Computer Science
Scientific paper
Feb 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009phdt.......111d&link_type=abstract
PhD Dissertation, University of Sheffield, 229 pages
Computer Science
Radio Galaxies, Infrared
Scientific paper
Until recently, the advantages of using mid- to far-infrared (MFIR) observations to investigate the thermal dust emission from active galaxies (AGN) had not been fully exploited. Therefore, this thesis presents the results from deep MFIR imaging observations (Spitzer/MIPS) of complete samples of 2Jy and 3CRR radio galaxies (0.03< z <0.7), as well as the results of two complementary radio imaging program (ATCA, VLA) for the 2Jy sample at high radio frequencies. The Spitzer data have been used to investigate the origin of the thermal MFIR emission, focussing on the correlations between 24, 70μm and [OIII]λ5007 luminosities. Because [OIII]λ5007 emission line luminosity acts as a proxy for intrinsic AGN power, the tight correlations found between MFIR and [OIII]λ5007 luminosities provide strong evidence that direct AGN illumination is the primary heating mechanism for the dust producing both the mid-IR and far-IR continuum. Using optical spectroscopic information about the level of recent star formation activity in the host galaxies, I deduce that starbursts significantly boost the far-IR emission in only a minority of radio-loud AGN (17-35%). This result is supported by analysis of polycyclic aromatic hydrocarbon star formation tracers in objects observed with Spitzer/IRS as well as the analysis of MFIR colours. Given the lack of optical and infrared evidence for starburst activity, it is unlikely that the majority of powerful radio-loud AGN are triggered at the peaks of major, gas rich mergers. In addition, at high [OIII] luminosities, the similarities between the MFIR properties (luminosities and colours) of broad-line radio galaxies (BLRG), quasars and narrow line radio galaxies support orientation-based unified schemes for powerful radio sources. However there remain groups of objects at low redshifts/luminosities, (e.g. weak line radio galaxies and low redshift 3C BLRG) whose characteristics at MFIR wavelengths are not readily explained in the simplest versions of such schemes. Overall, the results presented here underline the merits of observing complete samples of AGN at MFIR wavelengths. Much of the work presented in this thesis forms the basis of 3 publications (Tadhunter et al., 2007, Dicken et al., 2008, 2009).
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