The two SCUBA HAlf-Degree Extragalactic Survey (SHADES) fields are amongst the richest places in the sky in terms of multi-wavelength coverage. They comprise an eastern section of the Lockman Hole (LH) and the central portion of the Subaru- XMM/Newton Deep Field (SXDF). In this thesis, I have obtained extremely deep, multi-frequency radio imaging of the SHADES fields using the GiantMetre-wave Radio Telescope (GMRT) and the Very Large Array (VLA), at 610MHz and 1.4GHz, respectively. These data are used to analyse the nature of the sub-milliJansky (sub-mJy) radio population, which has been hotly debated in the last few years: are they powered by star-forming or nuclear activity? To tackle the problem, I employ different approaches making use of the large variety of multi-wavelength data in the SHADES fields. I begin by analysing the spectral index, α610MHz 1.4GHz , of radio sources detected in the LH, to explore the dominant emission mechanism. Based on a robust 10 σ detection criterion, I find a constantmedian spectral index of α610MHz 1.4GHz ≈ −0.6 to −0.7 for sources between S1.4GHz ≈ 200 μJy and 10mJy. This result suggests that the galaxy population in the sub-mJy regime is powered by optically-thin synchrotron emission – starforming galaxies or lobe-dominated active galactic nuclei (AGN). Making use of X-ray observations in the LH, I show that the fraction of radio sources detected in the hard X-ray band (between 2 and 10 keV) decreases from 50 to 15 per cent between S1.4GHz ≈ 1mJy and . 100 μJy, which strongly suggests a transition from AGN to star-forming galaxies. Based on the deep, multi-wavelength coverage of the SXDF, I explore the behaviour of the far-infrared (FIR)/radio correlation as a function of redshift. I combine the q24 factor – the logarithmic flux density ratio between Spitzer 24-μm and VLA 1.4- GHz flux densities – with available photometric redshifts and find strong evidence that the correlation holds out to z ≈ 3.5. Based on M82-like k-corrections and using a high-significance (S1.4GHz > 300 μJy) radio sub-sample, I find a mean and scatter of q24 = 0.71 ± 0.47. Monte-Carlo simulations based on these findings show that fewer sources deviate from the correlation at fainter flux densities (i.e. fewer radioloud AGN). I predict that the radio-loud fraction drops from 50 per cent at ∼ 1mJy to zero at . 100 μJy. The validity of the FIR/radio correlation out to very high redshifts adds credibility to identifications of sub-millimetre (submm) galaxies (SMGs) made at radio wavelengths. Based on a sample of 45 radio-identified SMGs in the LH, I find a median radio spectral index of α610MHz 1.4GHz = −0.72 ± 0.07, which suggests that optically-thin synchrotron is the dominant radio emission mechanism. Finally, as anAppendix I include a theoretical treatment that constrains the average geometry of the dusty, torus-like structures believed to obscure a large fraction of the AGN population. I use the distribution of column densities (NH) obtained from deep ∼ 1Msec X-ray observations in the Chandra Deep Field South. I find that to reproduce the wide observed range of NH, the best torus model is given by a classical “donut”- shaped distribution with an exponential angular dependency of the density profile.

Polarised synchrotron emission from the Milky Way is of interest for its role as a foreground to the polarised CMB and as a probe of the interstellar medium. The Galactic ALFA Continuum Transit Survey (GALFACTS) and the Global Magneto-Ionic Medium Survey (GMIMS) are two ongoing surveys of the diffuse polarised emission around 1.4 GHz, with wide bandwidths and high frequency-resolution. In this thesis, I use early data from GALFACTS to investigate the behaviour of polarised, diffuse Galactic synchrotron emission. I also analyse GMIMS total intensity data. I derive a rotation measure (RM) map of the GALFACTS sky using a combination of RM-synthesis and linear angle fitting, commenting on the structure of the maps in general and on specific regions in particular. Overall I find that the maps are rich in features, and probe the RM structure of the extended Galactic emission with reasonable accuracy. I also derive the Angular Power Spectrum (APS) of the polarised emission for thirty-one 15 by 15 degree subregions across the GALFACTS data. I compute the E- and B-modes (E+B) and the scalar APS of the polarised emission (PI). I parametrise the APS by fitting a power law to the data. Comparing the E+B APS to the PI APS shows that E+B is consistently steeper across the sky. The APS data is also used to estimate the level of foreground contamination of the CMB B-mode by the synchrotron emission. I find that the slope of the APS averaged over high-latitude, low-emission subregions agrees exactly with that of the Planck 30 GHz polarised emission, thus setting an upper limit to the synchrotron contamination of CMB B-modes. Finally, I evaluate the spurious, systematic, temperature zero-level offset and associated uncertainty in preliminary GMIMS total intensity maps, finding a lower limit of ±0.26 K. I also make spectral index maps made using the GMIMS data and the Haslam et al. (1982) 408 MHz map, improving upon previous spectral index maps in the literature.

