Gopal-Krishna

Bio: Gopal-Krishna is an academic researcher from Max Planck Society. The author has contributed to research in topics: Radio galaxy & Quasar. The author has an hindex of 34, co-authored 153 publications receiving 3249 citations. Previous affiliations of Gopal-Krishna include Tata Institute of Fundamental Research & Space Telescope Science Institute.

Reviving fossil radio plasma in clusters of galaxies by adiabatic compression in environmental shock waves

Abstract: 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.

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

Abstract: 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.

The Origin of X-shaped Radio Galaxies: Clues from the Z-symmetric Secondary Lobes

Abstract: Existing radio images of a few X-shaped radio galaxies reveal Z-symmetric morphologies in their weaker secondary lobes that cannot be naturally explained by either the galactic merger or radio-lobe backflow scenarios, the two dominant models for these X-shaped radio sources. We show that the merger picture can explain these morphologies provided one takes into account that, prior to the coalescence of their supermassive black holes, the smaller galaxy releases significant amounts of gas into the interstellar medium of the dominant active galaxy. This rotating gas, whose angular momentum axis will typically not be aligned with the original jets, is likely to provide sufficient ram pressure at a distance ~10 kpc from the nucleus to bend the extant jets emerging from the central engine, thus producing a Z-symmetry in the pair of radio lobes. Once the two black holes have coalesced some 107 yr later, a rapid reorientation of the jets along a direction close to that of the orbital angular momentum of the swallowed galaxy relative to the primary galaxy would create the younger primary lobes of the X-shaped radio galaxy. This picture naturally explains why such sources typically have powers close to the FR I/II break. We suggest that purely Z-symmetric radio sources are often en route to coalescence and the concomitant emission of substantial gravitational radiation, while X-shaped ones have already merged and radiated.

Multiband optical monitoring of the blazars S5 0716+714 and BL Lacertae

Abstract: We report results of multiband optical photometric monitoring of two well knownblazars, S5 0716+714 and BL Lacertae, carried out during 1996 and 2000−01 withan aim to study optical variations on time scales ranging from minutes to hours andlonger. The light curves were derived relative to comparison stars present on theCCD frames. Night to night intensity variations of > 0.1 mag were observed in S50716+714during a campaign of about 2 weeks in 1996. A good correlation between thelightcurves in different optical bands was found for both inter-night and intra-night ob-servations. In all, two prominent events of intra-night optical variability (INOV) weredetected in S5 0716+714. Each of these rapidly varying segments of the lightcurvescan be well fitted with an exponential intensity profile whose rate of variation is essen-tially the same in both cases. Our long-term monitoring data of S5 0716+714 showeda distinct flare around JD 2451875 which can be identified in the BVRI bands. Thisflare coincides with the brightest phase recorded during 1994−2001 in the long-termlightcurves reported by Raiteri et al. (2003). No evidence for the object to becomebluer when brighter was noticed on either inter-night or intra-night time scales. On theother hand, our essentially simultaneous multiband optical observations of BL Lac-ertae in October 2001 showed flux variations that were not achromatic. This blazardefinitely was found to become bluer when brighter on intra-night time scales andthere is a less significant trend of the same type on inter-night time scales. Based onfive nights of observations during a week, BL Lacertae showed a peak night-to-nightvariability of ∼ 0.6 mag in B. Thus, we found that the present optical observations ofthe two prominent blazars, made with similarly high sensitivity, reveal a contrastingbehaviour in terms of the dependence of spectral hardening with increasing brightness,at least on intra-night, and possibly also on inter-night, time scales.Key words: BL Lacertae objects: individual: S5 0716+714, BL Lacertae – galaxies:active – galaxies: photometry

Observational Evidence of Active Galactic Nuclei Feedback

Abstract: Radiation, winds, and jets from the active nucleus of a massive galaxy can interact with its interstellar medium, and this can lead to ejection or heating of the gas. This terminates star formation in the galaxy and stifles accretion onto the black hole. Such active galactic nuclei (AGN) feedback can account for the observed proportionality between the central black hole and the host galaxy mass. Direct observational evidence for the radiative or quasar mode of feedback, which occurs when AGN are very luminous, has been difficult to obtain but is accumulating from a few exceptional objects. Feedback from the kinetic or radio mode, which uses the mechanical energy of radio-emitting jets often seen when AGN are operating at a lower level, is common in massive elliptical galaxies. This mode is well observed directly through X-ray observations of the central galaxies of cool core clusters in the form of bubbles in the hot surrounding medium. The energy flow, which is roughly continuous, heats the hot intraclu...

Monthly Notices of the Royal Astronomical Society

Abstract: Monthly Notices is one of the three largest general primary astronomical research publications. It is an international journal, published by the Royal Astronomical Society. This article 1 describes its publication policy and practice.

The Compact Steep-Spectrum and Gigahertz Peaked-Spectrum Radio Sources

Abstract: I review the radio to X-ray properties of gigahertz peaked-spectrum (GPS) and compact steep- spectrum (CSS) sources, the current hypotheses for their origin, and their use to constrain the evolution of powerful radio galaxies. The GPS and CSS sources are compact, powerful radio sources with well-defined peaks in their radio spectra (near 1 GHz in the GPS and near 100 MHz in the CSS). The GPS sources are entirely contained within the extent of the narrow-line region (&1 kpc), while the CSS sources are contained entirely within the host galaxy (&15 kpc). The peaks in the spectra are probably due to synchrotron self-absorption, though free-free absorption through an inhomogeneous screen may also play a role. The turnover frequency varies with linear size l as , suggesting a simple physical relationship between these parameters. The radio 20.65 n / l m morphologies are strikingly like those of the large-scale classical doubles, though some sources can have very distorted morphologies suggestive of interactions. Radio polarization tends to be low, and in some cases the Faraday rotation measures can be extremely large. The IR properties are consistent with stellar populations and active galactic nucleus (AGN) bolometric luminosity similar to that of the 3CR classical doubles. The optical host galaxy properties (absolute magnitude, Hubble diagram, evidence for interaction) are consistent with those of the 3CR classical doubles. CSS sources at all redshifts exhibit high surface brightness optical light (most likely emission-line gas) that is aligned with the radio axis. The optical emission-line properties suggest (1) interaction of the radio source with the emission-line gas and (2) the presence of dust toward the emission-line regions. X- ray observations of high-redshift GPS quasars and a couple of GPS galaxies suggest the presence of significant columns of gas toward the nuclei. Searches for cold gas in the host galaxies have revealed large amounts of molecular gas and smaller amounts of atomic gas in several sources, though probably not enough to confine the radio sources. The main competing models for the GPS and CSS sources are that (1) they are frustrated by interaction with dense gas in their environments and (2) they are young and evolving radio sources that will become large-scale sources. Combining the bright GPS and CSS samples with the 3CR results in a sample spanning a range in source size of 10 5 that can be used to study source evolution. The number density versus linear size relation is consistent with a picture in which the sources expand with constant velocity and the radio power drops with linear size l according to . This strong evolution suggests that at least some of the 20.5 P / l GPS and CSS sources evolve to become lower luminosity FR 1 radio sources. The GPS and CSS sources are important probes of their host galaxies and will provide critical clues to the origin and evolution of powerful radio sources.