424 research outputs found

    Deflection of jets induced by jet-cloud & jet-galaxy interactions

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    The model first introduced by Raga & Canto (1996) in which astrophysical jets are deflected on passing through an isothermal high density region is generalised by taking into account gravitational effects on the motion of the jet as it crosses the high density cloud. The problem is also generalised for relativistic jets in which gravitational effects induced by the cloud are neglected. Two further cases, classical and relativistic, are discussed for the cases in which the jet is deflected on passing through the interstellar gas of a galaxy in which a dark matter halo dominates the gravitational potential. The criteria for the stability of jets due to the formation of internal shocks are discussed.Comment: 8 pages, 5 figures, Accepted for publication in MNRA

    Synchrotron Radiation from the Galactic Center in Decaying Dark Matter Scenario

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    We discuss the synchrotron radiation flux from the Galactic center in unstable dark matter scenario. Motivated by the anomalous excess of the positron fraction recently reported by the PAMELA collaboration, we consider the case that the dark matter particle is unstable (and long-lived), and that energetic electron and positron are produced by the decay of dark matter. Then, the emitted electron and positron becomes the source of the synchrotron radiation. We calculate the synchrotron radiation flux for models of decaying dark matter, which can explain the PAMELA positron excess. Taking the lifetime of the dark matter of O(10^26 sec), which is the suggested value to explain the PAMELA anomaly, the synchrotron radiation flux is found to be O(1 kJy/str) or smaller, depending on the particle-physics and cosmological parameters.Comment: 20 pages, 6 figure

    Radio constraints on dark matter annihilation in the galactic halo and its substructures

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    Annihilation of Dark Matter usually produces together with gamma rays comparable amounts of electrons and positrons. The e+e- gyrating in the galactic magnetic field then produce secondary synchrotron radiation which thus provides an indirect mean to constrain the DM signal itself. To this purpose, we calculate the radio emission from the galactic halo as well as from its expected substructures and we then compare it with the measured diffuse radio background. We employ a multi-frequency approach using data in the relevant frequency range 100 MHz-100 GHz, as well as the WMAP Haze data at 23 GHz. The derived constraints are of the order =10^{-24} cm3 s^{-1} for a DM mass m_chi=100 GeV sensibly depending however on the astrophysical uncertainties, in particular on the assumption on the galactic magnetic field model. The signal from single bright clumps is instead largely attenuated by diffusion effects and offers only poor detection perspectives.Comment: 12 pages, 7 figures; v2: some references added, some discussions enlarged; matches journal versio

    A jet-cloud interaction in 3C34 at redshift z = 0.69

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    We report the detection of a strong jet-cloud interaction at a distance of 120 kpc from the nucleus of the radio galaxy 3C34, which has redshift z=0.69. Hubble Space Telescope images of the radio galaxy show a long narrow region of blue emission orientated along the radio axis and directed towards a radio hotspot. The William Herschel Telescope has been used to provide long-slit spectroscopic data of this object, and infrared observations made with the United Kingdom InfraRed Telescope have enabled its spectral energy distribution to be modelled. We propose that the aligned emission is associated with a region of massive star-formation, induced by the passage of the radio jet through a galaxy within the cluster surrounding 3C34. A star-formation rate of about 100 solar masses per year is required, similar to the values necessary to produce the alignment effect in high-redshift radio galaxies. The consequences of this result for models of star formation in distant radio galaxies are discussed.Comment: 12 pages including 11 figures, LaTeX. To appear in MNRA

    Diffuse Background Radiation

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    A new determination of the upper limit to the cosmic diffuse background radiation, at ~110 nm, of 300 photons s-1 cm-2 sr-1 nm-1, is placed in the context of diffuse background measurements across the entire electromagnetic spectrum, including new optical, infrared, visible, and gamma-ray background measurements. The possibility that observed excess diffuse visible radiation is due to redshifted cosmological Lyman alpha recomination radiation is explored. Also, a new standard of units for the display of spectra is advocated.Comment: Nine pages and one figur

    Deep Spectroscopy in the Field of 3C 212

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    We present the results of longslit and multiaperture spectroscopy of faint galaxies in the field of the z=1.049 quasar 3C212. We show that an apparently aligned optical feature beyond the NW radio lobe has a redshift z=0.928, and a similarly aligned feature just beyond the SE radio lobe has a redshift z=1.053, quite close to that of the quasar. While the NW optical component is extremely well aligned with the radio jet and has a morphology that is very similar to that of the radio lobe lying 3" interior to it, the fact that we find three other field galaxies with closely similar redshifts indicates that it is most likely a member of an intervening group rather than an unusual example of true alignment with the radio structure. In addition, we have found two galaxies (besides the one near the SE radio lobe) having redshifts close to that of 3C212. We have firm or probable redshifts for 66 out of 82 galaxies we have observed in this field. Comparison with the redshift distribution of faint field galaxies indicates that a large fraction of the remaining 16 galaxies are likely to be at redshifts >1.4. There are at least two low-redshift dwarf starburst galaxies showing low metal abundance and high ionization in our sample; such galaxies are likely to be relatively common in very faint samples, and, under certain conditions, they could be confused with high-redshift objects.Comment: 15 pages, 4 figures, one of which is a separate JPEG plate. To be published in the April 1998 issue of The Astronomical Journa
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