AGN & Quasars

Quasars are among the most luminous sources in the universe, emitting radiation across the electromagnetic spectrum from radio to gamma-rays. Current models suggest the only objects capable of such prodigous energy output are massive black holes at the centre of galaxies, fed by stars, gas and dust. It is thought that Quasars, along with Seyfert Galaxies and Blazars are the same class of object viewed at different orientations and at different distances. Quasars are located at high redshifts in the distant universe. Blazars are nearer to us and are exactly what we would expect to see if a polar radio jet was angled directly towards us. In Seyfert galaxies the bright central source is partially obscured by a dusty torus and we receive only low energy gamma-rays and broad emission lines from highly excited hot gas.

Various groups in Australia are researching AGN and quasars, including the AGN group at ANU, who focus on the radio jets squirted out by most AGN, and the interaction of these jets with the surrounding gas and stars. The Astrophysics Group at the University of Melbourne has a wide-ranging research program in AGN and QSOs which includes detailed studies of the structure of AGN central regions using gravitational microlensing and the structure and emission mechanisms of radio-loud AGN. The group has been particularly interested in the physics of ‘red quasars’. The University of Sydney Institute of Astronomy utilise SUMSS to image the entire southern sky at 843 MHz with similar sensitivity and resolution to the northern NRAO VLA Sky Survey (NVSS). It will catalogue over 200,000 new radio galaxies and quasars, and the IoA are using the data to study many aspects of the physics and cosmic evolution of AGN, especially by combining radio data with optical spectra from the 2dFGRS and 6dFGS. The Theoretical Astrophysics group at the University of Sydney studies the radio-wave scintillation properties of active galaxies and quasars, using observations of variability to investigate both he structure of the sources themselves and the Galactic interstellar medium which gives rise to the observed intensity fluctuations. Particular attention is given to the extreme forms of variability – the fast IntraDay Variations and the Extreme Scattering Events – which reveal previously unstudied aspects of the interstellar medium, and probe the structure of active galaxies in unprecedented detail. The Astrophysics Theory Group at the University of Adelaide is interested in modeling the multi-wavelength emissions from blazars and quasars. Detailed studies of the rapid variability and flaring in AGN, as well as the radiation fields existing in the central AGN are also investigated; fluxes of neutrino and cosmic rays are predicted for some AGN in which protons are accelerated to ultra high energies in relativistic jets. The University of Tasmania is involved in studies of AGN and quasars at high resolution using VLBI and scintillation techniques. The UTas telescope at Mt Pleasant and Ceduna are part of the Australian Long Baseline Array and regularly are used to make milliarcsecond images of the central regions of active galaxies. Objects of interest include gravitational lenses and nearby FR-I, FR-II and GPS sources. Recently the group has commenced a project using the Ceduna antenna to continuously monitor a sample of intra-day variable sources. The results of this monitoring will reveal information on the source structure and intervening interstellar medium at microarcsecond resolutions. Another avenue of study of AGN is through observations of water megamasers, which enable the dynamics of central regions of nearby active galaxies to be probed on sub-parsec scales. A large collaboration involving the CfA, ATNF and University of Tasmania are using the Tidbinbilla antenna to search for new water megamasers sources and using the ATCA and VLBI for follow-up imaging of these interesting sources.

The 2dF QSO Redshift Survey (2QZ) is the world’s largest quasar redshift survey to date, having captured 23,000 quasar spectra – 45 times more than the largest previous survey. It was carried out during 1998-2002 with the 2dF instrument on the Anglo-Australian Telescope, and complements the 2dF Galaxy Redshift Survey. Early data from the survey indicated that QSOs are clustered to the same extent as local, optically selected galaxies – a result previously only hinted at. The survey findings will open up new cosmological studies, such as measurements of the Universal space curvature by through the extent to which foreground galaxies ‘gravitationally lens’ the light from background quasars.

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