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Giant gaseous layers(termed "superdisks") have been hypothesized in the past to account for the strip-like radio emission gap(or straight-edged central brightness depression) observed between twin radio lobes, in over a dozen relatively nearby powerful Fanaroff-Riley Class II radio galaxies. They could also provide a plausible alternative explanation for a range of observations. Although a number of explanations have been proposed for the origin of the superdisks, little is known about their material content. Some X-ray observations of superdisk candidates indicate the presence of hot gas, but a cool dusty medium also seems to be common. If they are entirely or partly composed of neutral gas, then it may be directly detectable and we report here a first attempt to detect/image any neutral hydrogen gas present in the superdisks that are inferred to be present in four nearby radio galaxies. We have not found a positive H I signal in any of the four sources, resulting in tight upper limits on the H I number density in the postulated superdisks,estimated directly from the central rms noise values of the final radio continuum subtracted image. The estimated ranges of the upper limit on neutral hydrogen number density and column density are 10-4-10-3 atoms per cm~3 and 10~(19)-10~(20) atoms per cm~2, respectively. No positive H I signal is detected even after combining all the four available H I images(with inverse variance weighting). This clearly rules out an H I dominated superdisk as a viable model to explain these structures, however, the possibility of a superdisk being composed of warm/hot gas still remains open. 相似文献
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Intranight optical variability of blazars 总被引：1，自引：0，他引：1
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A. Mangalam Gopal-Krishna Paul J. Wiita 《Monthly notices of the Royal Astronomical Society》2009,397(4):2216-2224
The recently discovered apparent dramatic expansion in the effective radii of massive elliptical galaxies from z ≃ 2 to ≃0.1 has been interpreted in terms of either galaxy mergers or the rapid loss of cold gas due to active galactic nuclei (AGN) feedback. In examining the latter case, we have quantified the extent of the expansion, which is uncertain observationally, in terms of the star formation parameters and time of the expulsion of the cold gas. In either case, the large global decrease in stellar density should translate into a major drop in the interstellar medium density and pressure with cosmic epoch. These cosmological changes are expected to have a major influence on the gas accretion mode, which will shift from 'cold' thin disc accretion at high redshifts towards 'hot' Bondi fed Advection Dominated Accretion Flow (ADAF) accretion at low redshifts. The decline of angular momentum inflow would then lead to a spin down of the black hole, for which we have calculated more precise time-scales; a value of about 0.2 Gyr is typical for a 109 M⊙ central black hole. These results have implications for the different cosmological evolutionary patterns found for the luminosity functions of powerful and weak radio galaxies. 相似文献
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Arti Goyal Gopal-Krishna G. C. Anupama D. K. Sahu R. Sagar S. Britzen M. Karouzos † M. F. Aller H. D. Aller 《Monthly notices of the Royal Astronomical Society》2009,399(3):1622-1632
We present the result of our extensive intranight optical monitoring of the well-known low-energy peaked BL Lac (LBL) object PKS 0735+178. This long-term follow-up consists of R -band monitoring for a minimum duration of ∼4 hours, on 17 nights spanning 11 years (1998–2008). Using the CCD as an N-star photometer, a detection limit of around 1 per cent was attained for the intranight optical variability (INOV). Remarkably, an INOV amplitude of ≥3 per cent on hour-like time-scale was not observed on any of the 17 nights, even though the likelihood of a typical LBL showing such INOV levels in a single session of ≳4 hours duration is known to be high (∼50 per cent) . Our observations have thus established a peculiar long-term INOV quiescence of this radio-selected BL Lac object. Moreover, the access to unpublished optical monitoring data of similarly high sensitivity, acquired in another programme, has allowed us to confirm the same anomalous INOV quiescence of this LBL all the way back to 1989, the epoch of its historically largest radio outburst. Here, we present observational evidence revealing the very unusual INOV behaviour of this classical BL Lac object and discuss this briefly in the context of its other known exceptional properties. 相似文献
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Conceivably, the centres of all massive galaxies emit continuous antipodal beams of relativistic particles. We shall argue that these beams consist of highly relativistic electrons and positrons. At formation, the beams plough a channel through the ambient medium; the swept-up matter repeatedly stalls the particles which thereby follow closely the ambient pressure gradient. Thereafter, when the relativistic beam particles traverse these swarms of swept-up filamentary matter, they are repeatedly forced to bypass individual filaments and radiate (in the direction of their instantaneous motion). This forward-peaked radiation pattern can explain the preferred one-sided-ness of the observed radio/optical/X-ray knots in the jets. Pressure confinement focusses the beams, and temporarily freezes the channels. Interaction with the intergalactic wind can bend the beams into the shape of a warpedU, on length scales above some 20 kpc. The observed morphologies of extragalactic radio sources find a common explanation.On leave from Tata Institute of Fundamental Research, Ootacamund, India. 相似文献
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Gopal-Krishna Paul J. Wiita Samir Dhurde 《Monthly notices of the Royal Astronomical Society》2006,369(3):1287-1292
Allowing for the conical shape of ultrarelativistic blazar jets with opening angles of a few degrees on parsec-scales, we show that their bulk Lorentz factors and viewing angles can be much larger than the values usually inferred by combining their flux-variability and proper-motion measurements. This is in accord with our earlier finding that such ultrarelativistic (Lorentz factor, Γ > 30 ) conical jets can reconcile the relatively slow apparent motions of Very Long Baseline Interferometry (VLBI) knots in TeV blazars with the extremely fast flows implied by their rapid γ-ray variability. This jet geometry also implies that de-projected jet opening angles will typically be significantly underestimated from VLBI measurements. In addition, de-projected jet lengths will be considerably overestimated if high Lorentz factors and significant opening angles are not taken into account. 相似文献
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Joydeep Bagchi Joe Jacob Gopal-Krishna Norbert Werner Nitin Wadnerkar Jaydeep Belapure A. C. Kumbharkhane 《Monthly notices of the Royal Astronomical Society》2009,399(2):601-614
We present detailed observations of MRC 0116+111, revealing a luminous, miniradio halo of ∼240-kpc diameter located at the centre of a cluster of galaxies at redshift z = 0.131 . Our optical and multiwavelength Giant Metrewave Radio Telescope and Very Large Array radio observations reveal a highly unusual radio source: showing a pair of giant (∼100-kpc diameter) bubble-like diffuse structures, that are about three times larger than the analogous extended radio emission observed in M87 – the dominant central radio galaxy in the Virgo cluster. However, in MRC 0116+111 we do not detect any ongoing active galactic nucleus (AGN) activity, such as a compact core or active radio jets feeding the plasma bubbles. The radio emitting relativistic particles and magnetic fields were probably seeded in the past by a pair of radio jets originating in the AGN of the central cD galaxy. The extremely steep high-frequency radio spectrum of the north-western bubble, located ∼100 kpc from cluster centre, indicates radiation losses, possibly because having detached, it is rising buoyantly and moving away into the putative hot intracluster medium. The other bubble, closer to the cluster centre, shows signs of ongoing particle re-acceleration. We estimate that the radio jets which inflated these two bubbles might have also fed enough energy into the intracluster medium to create an enormous system of cavities and shock fronts, and to drive a massive outflow from the AGN, which could counter-balance and even quench a cooling flow. Therefore, this source presents an excellent opportunity to understand the energetics and the dynamical evolution of radio jet inflated plasma bubbles in the hot cluster atmosphere. 相似文献