Disentangling active galactic nuclei and star formation in extragalactic radio sources

• Main Author: Rawlings, J. I.
• Published: University College London (University of London) 2014
• Subjects: 500
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.626542

This thesis presents a study of the extragalactic radio source population at infrared (IR) and radio wavelengths of the electromagnetic spectrum. The IR/radio emission from these sources can be the result of star−forming processes and/or the result of an accreting supermassive black hole, an active galactic nucleus (AGN). The purpose of this study has been to separate the two different types of emission, determine the dominant process and calculate star formation rates (SFRs) for this population. For the first sample of radio sources, high redshift radio galaxies (HzRGs), polycyclic aromatic hydrocarbon emission has been observed in their mid−IR spectra. For the second sample, the radio sources in the Extended Chandra Deep Field−South (ECDF−S), Herschel photometry was used to infer the contribution of cool dust associated with star formation to the IR emission. Overall, I find that extragalactic radio sources, both those that do and do not contain an AGN, can have a strong component of their IR and/or radio emission from star formation. From this result, I infer they can have exceptional SFRs ("1000sM! yr−1). For the HzRGs, it was also found that they have weak 9.7 μm silicate absorption (!9.7 μm < 0.8). This implies that their mid−IR obscuration is predominantly the result of the dusty torus that surrounds the central engine and not the result of dust in the host galaxy. For the ECDF−S radio sources, AGN and star−forming galaxy contributions to the radio source counts were also determined. I find that sources powered by star formation are responsible for the observed flattening of the counts at "1mJy and a significant portion of these sources host an AGN. The optical morphologies of low−redshift sources are also investigated and Ifind that the faintest sources are mostly either late−type galaxies or irregular/ mergers while the bulk of the bright sources are early−types.