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A long-standing question in extragalactic astrophysics is the composition of the relativistic jets of plasma that stream from the nuclei of quasars and active galaxiesÐdo they consist of a`normal' (electron±proton) plasma, or a`pair' (electron±positron) plasma? Distinguishing between these possibilities is crucial for understanding the physical processes occurring close to the putative supermassive black holes that are believed responsible for the jets. Here we report the detection of circularly polarized radio emission from the jets of the archtypal quasar 3C279. The circular polarization is produced by Faraday conversion, which requires the energy distribution of the radiating particles to extend to very low energies, indicating that electron±positron pairs are an important component of the jet plasma. Similar detections in three other radio sources suggest that, in general, extragalactic radio jets are composed mainly of an electron±positron plasma.The most luminous objects in the Universe are distant quasars and active galactic nuclei. A few per cent of these emit strongly at radio wavelengths, and are powered by twin jets of plasma streaming at close to the speed of light from the nucleus of the underlying galaxy 1 . The central`engine' in the nucleus is widely believed to be a supermassive black hole of some 10 8 ±10 9 solar masses 2 . At radio wavelengths the jets are visible from parsec to kiloparsec scales, emitting synchrotron radiation from ultra-relativistic electrons gyrating in a magnetic ®eld. The bulk kinetic energy carried by the jets powers the extended lobes of radio emission far outside the parent galaxy, while synchrotron and inverse Compton emission from the base of the jets generates radiation at optical, X-ray and even g-ray wavelengths 3,4 . The composition of the jet plasma has been an unresolved issue ever since the discovery of the jets. The two main candidates are a`normal' plasma consisting of protons and relativistic electrons (an e±p jet), and a`pair plasma' consisting only of relativistic electrons and positrons (an e + ±e -jet).Electrons and positrons emit synchrotron radiation of identical spectrum and linear polarization, so it is dif®cult to tell whether the radiating particles are mainly electrons or a mixture of electrons and positrons. Two lines of argument shed light on the problem. The ®rst comes from matching the total energy¯ux carried by the jet to the total dissipation observed in electromagnetic radiationÐthe energy stored in the extended radio lobes in magnetic ®elds and relativistic particlesÐand the mechanical work done against the surrounding medium 5,6 . The energy distribution of the radiating particles is assumed to be a power law, ng~g 2 2a1 for g min , g , g max , where n is the density, g 1 2 v 2 =c 2 2 1 2 is the Lorentz factor of a particle of velocity v, a is the observed spectral index of the synchrotron radiation, and c is the velocity of light. For an e±p jet, the energy distribution must cut off at g min < 100, or the jet may carry several orders of ...

We compute the ionizing radiation field at low redshift, arising from Seyferts, QSOs, and starburst galaxies. This calculation combines recent Seyfert luminosity functions, extrapolated ultraviolet fluxes from our IUE-AGN database, and a new intergalactic opacity model based on Hubble Space Telescope and Keck Ly-alpha absorber surveys. At z = 0 for AGN only, our best estimate for the specific intensity at 1 Ryd is I_0 = 1.3 (+0.8/-0.5) x 10^-23 ergs/cm^2/s/Hz/sr, independent of H_0, Omega_0, and Lambda. The one-sided ionizing photon flux is Phi_ion = 3400 (+2100/-1300) photons/cm^2/s, and the H I photoionization rate is Gamma_HI = 3.2 (+2.0/-1.2) x 10^-14 s^-1 for alpha_s = 1.8. We also derive Gamma_ HI for z = 0 - 4. These error ranges reflect uncertainties in the spectral indexes for the ionizing EUV (alpha_s = 1.8 +/- 0.3) and the optical/UV (alpha_UV = 0.86 +/- 0.05), the IGM opacity model, the range of Seyfert luminosities (0.001 - 100 L*) and the completeness of the luminosity functions. Our estimate is a factor of three lower than the most stringent upper limits on the ionizing background (Phi_ion < 10^4 photons/cm^2/s) obtained from H-alpha observations in external clouds, and it lies within the range implied by other indirect measures. Starburst galaxies with a sufficiently large Lyman continuum escape fraction, f_ esc > 0.05, may provide a comparable background to AGN, I_0 (z=0) = 1.1 (+1.5/-0.7) x 10^{-23). An additional component of the ionizing background of this magnitude would violate neither upper limits from H-alpha observations nor the acceptable range from other measurements.Comment: 30 pages, 9 figures, accepted for Astronomical J. (Oct. 1999

We present proper motions obtained from a dual frequency, six-epoch, VLBA polarization experiment monitoring a sample of 12 blazars. The observations were made at 15 GHz and 22 GHz at bi-monthly intervals over 1996. Ten of the eleven sources for which proper motion could be reliably determined are superluminal. Only J2005+77 has no superluminal components. Three sources (OJ 287, J1224 + 21, andJ1512-09)showmotionf asterthan10h −1 c, requiring γ pattern of at least 10h −1 (H 0 = 100h km s −1 Mpc −1 ). We compare our results to those in the literature and find motions outside the previously observed range for four sources. While some jet components exhibit significant non-radial motion, most motion is radial. In at least two sources there are components moving radially at significantly different structural position angles. In five of six sources (3C 120, J1224 + 21, 3C 273, 3C 279, J1512-09, andJ1927 + 73)thathavemultiplecomponentswithmeasurablepropermotion, theinnermostcomponentissignif icantlyslowerthantheother f requencyV LBIdataspanningseveralepochs.