Gamma-ray bursts (GRBs) are the most energetic electromagnetic events in the universe, but there are still unanswered questions about them, like the underlying radiation mechanisms that cause the different parts of their light curves. Given that Wolf-Rayet (WR) stars with circumburst rings could be the progenitor of GRBs with late flares \cite{complex}, the purpose of this thesis was to determine if the precursor and the main emission of GRBs with late time flares might originate due to different radiation mechanisms, and thereby if WR stars could be the progenitors. 271 of the longest GRBs with flux above 10 photons/cm$^2$/s were studied and all GRBs with a precursor and a defined quiescent period were chosen for further studies. The chosen 39 GRBs were divided into different categories depending on the appearance of their light curves. A gold sample with $R_{P, max}/R_{D, max} < 0.4$ and $T_Q/T_{tot} > 0.5$, for the maximum count rate of the precursor $R_{P, max}$, the dominant emission $R_{D, max}$, and for the normalised quiescent period $T_Q/T_{tot}$ was concluded to have similar characteristics and to fit what would be expected if WR stars would be the progenitors. This group of GRBs all have a short and less bright precursor, a long quiescent period and a main emission which is brighter and longer than the precursor. The distributions of the photon index $\alpha$ for the precursor and the dominant emission for the gold sample indicate that the precursor is due to photospheric emission and the dominant emission is due to synchrotron emission. This is consistent with the interpretation that the precursor is a result of the jet interacting with the photosphere and the dominant emission is a result of interactions with the circumburst ring of a star like the WR stars. The next step in this investigation would be to study GRBs with more than one precursor that otherwise fit the description of the gold sample, to determine if these fit into the gold sample as well. / Gammablixtrar (GRB) är de mest kraftfulla elektromagnetiska eventen i universum men det finns fortfarande obesvarade frågor om dem, som de underliggande strålningsmekanismerna som orsakar de olika delarna av deras ljuskurvor. För en del av alla GRBs tar det upp till eller mer än 100 sekunder från utlösningstiden till det att en topp ses i ljuskurvan. För dessa finns då ofta en liten svag topp, som följs av en lång lugn period och sedan den dominant, starkare utstrålningen. GRBs tros kunna härstamma från Wolf-Rayet-stjärnor (WR-stjärnor), som är massiva, döende stjärnor som kan vara omringade av bubblor, nebulosor, och ringar. Om GRBs härstammar från dessa förväntas den första mindre toppen och den andra större toppen uppkomma på grund av olika strålningsprocesser. Syftet med detta projekt var därför att undersöka huruvida dessa toppar uppkommer på grund av olika processer eller ej. Sammanfattningsvis så hittades en distinkt och homogen grupp av GRBs med likande egenskaper. Resultaten påvisar att den första svaga toppen är fotosfärisk strålning, så att den uppkommer på grund av att jetstrålen från GRBn interagerar med fotosfären. Därtill tyder resultaten på att den dominanta starkare toppen är synkrotronstrålning, som kan uppkomma när jetstrålen interagerar med en ring runt en WR-stjärna. Nästa steg i detta projekt skulle vara att studera GRBs med fler än en mindre topp innan den dominant utstrålningen, för att se om dessa också har liknande egenskaper som de som hittades i den homogena gruppen.