HLS is a blend of two cluster-lensed DSFGs, one at z = 2.0 (an optically faint arc) and the other at z = 4.7 (triply-imaged galaxy), implying that a sample of bright Herschel sources may have such multiplicity. In the z = 2.0 arc, the sub-arcsecond clumps detected in the SMA image surprisingly do not correspond to the clumps in the JVLA CO(1-0) image. When investigating the CO(1-0) velocity structure, there is a substantial amount of molecular gas (likely a molecular wind/outflow) we find that we find is not associated with star formation. This suggests that the CO morphology in DSFGs could be strongly influenced by molecular outflows resulting in the over-prediction of the amount of the molecular gas available for star formation. In the z = 2.0 arc, we also constrain αCO~4. While this value is normal for galaxies like the Milky Way, it is quite unusual for ULIRGs. This hints that the physical conditions may be much different in the arc from other ULIRGs, which usually have αCO ≈ 0.8.(3) We successfully detect rest-frame optical emission lines in 8 gravitationally lensed DSFGs at z = 1−4 using ground-based near-IR spectroscopy with Keck, LBT and Magellan. The luminosities of these lines are substantially less than what the far-IR derived star formation rates predict, suggesting that these DSFGs have large dust attenuations. The difference in the star formation rates is a factor of 30 x (AV= 4), which is larger than previously reported for DSFGs at z > 1. One galaxy (z = 1.5) in the sample showed the largest suppression with a factor of 550x (AV = 7), which is similar to local ULIRGs. Future prospects: Herschel provided a glimpse into the star formation of DSFGs, but only the brightest at z > 2 could be studied in detail without gravitational lensing. ALMA will revolutionize the study of DSFGs with its high spatial resolution submm/mm imaging of their dust continuum and molecular gas, and it will begin to unravel their physical properties. In order to detect nebular emission lines in fainter higher redshift sources, 20-30 meter class telescopes, with next generation near-IR spectrographs, will be necessary. JWST will play a significant role as it will target rest-frame optical nebular emission lines in DSFGs unobtainable from the ground as well as weaker Hydrogen series lines (such as Paschen and Brackett series) to better understand their instantaneous star formation and dust attenuation.

With the Hubble Space Telescope and its near-infrared capabilities, it is now possible to probe deep into the epoch of reionization, improving our understanding of galaxy evolution through cosmic history. Whether it is via colour-selection or fitting the spectral energy distribution, it has now become routine to amass large samples of galaxies as distant as redshift z = 8, with the current frontier of observations at z = 9 - 10. The new Hubble Frontier Fields (HFF) programme provides the potential to study the most distant, intrinsically faint background galaxies through the gravitational lensing provided by a foreground galaxy cluster. This thesis presents a study of the galaxy population at z = 9 - 10 that exploits this phenomenon. In an initial search of the first two HFF cluster+parallel pointings, Abell 2744 and MACS J0416.1-240, we unveil twelve candidate high-redshift galaxies at 8:4 < z < 9:5, and are thus able to place constraints on the galaxy UV luminosity function at z = 9. For this study, we employ the "blank-field" method, whereby we confine attention to only the homogeneously deep, relatively low-magnification regions of the imaging. We are able to demonstrate evidence for a smooth decline in UV luminosity density between z ≃ 8 and z ≃ 9, in contrast to reports in the recent literature of a steep drop-off at these redshifts. We extend this study to include the new MACS J0717.5+3745 and MACS J1149+2223 cluster+parallel pointings, and supplement the search for z ≃ 9-10 galaxies with twenty CLASH cluster pointings. From a search over an area ≃ 130 sq. arcmin, we are able to present 33 galaxy candidates with photometric redshift solutions in the range 8:4 < zphot < 11:2. Our new results reinforce the argument for a smoothly-evolving LF between z ≃ 8 and z ≃ 9, which can be equally well modelled by a factor ≃ 2 drop in Φ* or a dimming of ≃ 0:5 mag in M*. We also find evidence that this smooth decline in the UV luminosity function, and hence UV luminosity density, continues to z ≃ 10. As well as considering the galaxy population at z = 9 - 10, this thesis presents a study of the stellar populations of galaxies at z ≥ 5. We are able to extend the luminosity baseline and measure the colour-magnitude relation at z = 5 - 8, through a combination of probing intrinsically faint galaxies behind cluster fields, in conjunction with both ultra-deep, pencil beam imaging such as the Hubble Ultra Deep Field (HUDF) and wider, shallower imaging from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS). This allows us to make inferences about the underlying stellar populations of galaxies at these epochs, and provides a unique insight into the colours of intrinsically faint, lensed galaxies as faint as M1500 ~ -14. We find that the data is consistent with an essentially unchanged average UV slope (β) for a given luminosity across the redshift range z = 5-8. We also find that the data favours a mild flattening of the colour-magnitude relation with redshift between z = 5 and z = 8.

