Extremely relativistic electron–positron twin-jets form extragalactic radio sources

Abstract

The extended extragalactic double radio sources—quasars, BL Lac-type-objects (blazars), and radio galaxies—are commonly interpreted as synchrotron sources in which extremely relativistic electrons gyrate in enhanced magnetic fields. The detection of narrow emission bridges between hot spots inside these sources and the centre of an intermediate galaxy has strongly favoured the existence of a continuous power-line feeding the extended source^1,3,5. Such beams may be a universal phenomenon occurring in many—if not all—massive galaxies, including our own⁴. But whereas early interpretations involved low-frequency electromagnetic waves and/or relativistic particle beams, more recent work favoured non-relativistic (β <10⁻¹) (refs 3, 6–8), or mildly relativistic (γ <10) (refs 9–11) bulk velocities for the power supply. In particular, non-relativistic bulk velocities c β have been derived from estimates of the involved kinetic energy densities u and mass densities ρ in the form β≍(2u/ρ)^1/2. This can lead to large underestimates of β when ρ is derived from Faraday rotation and depolarization data, because the observed jets are likely to have a two-fluid structure, with light relativistic plasma streaming inside of heavy ‘walls’ of thermal matter, or traversing ‘swarms’ of heavy quasistatic filaments¹². We suggest here that these beams consist of extremely relativistic electrons and positrons, of typical Lorentz factor γ ≳10².