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Register a new userRecurrent Activity in Active Galactic Nuclei
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Authors
D.J. Saikia NCRA
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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 episodic, 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 episodic 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 episodic activity, and present a few cases of quasars which require further observations to confirm their episodic nature. We also illustrate evidence of episodic AGN activity in radio sources in clusters of galaxies.
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There is increasing evidence to suggest that AGN activity may be episodic, with a wide range of possible time scales. Radio galaxies exhibit the most striking examples of episodic activity, with two or three distinct pairs of lobes on opposite sides of the active nucleus. Radio emission from earlier cycles of activity are expected to have steep radio spectra due to radiative losses, and hence be detected more easily at low radio frequencies. Inverse-Compton scattered cosmic microwave background radiation could in prinicple probe even lower Lorentz-factor particles, revealing an older population. We illustrate the time scales of episodic activity by considering different radio galaxies, discuss the possiblity of episodic activity in cluster radio sources, and a possible trend for a high incidence of H{sc i} absorption in sources with evidence of episodic activity.
We analyze the long-term evolution of the fluxes of six active galactic nuclei (AGN) - 0923+392, 3C 111, 3C 273, 3C 345, 3C 454.3, and 3C 84 - in the frequency range 80-267 GHz using archival calibration data of the IRAM Plateau de Bure Interferometer. Our dataset spans a long timeline of ~14 years with 974-3027 flux measurements per source. We find strong (factors ~2-8) flux variability on timescales of years for all sources. The flux density distributions of five out of six sources show clear signatures of bi- or even multimodality. Our sources show mostly steep (alpha~0.5-1), variable spectral indices that indicate outflow dominated emission; the variability is most probably due to optical depth variations. The power spectra globally correspond to red-noise spectra with five sources being located between the cases of white and flicker noise and one source (3C 111) being closer to the case of random walk noise. For three sources the low-frequency ends of their power spectra appear to be upscaled in spectral power by factors ~2-3 with respect to the overall powerlaws. In two sources, 3C 454.3 and 3C 84, the 1.3-mm emission preceeds the 3-mm emission by ~55 and ~300 days, respectively, probably due to (combinations of) optical depth and emission region geometry effects. We conclude that the source emission cannot be described by uniform stochastic emission processes; instead, a distinction of quiescent and (maybe multiple) flare states of the source emission appears to be necessary.
We study the incidence of nuclear obscuration on a complete sample of 1310 AGN selected on the basis of their rest-frame 2-10 keV X-ray flux from the XMM-COSMOS survey, in the redshift range 0.3<z<3.5. We classify the AGN as obscured or un-obscured on the basis of either the optical spectral properties and the overall SED or the shape of the X-ray spectrum. The two classifications agree in about 70% of the objects, and the remaining 30% can be further subdivided into two distinct classes: at low luminosities X-ray un-obscured AGN do not always show signs of broad lines or blue/UV continuum emission in their optical spectra, most likely due to galaxy dilution effects; at high luminosities broad line AGN may have absorbed X-ray spectra, which hints at an increased incidence of small-scale (sub-parsec) dust-free obscuration. We confirm that the fraction of obscured AGN is a decreasing function of the intrinsic X-ray luminosity, while the incidence of absorption shows significant evolution only for the most luminous AGN, which appear to be more commonly obscured at higher redshift. We find no significant difference between the mean stellar masses and star formation rates of obscured and un-obscured AGN hosts. We conclude that the physical state of the medium responsible for obscuration in AGN is complex, and mainly determined by the radiation environment (nuclear luminosity) in a small region enclosed within the gravitational sphere of influence of the central black hole, but is largely insensitive to the wider scale galactic conditions.
The inevitable spread in properties of the toroidal obscuration of active galactic nuclei (AGNs) invalidates the widespread notion that type 1 and 2 AGNs are intrinsically the same objects, drawn randomly from the distribution of torus covering factors. Instead, AGNs are drawn emph{preferentially} from this distribution; type 2 are more likely drawn from the distribution higher end, type 1 from its lower end. Type 2 AGNs have a higher IR luminosity, lower narrow-line luminosity and a higher fraction of Compton thick X-ray obscuration than type 1. Meaningful studies of unification statistics cannot be conducted without first determining the intrinsic distribution function of torus covering factors.
Observing programs comprising multiple scientific objectives will enhance the productivity of NASAs next UV/Visible mission. Studying active galactic nuclei (AGN) is intrinsically important for understanding how black holes accrete matter, grow through cosmic time, and influence their host galaxies. At the same time, the bright UV continuum of AGN serves as an ideal background light source for studying foreground gas in the intergalactic medium (IGM), the circumgalactic medium (CGM) of individual galaxies, and the interstellar medium (ISM) and halo of the Milky Way. A well chosen sample of AGN can serve as the observational backbone for multiple spectroscopic investigations including quantitative measurements of outflows from AGN, the structure of their accretion disks, and the mass of the central black hole.
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