Masters of Science / The study of the faint radio universe and of its properties has recently become a very active field of research not only because of the much improved capabilities of the SKA pathfinders but also because of the need to better plan for SKA surveys. These new facilities will map large areas of the sky to unprecedented depths and transform radio astronomy into the leading technique for investigating the complex processes which govern the formation and evolution of galaxies. This thesis combines multi-wavelength techniques, highly relevant to future deep radio surveys, to study the properties of faint radio sources. The nature of the faint radio sources is presented, over a large GMRT survey area of an area of 1.2 deg2 comprising 2800 sources. Utilising multi-wavelength data we have matched 85% of the radio population to Spitzer/IRAC and obtained a redshift estimate for 63%. The redshift associations are a combination of photometric and spectroscopic redshift estimates. This study investigates several multi-wavelength diagnostics used to identify AGN, using radio, infrared, optical and x-ray data . This analysis shows that various diagnostics (from the radio through the X-ray ones) select fairly different types of AGNs, with an evidence of a disagreement in the number of AGNs selected by each individual diagnostics. For the sources with redshift we use a classification scheme based on radio luminosity, x-ray emission, BOSS/SDSS spectroscopy, IRAC colors satisfying the Donley criterion, and MIPS 24ɥm radio-loud AGN criteria to separate them into AGNs and SFGs. On the basis of this classification, we find that at least 12.5% of the sources with redshifts are AGNs while the remaining 87.5% are adopted as SFGs. We explore the nature of the classified sources through the far-infrared radio correlation. We measure a median qIR value of 2:45± 0:01 for the SFGs and qIR value of 2:27 ± 0:05 for the AGNs. The decrease in the median value of qIR for the AGNs is a result of the additional AGN component to radio emission for the AGN-powered sources and find tentative evidence of an evolution of the qIR with redshift.

Spatially-resolved studies of nearby star-forming galaxies are essential to understand various physical and chemical phenomena at play in the interstellar medium in the galaxies, and consequently to obtain a comprehensive picture of galaxy formation and evolution. In this thesis, I perform spatially-resolved analyses of chemical abundances and star-formation in nearby star-forming galaxies - blue compact dwarf galaxies (BCDs) and spiral galaxies. I map various properties of H II regions and the surrounding gas within three BCDs, using integral field spectroscopic (IFS) data from the Gemini Multi-Object Spectrograph-North. While answering questions related to chemical homogeneity, ionisation mechanisms and stellar populations within BCDs, I address more profound issues, which go beyond the characterisation of studied BCDs and aim to explain global phenomena with broader implications. The BCD NGC 4449 hosts a metal-poor central star-forming region, which I explain by various scenarios related to the interplay between star-formation, metal-distribution and gas dynamics within galaxies. The BCD NGC 4670 shows an unusual negative relationship between the nitrogen-to-oxygen ratio and oxygen abundance at spatially-resolved scales. I explore this relation with chemical evolution models and by comparison to other star-forming galaxies and suggest that nitrogen enrichment, variations in star-formation efficiency or hydrodynamical effects may be responsible for the observed relation. For another BCD, SBS 1415+437, the spatially-resolved abundances on average agree with the integrated abundance, implying that low-redshift spatially-resolved results may be directly compared with unresolved high-redshift results. I study spiral galaxies to address long-standing issues related to the reliability of metallicity calibrators and the Schmidt Law of star-formation. Using IFS data of twenty-four spiral galaxies taken with the Multi-Unit Spectroscopic Explorer, I find that the current strong-line metallicity calibrators for H II regions are unsuitable for regions dominated by diffuse ionised gas (DIG). I devise new recipes for estimating the metal-content of the DIG. For another set of nine spiral galaxies, I use multi-wavelength data to show that the spatially-resolved Schmidt relation is very sensitive to the consideration of diffuse background, which is a component unrelated to the current star-formation. Removal of this component from the SFR tracers and the atomic gas results in similar local and global Schmidt relation. To conclude, the spatially-resolved analyses presented in this thesis have led to discoveries and further questions, which I will address in my ongoing and future works.

