arXiv subjects

Paul J. Wiita

Publications and source records attributed to Paul J. Wiita.

154 records · Page 9Linked to original sources

arXiv2002

Radio galaxies and the star formation history of the universe

Multi-wavelength observations made in the last decade suggest that the universe underwent an intense phase of star formation in the past (z > 1). This intensive activity is commonly attributed to a higher galaxy merger rate when the universe was a fraction of its present age. We examine the role of the powerful radio sources whose comoving density is known to be a few orders of magnitude higher at z ~ 2, the `quasar era'. Taking into account recent models for the temporal evolution of the size and luminosity of a powerful double radio source, as well as Lambda-CDM simulations of the cosmic web of baryonic material at different redshifts, we argue that during the quasar era a high fraction of the volume of the web was occupied by the lobes of double radio sources. Widespread compression of protostellar clouds, triggered by the high pressure of the synchrotron plasma of the radio lobes, can thus be expected to have played a significant role in the global star formation history of the universe. These lobes can also yield a rather high level of magnetization of the intergalactic medium at these early cosmic epochs.

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arXiv2001

Are the hotspots of radio galaxies the sites of in-situ acceleration of relativistic particles?

Using a large set of optically detected hotspots in powerful extragalactic double radio sources, we examine the basic question of whether the detection of optical synchrotron radiation requires in-situ acceleration of relativistic electrons within the hotspots/lobes. For this, we take into account the jet's bulk relativistic motion, as well as its likely misalignment from the plane of the sky. Together, both these factors can drastically reduce the apparent range of the ultra-relativistic electrons ejected from the nucleus in the form of a jet. The conventionally adopted parameter space for the fundamental variables, namely, the hotspot magnetic field, radio source orientation angle relative to the line-of-sight and the bulk speed of the jet plasma, is considered. We find that the observed optical/near-IR synchrotron emission of the hotspots can be explained even if the radiating relativistic electrons were accelerated exclusively within the nuclear region, provided the energy losses incurred by the electrons during their transport down the jet are dominated by inverse Compton upscatterings of the cosmic microwave background photons. Under this circumstance, in-situ acceleration of relativistic electrons inside the hotspots or lobes is not found to be mandated by their reported optical/near-infrared detections.

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arXiv2001

Was the Cosmic Web of Protogalactic Material Permeated by Lobes of Radio Galaxies During the Quasar Era?

Evidence for extended active lifetimes (> 10^8 yr) for radio galaxies implies that many large radio lobes were produced during the `quasar era', 1.5 < z < 3, when the comoving density of radio sources was 2 -- 3 dex higher than the present level. However, inverse Compton losses against the intense microwave background substantially reduce the ages and numbers of sources that are detected in flux-limited surveys. The realization that the galaxy forming material in those epochs was concentrated in filaments occupying a small fraction of the total volume then leads to the conclusion that radio lobes permeated much of the volume occupied by the protogalactic material during that era. The sustained overpressure in these extended lobes is likely to have played an important role in triggering the high inferred rate of galaxy formation at z > 1.5 and in the magnetization of the cosmic network of filaments.

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arXiv2001

The Fanaroff-Riley Transition and the Optical Luminosity of the Host Elliptical Galaxy

We show that a model for radio source dynamics we had earlier proposed can readily reproduce the relationship between the radio power division separating the two Fanaroff-Riley classes of extragalactic radio sources and the optical luminosity of the host galaxy, as found by Owen and Ledlow (1994). In our scenario, when less powerful jets eventually slow down to the point that the advance of the working surface (i.e., hotspot) becomes subsonic with respect to the external gas, the jet's collimation is severely weakened. This criterion distinguishes the powerful and well collimated FR II sources from the weaker sources producing the less collimated FR I type morphologies.

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arXiv2001

Cosmic Radio Jets

Extragalactic radio sources, including quasars, are now typically understood as being produced by a pair of nearly symmetric, oppositely directed relativistic jets. While some these sources span megaparsecs, and are thus the largest physically connected structures in the universe, emitting regions identified as jets have now been found on all scales down to fractions of a parsec, and jets appear to be a common element of most (maybe all) types of active galactic nuclei (AGN). We first summarize key observations of different classes of cosmic radio jets, and describe how they may be connected. Theoretical models for the launching and propagation of extragalactic jets are briefly described. All of these models assume a magnetized plasma, which typically amounts to only a small fraction of the accreted gas, is ejected from the vicinity of a supermassive black hole. The extreme complexity of the relevant physics has demanded numerical simulations to examine non-linear effects on the stability of propagating jets, and some recent results from these efforts are summarized.

