arXiv subjects

Gopal-Krishna

Publications and source records attributed to Gopal-Krishna.

117 records · Page 7Linked to original sources

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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arXiv2000

Reviving Fossil Radio Plasma in Clusters of Galaxies by Adiabatic Compression in Environmental Shock Waves

We give for a plasma with a history of several expansion and contraction phases an analytical model of the evolution of a contained relativistic electron population under synchrotron, inverse Compton and adiabatic energy losses or gains. This is applied to different scenarios for evolution of radio plasma inside the cocoons of radio galaxies, after the activity of the central engine has ceased. It is demonstrated that fossil radio plasma with an age of even up to 2 Gyr can be revived by compression in a shock wave of large-scale structure formation, caused during the merging events of galaxy clusters, or by the accretion onto galaxy clusters. We argue, that this is a highly plausible explanation for the observed cluster radio relics, which are the regions of diffuse radio emission found in clusters of galaxies, without any likely parent radio galaxy seen nearby. An implication of this model is the existence of a population of diffuse, ultra-steep spectrum, very low frequency radio sources located inside and possibly outside of clusters of galaxies, tracing the revival of aged fossil radio plasma by the shock waves associated with large-scale structure formation.

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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

Are Cluster Radio Relics Revived Fossil Radio Cocoons?

A new model for the, so called, `cluster radio relics' is presented (Ensslin & Gopal-Krishna 2000). Fossil radio cocoons, resulting from the former activity of radio galaxies, should contain a low energy relativistic electron population and magnetic fields. Electrons with an age of even up to 2 Gyr can be re-accelerated adiabatically to radio emitting energies, if the fossil radio plasma gets compressed in an environmental shock wave. Such a wave can be caused by a merging event of galaxy clusters, or by the accretion onto galaxy clusters. An implication of this model is the existence of a population of diffuse, ultra-steep spectrum, very low-frequency radio sources located inside and possibly outside of clusters of galaxies, tracing the revival of aged fossil radio plasma by the shock waves associated with large-scale structure formation.

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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

Radio Jet-blown Neutral Hydrogen Supershells in Spiral Galaxies?

Taking a clue from the pair of HI supershells found in the Scd galaxy NGC 3556 (M 108), we propose a new mechanism for the origin of HI supershells in gas-rich massive galaxies. In this scenario, the two supershells were inflated out of the neutral hydrogen disk due to the localised flaring of a pair of radio lobes formed by the jets ejected from the nucleus during an active phase about 10^7 years ago, but have faded away by now. It is shown that the salient features of this supershell pair, such as their symmetrical locations about the galactic centre, the anomalously large energy requirements, the large galactocentric distances, as well as the Z-symmetric hemispherical shapes, find a more natural explanation in terms of this scenario, as compared to the standard models which postulate either a massive starburst, or the infall of external gas clouds. Other possible implications of this hypothesis are briefly discussed.

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arXiv1998

Do the central engines of quasars evolve by accretion ?

According to a currently popular paradigm, nuclear activity in quasars is sustained via accretion of material onto super-massive black holes located at the quasar nuclei. A useful tracer of the gravitational field in the vicinity of such central black holes is available in the form of extremely dense gas clouds within the broad emission-line region (BLR) on the scale of $\sim 1~$parsec. Likewise, the radio sizes of the lobe-dominated radio sources are believed to provide a useful statistical indicator of their ages. Using two homogeneously observed (and processed) sets of lobe-dominated radio-loud quasars, taken from literature, we show that a positive correlation exists between the radio sizes of the quasars and the widths of their broad $Hβ$ emission lines, and this correlation is found to be significantly stronger than the other well known correlations involving radio size. This statistical correlation is shown to be consistent with the largest (and, hence, very possibly the oldest) radio sources harboring typically an order-of-magnitude more massive central engines, as compared to the physically smaller and, hence, probably much younger radio sources. This inference is basically in accord with the "accreting central engine" picture for the radio-loud quasars.

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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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arXiv1994

Unified schemes for active galaxies: a clue from the missing Fanaroff-Riley type I quasar population

We link the lack of FR I type structure among quasars to the void of radio loud quasars below a critical disk luminosity of ~10^46 erg/sec in the PG sample. We argue that the opening angle of the obscuring torus in radio loud quasars depends on the power of the central engine, approaching the jet's beaming angle near the FR I/FR II break. Consequently, low power radio quasars would either be classified as radio galaxies (FR I) or strongly core-boosted (BL Lac) object, depending on the aspect angle, and no conspicuous transitional population would be expected for FR I sources. A closing torus with decreasing power would not only obscure the optical nucleus for most aspect angles but would also enhance the entrainment of the cool torus material into the jet stream, causing obscuration along the jet's periphery, as well as the jet's deceleration to form a FR I source. Above a critical luminosity, the wider torus allows for FR II type jets and visibility of the nuclear optical emission, characteristic of radio loud quasars. Apparently, at the same engine power the torus opening in radio weak quasars and Seyferts is substantially wider than in radio loud quasars, probably because of different dynamics or feeding mechanisms in disk and elliptical galaxies. This provides a clue for the radio-loud/radio-quiet dichotomy of quasars if the the jet/torus interaction leads to injection of relativistic e^+/- pairs via p-p collisions. Strong jet/torus interaction may lead to a substantial injection of secondary pairs and collimation in radio loud quasars, while weak interaction in radio weak quasars leads neither to pair injection nor to good collimation.

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