In recent years, advances in deep optical, and especially deep near-infrared imaging with the Hubble Space Telescope (HST) and wide-field ground-based telescopes such as VISTA, have revolutionized our understanding of the cosmological evolution of galaxies and supermassive black-holes (as manifest through active galactic nuclei; AGN). In particular, the dynamic range provided by the survey `wedding cake' of available HST+ground-based optical/IR data offers new opportunities to push the meaningful statistical study of galaxy and AGN evolution out to high redshifts. Much recent attention has focused, unsurprisingly, on using these new data to push studies of galaxy formation back to within a billion years of the Big Bang, and exploring the role of young galaxies in driving cosmic hydrogen reionization during the crucial era corresponding to redshifts z ≃ 6-10. However, these data have not been as thoroughly exploited at intermediate redshifts, and have only recently been used to explore black-hole/AGN evolution. In this thesis I utilise the latest deep optical/near-infrared imaging and spectroscopy to explore three key facets of cosmological evolution. First, I present a new, robust measurement of the evolving rest-frame ultraviolet (UV) galaxy luminosity function (LF) over the key redshift range from z ≃ 2 to z ≃ 4. My results are based on the high dynamic range provided by combining the Hubble Ultra Deep Field (HUDF), CANDELS/GOODS-South, and UltraVISTA/COSMOS surveys. I utilise the unparalleled multi-frequency photometry available in this survey `wedding cake' to compile complete galaxy samples at z ≃ 2; 3; 4 via photometric redshifts (calibrated against the latest spectroscopy). This study is important as the peak of star-formation is shown to happen within a redshift range z = 2 - 4 and determining the exact epoch that the galaxies were forming most of their stars depends significantly on the UV luminosity density which requires robust measurements of the galaxy UV luminosity function and its accurate parameterization. My new determinations of the UV LF extend from M1500 ≃ -22 (AB mag) down to M1500 =-14.5, -15.5 and -16 at z ≃2, 3 and 4 respectively (thus reaching ≃ 3-4 magnitudes fainter than previous blank-field studies at z ≃ 2 - 3). At z ≃ 2 - 3 I find a much shallower faint-end slope (α = -1:32 ± 0:03) than the steeper values (α ≃ -1:7) reported in the literature, and show that this new measurement is robust. By z ≃ 4 the faint-end slope has steepened slightly, to α = -1:43 ± 0:04, and I show that these measurements are consistent with the overall evolutionary trend from z = 0 to z = 8. I then calculate the UV luminosity density (and hence unobscured star-formation density) and show that it peaks at z ≃ 2:5 - 3, when the Universe was ≃ 2:5 Gyr old. Second, I have used these data to revisit the possibility that X-ray AGN played a significant role in cosmic hydrogen reionization which is one of the major processes in the formation of the Universe we see today. Hence, it is really important to understand this phenomenon thoroughly by studying the properties of sources capable of ionising photons, such as star-forming galaxies and high redshift AGNs. Although most recent studies have suggested that the emerging population of young star-forming galaxies can bathe the Universe in sufficient high-energy photons to complete reionization by z ≃ 6, some authors have reasserted the potentially important role of high-redshift AGN in the hydrogen reionization process. In an effort to clarify this situation, I reinvestigate a claimed sample of 22 X-ray detected active galactic nuclei (AGN) at redshifts z > 4, which has reignited the debate as to whether young galaxies or AGN reionized the Universe. These sources lie within the GOODS-S/CANDELS field, and I examine both the robustness of the claimed X-ray detections (within the Chandra 4Ms imaging) and perform an independent analysis of the photometric redshifts of the optical/infrared counterparts. I confirm the reality of only 15 of the 22 reported X-ray detections, and moreover find that only 12 of the 22 optical/infrared counterpart galaxies actually lie robustly at z > 4. I recalculate the evolving far-UV (1500Å) luminosity density produced by AGN at high redshift, and find that it declines rapidly from z ≃ 4 to z ≃ 6, in agreement with several other recent studies of the evolving AGN luminosity function. The associated rapid decline in inferred hydrogen-ionizing emissivity contributed by AGN falls an order-of-magnitude short