Recurrent activity in Active Galactic Nuclei

D.J. Saikia1,2 and M. Jamrozy3
1National Centre for Radio Astrophysics, Post Bag 3, Ganeshkhind, Pune 411 007
2ICRAR, University of Western Australia, Crawley, WA 6009, Australia
3 Obserwatorium Astronomiczne, Uniwersytet Jagiello'nski, ul. Orla 171, PL-30244 Krakow, Poland

Abstract. There has been a growing body of evidence to suggest that AGN activity, which is powered by mass accretion on to a supermasive black hole, could be , although the range of time scales involved needs to be explored further. The structure and spectra of radio emission from radio galaxies, whose sizes range up to ~5 Mpc, contain information on the history of AGN activity in the source. They thus provide a unique opportunity to study the time scales of recurrent AGN activity. The most striking examples of recurrent activity in radio galaxies and quasars are the double-double or triple-double radio sources which contain two or three pairs of distinct lobes on opposite sides of the parent optical object. Spectral and dynamical ages of these lobes could be used to constrain time scales of activity. Inverse-Compton scattered cosmic microwave background radiation could in principle probe lower Lorentz-factor particles than radio observations of synchrotron emission, and thereby reveal an older population. We review briefly the radio continuum as well as molecular and atomic gas properties of radio sources which exhibit recurrent or There has been a growing body of evidence to suggest that AGN activity, which is powered by mass accretion on to a supermasive black hole, could be , although the range of time scales involved needs to be explored further. The structure and spectra of radio emission from radio galaxies, whose sizes range up to $sim$5 Mpc, contain information on the history of AGN activity in the source. They thus provide a unique opportunity to study the time scales of recurrent AGN activity. The most striking examples of recurrent activity in radio galaxies and quasars are the double-double or triple-double radio sources which contain two or three pairs of distinct lobes on opposite sides of the parent optical object. Spectral and dynamical ages of these lobes could be used to constrain time scales of activity. Inverse-Compton scattered cosmic microwave background radiation could in principle probe lower Lorentz-factor particles than radio observations of synchrotron emission, and thereby reveal an older population. We review briefly the radio continuum as well as molecular and atomic gas properties of radio sources which exhibit recurrent or activity, and present a few cases of quasars which require further observations to confirm their nature. We also illustrate evidence of AGN activity in radio sources in clusters of galaxies.

Keywords: galaxies: active -- galaxies: nuclei -- radio continuum: galaxies -- galaxies: quasars: general -- galaxies: clusters: general

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