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arXiv2000

On the Origin of the Fanaroff-Riley Dichotomy

A small fraction of double radio sources show a peculiar and striking hybrid morphology; they have a distinctly FR I structure on one side of the nucleus, and a FR II structure on the other. We argue that the mere existence of these HYMORS is quite incompatible with the theoretical explanations for the Fanaroff-Riley dichotomy that are based upon the nature of the jet plasma, or those invoking an intrinsic property of the central engine. Rather, these HYMORS strongly support models that explain the difference between FR I and FR II sources in terms of asymmetry of interaction of the jets with the external environments. We further show that a model for radio source dynamics we had earlier proposed can neatly reproduce the observed dependence of the radio power dividing the two FR classes on the optical luminosity of the host galaxy, as found by Owen & White and Ledlow & Owen

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arXiv2000

Extragalactic radio sources with hybrid morphology: implications for the Fanaroff-Riley dichotomy

We provide observational and theoretical perspectives on the currently much debated issue of the Fanaroff-Riley (FR) morphological dichotomy of extragalactic radio sources. In this context we introduce a new, albeit rare, class of double radio sources in which the two lobes exhibit clearly different FR morphologies. It is argued that such `HYbrid MOrphology Radio Sources', or HYMORS, could be used to effectively constrain the theoretical mechanisms proposed for the FR dichotomy. Basically, the existence of HYMORS supports explanations for the FR dichotomy based upon jet interaction with the medium external to the central engine, and appears quite difficult to reconcile with the class of explanations that posit fundamental differences in the central engine, such as black hole spin or jet composition, to be responsible for the two FR classes of double radio sources.

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arXiv2000

Models for Accretion Disk Fluctuations through Self-Organized Criticality Including Relativistic Effects

The possibility that some of the observed X-ray and optical variability in active galactic nuclei and galactic black hole candidates is produced in accretion disks through the development of a self-organized critical state is reconsidered. New simulations, including more complete calculations of relativistic effects, do show that this model can produce light-curves and power-spectra for the variability which agree with the range observed in optical and X-ray studies of AGN and X-ray binaries. However, the universality of complete self-organized criticality is not quite achieved. This is mainly because the character of the variations depend quite substantially on the extent of the unstable disk region. If it extends close to the innermost stable orbit then a physical scale is introduced and the scale-free character of self-organized criticality is vitiated. Significant dependence of the power spectrum density slope on the type of diffusion within the disk and a weaker dependence on the amount of differential rotation are noted. When general relativistic effects are incorporated in the models, additional substantial differences are produced if the disk is viewed from directions far from the accretion disk axis.

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arXiv2000

Effect of Beam-Plasma Instabilities on Accretion Disk Flares

We show that a certain class of flare models for variability from accretion disk coronae are subject to beam-plasma instabilities. These instabilities can prevent significant direct acceleration and greatly reduce the variable X-ray emission argued to arise via inverse Compton scattering involving relativistic electrons in beams and soft photons from the disk.

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arXiv1996

The Linear Sizes of Quasars and Radio Galaxies in the Unified Scheme

A key argument in favor of orientation based unification schemes is the finding that among the most powerful 3CRR radio sources the (apparent) median linear size of quasars is smaller than that of radio galaxies, which supports the idea that quasars are a subset of radio galaxies, distinguished by being viewed at smaller angles to the line of sight. Recent measurements of radio sizes for a few other low frequency samples are, however, not in accord with this trend, leading to the claim that orientation may not be the main difference between radio galaxies and quasars. We point out that this ``inconsistency'' can be removed by making allowance for the temporal evolution of sources in both size and luminosity, as inferred from independent observations. This approach can also readily explain the other claimed ``major discrepancy'' with the unified scheme, namely, the difference between the radio luminosity--size correlations for quasars and radio galaxies.

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