of the level required to maintain hydrogen ionization at z ≃ 6. I conclude that AGNs make a very minor contribution to cosmic hydrogen reionization. Finally, I have utilized the deep optical/near-infrared survey data to explore the prevalence of quenched/passive galaxies at high redshift. Applying a robust method to isolate passive galaxies from star-forming galaxies is the key to improving our understanding of the quenching process. Focusing primarily on the deep HUDF data-set, I have revisited the effectiveness of simple colour-colour (UVJ) selection techniques in isolating robust samples of quenched galaxies, and find that dust plays a more important role in this selection process than has been previously appreciated. Through careful SED fitting I successfully isolate a sample of apparently dust-free quiescent galaxies in the redshift range 0:5 < z < 4:5 but (at least in the HUDF) fail to find any galaxy which has remained truly quiescent for > 1 Gyr. I conclude by focusing on the properties of a refined/robust sample of apparently quenched galaxies at z > 3, and in particular establishing the contribution of quenched galaxies to stellar-mass density at early times. I conclude with a summary of my findings, and a brief discussion of the most promising avenues for future advances with the next generation of facilities, such as the James Webb Space Telescope (JWST).

Magister Scientiae - MSc / We determine the stellar mass of star forming galaxies in the X-ray luminous cluster MS 0451.6-0305 at z ∼ 0.54. The stellar masses are estimated from fitting model spectral energy distributions (SEDs) to deep, optical UBRIz observations obtained from WIYN 3.5m telescope and public NIR K-band image from Palomar Observatory telescope. The model SEDs are based on the stellar population synthesis (SPS) model of Bruzual & Charlot (2003) and Conroy et al. (2009) that span a wide range of age, star formation history, Initial Mass Function (IMF), metallicity and dust content. We measure stellar masses for galaxies down to M∗∼2×10⁸M(.) We find a tight correlation between stellar masses derived from the two SPSs. We compare the derived stellar masses to the dynamical masses for a set of 25 star forming galaxies. The dynamical masses are derived from high resolution, spectroscopic observations of emission lines from the DEIMOS spectrograph on the Keck telescope. A strong correlation is seen between the dynamical and stellar mass for the galaxies; and the star forming galaxies show fairly constant ratio between stellar and dynamical mass. When comparing to the field sample of Guzm ́an et al. (2003) of luminous compact blue galaxies, we see an excess of low mass galaxies in the cluster. / South Africa

The nature of high-energy gamma-ray emission from Star-Forming Galaxies is of utmost importance for understanding both the origin of Cosmic Rays and the high-energy processes that shape galaxy formation. Observations from the gamma-ray telescope Fermi-LAT have detected gamma-ray emission from a handful of nearby Star-Forming Galaxies. Interestingly, observations of the Small Magellanic Cloud show evidence for a spectral cutoff at energies of approximately 10 GeV. This has raised the question as to whether some Star-Forming Galaxies are unable to contain their Cosmic Ray population. Using the Fermitools to analyse the gamma-ray emission from a selection of bright nearby Star-Forming Galaxies, this study intends to explore the possibility of finding further evidence for exponential cutoffs in the gamma-ray spectrum of Star-Forming Galaxies. The shape of the combined spectrum of the 49 galaxies in the sample was determined using least-square fitting of a single power law, a broken power law and a power law with an exponential cutoff. No evidence of an exponentialcutoff was found and the shape of the spectrum was best described by a broken power law with indices Γ1 = -2.48 ± 0.05 and Γ2 = -0.88 ± 0.09. This is in poor agreement with previous observations, which favour a simple power law with an index in the range -2.2 to -2.4. Interestingly, the single power law, while disfavoured over the broken power law at ~7σ, was best fit with the index Γ = -2.35 ± 0.06, which is surprisingly well in agreement with previous observations. The discrepancy between the results presented here and those found in the literature is interpreted as due to insufficient treatment of background fluctuations and the possible existence of bright sources at the unverified blank sky locations used for modelling the background.

