Relativistic models of two low-luminosity radio jets: B2 0326+39and B2 1553+24

Abstract

We apply the intrinsically symmetrical, decelerating relativistic jet model developed by Laing & Bridle for 3C 31 to deep, full-synthesis 8.4-GHz VLA imaging of the two low-luminosity radio galaxies B2 0326+39 and B2 1553+24. After some modifications to the functional forms used to describe the geometry, velocity, emissivity and magnetic-field structure, these models can accurately fit our data in both total intensity and linear polarization. We conclude that the jets in B2 0326+39 and B2 1553+24 are at angles of 64° ± 5° and 7.°7 ± 1.°3 to the line of sight, respectively. In both objects, we find that the jets decelerate from 0.7-0.8c to < 0.2c over a distance of approximately 10 kpc, although in B2 1553+24 this transition occurs much further from the nucleus than in B2 0326+39 or 3C 31. The longitudinal emissivity profiles can be divided into sections, each fitted accurately by a power law; the indices of these power laws decrease with distance from the nucleus. B2 0326+39 also requires a discontinuity in emissivity to in order to fit a region with several bright knots of emission. In B2 1553+24, the sudden brightening of the jet can be explained by a combination of rapid expansion of the jet and a continuous variation of emissivity. The magnetic fields in both objects are dominated by the longitudinal component in the high-velocity regions close to the nucleus and by the toroidal component further out, but B2 0326+39 also has a significant radial component at large distances, whereas B2 1553+24 does not. Simple adiabatic models fail to fit the emissivity variations in the regions of high velocity but provide good descriptions of the emissivity after the jets have decelerated. Given the small angle to the line of sight inferred for B2 1553+24, there should be a significant population of similar sources at less extreme orientations. Such objects should have long (?200 kpc), straight, faint jets and we show that their true sizes are likely to have been underestimated in existing images.