A relativistic model of the radio jets in NGC 315

ABSTRACT

We apply our intrinsically symmetrical, decelerating relativistic jet model to deep Very Large Array imaging of the inner ±70 arcsec of the giant low-luminosity radio galaxy NGC 315. An optimized model accurately fits the data in both total intensity and linear polarization. We infer that the velocity, emissivity and field structure in NGC 315 are very similar to those of the other low-luminosity sources we have modelled, but that all of the physical scales are larger by a factor of about 5. We derive an inclination to the line of sight of 38°± 2° for the jets. Where they first brighten, their on-axis velocity is  β=v/c≈ 0.9 . They decelerate to β≈ 0.4 between 8 and 18 kpc from the nucleus and the velocity thereafter remains constant. The speed at the edge of the jet is ≈0.6 of the on-axis value where it is best constrained, but the transverse velocity profile may deviate systematically from the Gaussian form we assume. The proper emissivity profile is split into three power-law regions separated by shorter transition zones. In the first of these, at ≈3 kpc (the flaring point) the jets expand rapidly at constant emissivity, leading to a large increase in the observed brightness on the approaching side. At ≈10 kpc, the emissivity drops abruptly by a factor of 2. Where the jets are well resolved, their rest-frame emission is centre brightened. The magnetic field is modelled as random on small scales but anisotropic and we rule out a globally ordered helical configuration. To a first approximation, the field evolves from a mixture of longitudinal and toroidal components to predominantly toroidal, but it also shows variations in structure along and across the jets, with a significant radial component in places. Simple adiabatic models fail to fit the emissivity variations.