This thesis presents studies of cosmic populations of extragalactic radio sources. The problems selected for this thesis are 1) the derivation of constraints on the emergence of new sub-mJy populations at flux density below about 1mJy (at1.4 GHz) paying careful attention to including sources with low surface brightness and counting sources rather than components 2) development of a new method to estimate the asymmetry in the large scale galaxy environment with respect to the axes of extended radio sources and use this to examine for evidence of impact of the environment on the morphology of radio sources. The studies presented herein have been carried out using the Australia Telescope Low Brightness Survey (ATLBS), which is a sensitive radio survey at 1.4 GHz, imaging 8.42 square degrees of the sky along with accompanying optical observations of the same region. There are multiple populations of extragalactic radio sources in the cosmos. These consist of populations of powerful radio-loud quasars and radio galaxies to populations of weaker radio sources such as star-forming galaxies. These populations of radio sources show evidence of evolution with cosmic epoch. Because the radio galaxy phenomenon and the AGNs at the centers of their host galaxies may influence the evolution of the galaxy via feedbacks, examinations of these source populations over cosmic time are a necessary complementary study to understanding the process of galaxy formation and in general, cosmology. Below we give brief introduction to the problems studied in this thesis. Sub-mJy Radio Source Counts Radio source counts, which have historically been a key probe of cosmology, now represent a measure of cosmological evolution in radio source populations. Currently, the estimation of source counts at sub-mJy flux density as well as the nature and evolution of these sources is undetermined. At flux densities ≤1.0 mJy a ‘flattening’ of normalized differential source counts has been widely reported in literature( Windhorst et al.(1985),Hopkins et al.(2003),Huynh et al.(2005) and references therein). The flattening is observed as an apparent change of slope for the normalized differential source counts from ∼0.7 at5.0 −100.0 mJy to about 0.4 in the 0.25 −5.0 mJy range. Attempts to understand the nature of the sub-mJy population have arrived at discordant results and identify the sub-mJy sources with different populations: starburst galaxies(Condon(1989), Benn et al. (1993), Huynhet al. (2005)), early type galaxies (Gruppioniet al., 1999),low(radio) luminosity activegalactic nuclei(AGNs)(Huynh et al.,2008) or a mixture of these. Due to unavailability of spectroscopically complete samples of hosts of sub-mJy sources, the exact nature of the sub-mJy radio source population is currently uncertain. However, the presence of a population which emerges at sub-mJy flux density and is different from the AGN-dominated radio source population is not in doubt. The studies in the literature are inconsistent in identifying the precise location of the emergence of flattening in counts at sub-mJy flux density. Several studies show that the source counts are consistent with a continuation in the slope of the differential counts below mJy flux density (Prandoniet al.(2001) and Subrahmanyan et al.(2010)). The scatter in the sub-mJy counts from different studies may be because of the relatively small areas covered by deep surveys(in many cases, a single pointing of an interferometric array) which may have relatively large errors arising from large scale clustering in the spatial distribution of cosmic radio sources(however the study by Condon (2007) concludes that the scatter in the source counts stems from variations in corrections and sensitivity in different studies)In contrast, wide-field surveys may not reach the depth to probe sub-mJy counts. Another reason is the correction applicable to the observed source counts necessary to estimate the true source counts; these are especially pertinent at low flux densities. To resolve these is-sues, a survey which combines the attributes of wide spatial coverage as well as excellent sensitivity and a procedure which accounts for the biases in estimation of the sub-mJy source counts is needed. In conclusion, accurate measurements of the source counts at sub-mJy flux densities are needed to correctly estimate the cosmic evolution of radio sources. Environments of Extended Radio Sources Another issue of importance in the study of extragalactic radio sources is their interaction with their environments. The gas environments in which radio sources reside and evolve ought to have an influence on the morphology of the radio sources. This has been shown in many case studies where the radio structures have been compared with the X-ray gas environments (Blanton et al. (2011); Boehringer et al.(1993)). Studies of the optical environments of radio galaxies have also been carried out previously (Longair & Seldner, 1979; Yee & Green, 1984; Hill & Lilly, 1991; Zirbel, 1997). The motivation behind these studies has been to examine differences between different classes of radio sources, the evolution of environments with cosmic epoch as well as the possibility of identifying clusters/groups of galaxies using radio sources as a tracer(Wing &Blanton, 2011). Many previous studies have found that the environments of FRI/FRII sources are different and are dependent on the cosmic epoch. FRI sources, typically, are found in rich environments. FRII sources in the local universe are generally hosted by field galaxies, but at higher red shifts are found in richer environments(Hatch et al.,2011;Best et al.,2003;Overzier et al.,2008). However, there have been fewer studies that relate the richness of the environments and morphological asymmetries of radio galaxies. Earlier investigations by Subrahmanyan et al. (2008) and Safouris et al.(2009) are noteworthy in this regard where the radio structures of two giant radio galaxies were examined in the context of the large-scale galaxy distributions in their vicinity(also see Chen et al.(2012) and references therein). The study was also used to infer properties of the ambient thermal gas medium in which the structures evolved. Clear correlations between structural asymmetries and associated extended emission-line gas were also found for radio galaxies that have relatively smaller sizes of a few hundred kpc(McCarthy et al., 1991). Thesis Work To progress the field in the problems highlighted above, the following work has been done in this thesis. Radio Imaging of ATLBS Survey To characterize the cosmic evolution of radio sources and their properties, observations and imaging of faint radio sources is essential. The Australia Telescope Low Brightness Survey(ATLBS), which has been used in the studies presented in this thesis, has been designed specifically to image diffuse radio emission to relatively high red shifts(z ∼1−1.5). Therefore obtaining good surface brightness sensitivity was a prime objective in planning the radio observations and in imaging the data obtained from these observations. This requires a nearly complete synthesized aperture and observations of a representative patch of the extra galactic sky. These requirements have been fulfilled in ATLBS survey, which has excellent uv coverage, especially at short spacings, and images a region off the galactic plane that is devoid of strong radio sources. The observations were carried out for two adjacent fields, designated as A and B with their centers at RA:00h 35m 00s,DEC:−67◦00 00 and RA:00h 59m 17s,DEC:−67◦00 00 ,in the 20 cm band, with a center frequency of1388MHz,infullpolarization mode. The radio data was imaged by using techniques such as multi-frequency deconvolution and self-calibration to make two mosaics of region A and B which are free of artefacts. These high-resolution radio images(with beamFWHM of 6 “)of the ATLBS survey regions cover 8.42 square degrees sky area with rms noise 72 µJy beam−1 and are of exceptional quality in that there are no imaging errors or artifacts above the thermal noise over the entire field of view. The images have excellent surface brightness sensitivity and hence provide good representation of extended emission components associated with radio sources. Optical Imaging of ATLBS Survey The ATLBS survey region has been also observed in SDSS r band, specifically for providing information about the galaxies hosting radio sources observed in ATLBS survey as well as galaxies in the neighbourhood of the radio sources. The optical observations were carried out using the CTIO 4 meter Blanco Telescope in Chile and using theMOSAICIIimager,whichisamosaicof8CCDs. In total, 28 optical images were created from the optical data. Each image was formed from a set of 5 dithers, using which spurious sources in the images were rejected. The final images are complete down to a magnitude of 22.75. Radio Source Counts Using the sensitive radio and optical images, a study of radio source counts was carried out. This study made use of some novel strategies and algorithms to generate a source list and correct it for various biases to obtain the radio source counts. More specifically, care was taken to identify sources with low surface brightness by making use of low resolution images for initial identification, and using multiple indicators (including optical images) to identify components of sources. The blending issues inherent in using low resolution images has been avoided using higher resolution images to identify blended sources. Thus, use of low resolution images( beam FWHM =50”′) almost completely removes effects of resolution bias and the use of high resolution images avoids blending issues. These strategies, together with use of optical images to locate candidate galaxy hosts and a careful visual examination of resolved and complex sources instead of automated classification ensures that the ATLBS catalog is a ‘source catalog’ as opposed to a ‘component catalog’. The distinction between ‘sources’ (which are single sources) as opposed to components(parts of a single source appearing separate) is crucial in estimating the true source counts. The source list was used toestimatetheradiosourcecountsdownto0.4 mJy. Comparing the counts with previous work shows that the ATLBS counts are systematically lower and the upturn in sub-mJy source counts has not been found down to the noise limited flux densities probed. The systematically low counts for ATLBS relative to most previous studies are attributed to the ATLBS counts representing sources as opposed to components, as well as corrections for noise bias as well as clustering effects that may affect source counts derived from the small sky coverage typical of deep surveys. This study also demonstrates the substantial difference in counts that result from using component catalogs as opposed to source catalogs: at 1 mJy flux density component counts may be as much as 50% above true source counts. This implies that automated image analysis for counts may be dependent on the ability of the imaging to reproduce connecting emission with low surface brightness as well as the ability of the algorithm to recognize sources, which require that source finding algorithms effectively work with multi-resolution and multi-wavelength data. Galaxy Environments of Extended Radio Sources in ATLBS Survey A study of the galaxy environments of the extended sources in the ATLBS survey was carried out using the optical images. This study of the environments of radio sources from the ATLBS survey is restricted to those that are extended and hence to a subset of the ATLBS-ESS(Extended Source Sample) sources. Briefly, the ATLBS-ESS subsample consists of 119 radio sources that have angular size ex-ceeding0’.5. Applying a red shift cut(to exclude sources with high red shifts whose optical environment may be beyond the depth of the optical images) as well as other constraints(such as availability of optical magnitudes of the host galaxy), a sub-sample of 43 sources was formed, including sources of diverse radio morphologies(FRI/FRII, WATs and HTs)as well as7 radio sources which are highly asymmetric in their radio morphology. For these sources, where no spectroscopic data was available, a red shift estimate was obtained from a magnitude-red shift relation derived from other sources in the ATLBS survey. Using the optical images convolved with a matched filter(following the prescription from Postman et al. (1996))consisting of a radial and magnitude filter, smoothed maps were formed for each source in the sample. These give the likelihood of a cluster being present in a given position in the map (in this case the location of interest being the position of the radio source in the map). Further, five parameters were defined in this study, which give estimates of the angular anisotropy of galaxy density around the axis of the radio source. This method used to quantify environmental asymmetry for the study presented in the thesis is new. The parameters defined thus were used to examine the environments of radio sources in the sample over a wide range in red shift. Specifically a comparison of FRI/FRII environments was made in two different red shift regimes(above and below z = 0.5) and it was found that the FRI and FRII sources inhabit environments of similar richness at low and high red shifts, with no evidence for red shift evolution. The WAT and HT sources were(as expected from earlier studies in literature)found in the most dense environments. Examination of the anisotropy parameters for the asymmetric radio sources clearly showed the influence environment has on radio source morphology, specifically in that the higher density of galaxies was found on the shorter side of the radio sources in almost all cases. Images and Other Resources The radio and optical images are an excellent resource for examining with auto-mated algorithms for source finding, parameter fitting, and morphological classification, and as a resource for testing such algorithms that would be used on upcoming all-sky continuum surveys with the LOFAR and ASKAP/SKA. The techniques and methods developed and presented in the thesis may be used in future studies of radio source populations.